linux/drivers/block/drbd/drbd_req.c

// SPDX-License-Identifier: GPL-2.0-or-later
/*
   drbd_req.c

   This file is part of DRBD by Philipp Reisner and Lars Ellenberg.

   Copyright (C) 2001-2008, LINBIT Information Technologies GmbH.
   Copyright (C) 1999-2008, Philipp Reisner <philipp.reisner@linbit.com>.
   Copyright (C) 2002-2008, Lars Ellenberg <lars.ellenberg@linbit.com>.


 */

#include <linux/module.h>

#include <linux/slab.h>
#include <linux/drbd.h>
#include "drbd_int.h"
#include "drbd_req.h"


static bool drbd_may_do_local_read(struct drbd_device *device, sector_t sector, int size);

/* Update disk stats at start of I/O request */
static void _drbd_start_io_acct(struct drbd_device *device, struct drbd_request *req)
{
	struct request_queue *q = device->rq_queue;

	generic_start_io_acct(q, bio_op(req->master_bio),
				req->i.size >> 9, &device->vdisk->part0);
}

/* Update disk stats when completing request upwards */
static void _drbd_end_io_acct(struct drbd_device *device, struct drbd_request *req)
{
	struct request_queue *q = device->rq_queue;

	generic_end_io_acct(q, bio_op(req->master_bio),
			    &device->vdisk->part0, req->start_jif);
}

static struct drbd_request *drbd_req_new(struct drbd_device *device, struct bio *bio_src)
{
	struct drbd_request *req;

	req = mempool_alloc(&drbd_request_mempool, GFP_NOIO);
	if (!req)
		return NULL;
	memset(req, 0, sizeof(*req));

	drbd_req_make_private_bio(req, bio_src);
	req->rq_state = (bio_data_dir(bio_src) == WRITE ? RQ_WRITE : 0)
		      | (bio_op(bio_src) == REQ_OP_WRITE_SAME ? RQ_WSAME : 0)
		      | (bio_op(bio_src) == REQ_OP_WRITE_ZEROES ? RQ_ZEROES : 0)
		      | (bio_op(bio_src) == REQ_OP_DISCARD ? RQ_UNMAP : 0);
	req->device = device;
	req->master_bio = bio_src;
	req->epoch = 0;

	drbd_clear_interval(&req->i);
	req->i.sector     = bio_src->bi_iter.bi_sector;
	req->i.size      = bio_src->bi_iter.bi_size;
	req->i.local = true;
	req->i.waiting = false;

	INIT_LIST_HEAD(&req->tl_requests);
	INIT_LIST_HEAD(&req->w.list);
	INIT_LIST_HEAD(&req->req_pending_master_completion);
	INIT_LIST_HEAD(&req->req_pending_local);

	/* one reference to be put by __drbd_make_request */
	atomic_set(&req->completion_ref, 1);
	/* one kref as long as completion_ref > 0 */
	kref_init(&req->kref);
	return req;
}

static void drbd_remove_request_interval(struct rb_root *root,
					 struct drbd_request *req)
{
	struct drbd_device *device = req->device;
	struct drbd_interval *i = &req->i;

	drbd_remove_interval(root, i);

	/* Wake up any processes waiting for this request to complete.  */
	if (i->waiting)
		wake_up(&device->misc_wait);
}

void drbd_req_destroy(struct kref *kref)
{
	struct drbd_request *req = container_of(kref, struct drbd_request, kref);
	struct drbd_device *device = req->device;
	const unsigned s = req->rq_state;

	if ((req->master_bio && !(s & RQ_POSTPONED)) ||
		atomic_read(&req->completion_ref) ||
		(s & RQ_LOCAL_PENDING) ||
		((s & RQ_NET_MASK) && !(s & RQ_NET_DONE))) {
		drbd_err(device, "drbd_req_destroy: Logic BUG rq_state = 0x%x, completion_ref = %d\n",
				s, atomic_read(&req->completion_ref));
		return;
	}

	/* If called from mod_rq_state (expected normal case) or
	 * drbd_send_and_submit (the less likely normal path), this holds the
	 * req_lock, and req->tl_requests will typicaly be on ->transfer_log,
	 * though it may be still empty (never added to the transfer log).
	 *
	 * If called from do_retry(), we do NOT hold the req_lock, but we are
	 * still allowed to unconditionally list_del(&req->tl_requests),
	 * because it will be on a local on-stack list only. */
	list_del_init(&req->tl_requests);

	/* finally remove the request from the conflict detection
	 * respective block_id verification interval tree. */
	if (!drbd_interval_empty(&req->i)) {
		struct rb_root *root;

		if (s & RQ_WRITE)
			root = &device->write_requests;
		else
			root = &device->read_requests;
		drbd_remove_request_interval(root, req);
	} else if (s & (RQ_NET_MASK & ~RQ_NET_DONE) && req->i.size != 0)
		drbd_err(device, "drbd_req_destroy: Logic BUG: interval empty, but: rq_state=0x%x, sect=%llu, size=%u\n",
			s, (unsigned long long)req->i.sector, req->i.size);

	/* if it was a write, we may have to set the corresponding
	 * bit(s) out-of-sync first. If it had a local part, we need to
	 * release the reference to the activity log. */
	if (s & RQ_WRITE) {
		/* Set out-of-sync unless both OK flags are set
		 * (local only or remote failed).
		 * Other places where we set out-of-sync:
		 * READ with local io-error */

		/* There is a special case:
		 * we may notice late that IO was suspended,
		 * and postpone, or schedule for retry, a write,
		 * before it even was submitted or sent.
		 * In that case we do not want to touch the bitmap at all.
		 */
		if ((s & (RQ_POSTPONED|RQ_LOCAL_MASK|RQ_NET_MASK)) != RQ_POSTPONED) {
			if (!(s & RQ_NET_OK) || !(s & RQ_LOCAL_OK))
				drbd_set_out_of_sync(device, req->i.sector, req->i.size);

			if ((s & RQ_NET_OK) && (s & RQ_LOCAL_OK) && (s & RQ_NET_SIS))
				drbd_set_in_sync(device, req->i.sector, req->i.size);
		}

		/* one might be tempted to move the drbd_al_complete_io
		 * to the local io completion callback drbd_request_endio.
		 * but, if this was a mirror write, we may only
		 * drbd_al_complete_io after this is RQ_NET_DONE,
		 * otherwise the extent could be dropped from the al
		 * before it has actually been written on the peer.
		 * if we crash before our peer knows about the request,
		 * but after the extent has been dropped from the al,
		 * we would forget to resync the corresponding extent.
		 */
		if (s & RQ_IN_ACT_LOG) {
			if (get_ldev_if_state(device, D_FAILED)) {
				drbd_al_complete_io(device, &req->i);
				put_ldev(device);
			} else if (__ratelimit(&drbd_ratelimit_state)) {
				drbd_warn(device, "Should have called drbd_al_complete_io(, %llu, %u), "
					 "but my Disk seems to have failed :(\n",
					 (unsigned long long) req->i.sector, req->i.size);
			}
		}
	}

	mempool_free(req, &drbd_request_mempool);
}

static void wake_all_senders(struct drbd_connection *connection)
{
	wake_up(&connection->sender_work.q_wait);
}

/* must hold resource->req_lock */
void start_new_tl_epoch(struct drbd_connection *connection)
{
	/* no point closing an epoch, if it is empty, anyways. */
	if (connection->current_tle_writes == 0)
		return;

	connection->current_tle_writes = 0;
	atomic_inc(&connection->current_tle_nr);
	wake_all_senders(connection);
}

void complete_master_bio(struct drbd_device *device,
		struct bio_and_error *m)
{
	m->bio->bi_status = errno_to_blk_status(m->error);
	bio_endio(m->bio);
	dec_ap_bio(device);
}


/* Helper for __req_mod().
 * Set m->bio to the master bio, if it is fit to be completed,
 * or leave it alone (it is initialized to NULL in __req_mod),
 * if it has already been completed, or cannot be completed yet.
 * If m->bio is set, the error status to be returned is placed in m->error.
 */
static
void drbd_req_complete(struct drbd_request *req, struct bio_and_error *m)
{
	const unsigned s = req->rq_state;
	struct drbd_device *device = req->device;
	int error, ok;

	/* we must not complete the master bio, while it is
	 *	still being processed by _drbd_send_zc_bio (drbd_send_dblock)
	 *	not yet acknowledged by the peer
	 *	not yet completed by the local io subsystem
	 * these flags may get cleared in any order by
	 *	the worker,
	 *	the receiver,
	 *	the bio_endio completion callbacks.
	 */
	if ((s & RQ_LOCAL_PENDING && !(s & RQ_LOCAL_ABORTED)) ||
	    (s & RQ_NET_QUEUED) || (s & RQ_NET_PENDING) ||
	    (s & RQ_COMPLETION_SUSP)) {
		drbd_err(device, "drbd_req_complete: Logic BUG rq_state = 0x%x\n", s);
		return;
	}

	if (!req->master_bio) {
		drbd_err(device, "drbd_req_complete: Logic BUG, master_bio == NULL!\n");
		return;
	}

	/*
	 * figure out whether to report success or failure.
	 *
	 * report success when at least one of the operations succeeded.
	 * or, to put the other way,
	 * only report failure, when both operations failed.
	 *
	 * what to do about the failures is handled elsewhere.
	 * what we need to do here is just: complete the master_bio.
	 *
	 * local completion error, if any, has been stored as ERR_PTR
	 * in private_bio within drbd_request_endio.
	 */
	ok = (s & RQ_LOCAL_OK) || (s & RQ_NET_OK);
	error = PTR_ERR(req->private_bio);

	/* Before we can signal completion to the upper layers,
	 * we may need to close the current transfer log epoch.
	 * We are within the request lock, so we can simply compare
	 * the request epoch number with the current transfer log
	 * epoch number.  If they match, increase the current_tle_nr,
	 * and reset the transfer log epoch write_cnt.
	 */
	if (op_is_write(bio_op(req->master_bio)) &&
	    req->epoch == atomic_read(&first_peer_device(device)->connection->current_tle_nr))
		start_new_tl_epoch(first_peer_device(device)->connection);

	/* Update disk stats */
	_drbd_end_io_acct(device, req);

	/* If READ failed,
	 * have it be pushed back to the retry work queue,
	 * so it will re-enter __drbd_make_request(),
	 * and be re-assigned to a suitable local or remote path,
	 * or failed if we do not have access to good data anymore.
	 *
	 * Unless it was failed early by __drbd_make_request(),
	 * because no path was available, in which case
	 * it was not even added to the transfer_log.
	 *
	 * read-ahead may fail, and will not be retried.
	 *
	 * WRITE should have used all available paths already.
	 */
	if (!ok &&
	    bio_op(req->master_bio) == REQ_OP_READ &&
	    !(req->master_bio->bi_opf & REQ_RAHEAD) &&
	    !list_empty(&req->tl_requests))
		req->rq_state |= RQ_POSTPONED;

	if (!(req->rq_state & RQ_POSTPONED)) {
		m->error = ok ? 0 : (error ?: -EIO);
		m->bio = req->master_bio;
		req->master_bio = NULL;
		/* We leave it in the tree, to be able to verify later
		 * write-acks in protocol != C during resync.
		 * But we mark it as "complete", so it won't be counted as
		 * conflict in a multi-primary setup. */
		req->i.completed = true;
	}

	if (req->i.waiting)
		wake_up(&device->misc_wait);

	/* Either we are about to complete to upper layers,
	 * or we will restart this request.
	 * In either case, the request object will be destroyed soon,
	 * so better remove it from all lists. */
	list_del_init(&req->req_pending_master_completion);
}

/* still holds resource->req_lock */
static void drbd_req_put_completion_ref(struct drbd_request *req, struct bio_and_error *m, int put)
{
	struct drbd_device *device = req->device;
	D_ASSERT(device, m || (req->rq_state & RQ_POSTPONED));

	if (!put)
		return;

	if (!atomic_sub_and_test(put, &req->completion_ref))
		return;

	drbd_req_complete(req, m);

	/* local completion may still come in later,
	 * we need to keep the req object around. */
	if (req->rq_state & RQ_LOCAL_ABORTED)
		return;

	if (req->rq_state & RQ_POSTPONED) {
		/* don't destroy the req object just yet,
		 * but queue it for retry */
		drbd_restart_request(req);
		return;
	}

	kref_put(&req->kref, drbd_req_destroy);
}

static void set_if_null_req_next(struct drbd_peer_device *peer_device, struct drbd_request *req)
{
	struct drbd_connection *connection = peer_device ? peer_device->connection : NULL;
	if (!connection)
		return;
	if (connection->req_next == NULL)
		connection->req_next = req;
}

static void advance_conn_req_next(struct drbd_peer_device *peer_device, struct drbd_request *req)
{
	struct drbd_connection *connection = peer_device ? peer_device->connection : NULL;
	if (!connection)
		return;
	if (connection->req_next != req)
		return;
	list_for_each_entry_continue(req, &connection->transfer_log, tl_requests) {
		const unsigned s = req->rq_state;
		if (s & RQ_NET_QUEUED)
			break;
	}
	if (&req->tl_requests == &connection->transfer_log)
		req = NULL;
	connection->req_next = req;
}

static void set_if_null_req_ack_pending(struct drbd_peer_device *peer_device, struct drbd_request *req)
{
	struct drbd_connection *connection = peer_device ? peer_device->connection : NULL;
	if (!connection)
		return;
	if (connection->req_ack_pending == NULL)
		connection->req_ack_pending = req;
}

static void advance_conn_req_ack_pending(struct drbd_peer_device *peer_device, struct drbd_request *req)
{
	struct drbd_connection *connection = peer_device ? peer_device->connection : NULL;
	if (!connection)
		return;
	if (connection->req_ack_pending != req)
		return;
	list_for_each_entry_continue(req, &connection->transfer_log, tl_requests) {
		const unsigned s = req->rq_state;
		if ((s & RQ_NET_SENT) && (s & RQ_NET_PENDING))
			break;
	}
	if (&req->tl_requests == &connection->transfer_log)
		req = NULL;
	connection->req_ack_pending = req;
}

static void set_if_null_req_not_net_done(struct drbd_peer_device *peer_device, struct drbd_request *req)
{
	struct drbd_connection *connection = peer_device ? peer_device->connection : NULL;
	if (!connection)
		return;
	if (connection->req_not_net_done == NULL)
		connection->req_not_net_done = req;
}

static void advance_conn_req_not_net_done(struct drbd_peer_device *peer_device, struct drbd_request *req)
{
	struct drbd_connection *connection = peer_device ? peer_device->connection : NULL;
	if (!connection)
		return;
	if (connection->req_not_net_done != req)
		return;
	list_for_each_entry_continue(req, &connection->transfer_log, tl_requests) {
		const unsigned s = req->rq_state;
		if ((s & RQ_NET_SENT) && !(s & RQ_NET_DONE))
			break;
	}
	if (&req->tl_requests == &connection->transfer_log)
		req = NULL;
	connection->req_not_net_done = req;
}

/* I'd like this to be the only place that manipulates
 * req->completion_ref and req->kref. */
static void mod_rq_state(struct drbd_request *req, struct bio_and_error *m,
		int clear, int set)
{
	struct drbd_device *device = req->device;
	struct drbd_peer_device *peer_device = first_peer_device(device);
	unsigned s = req->rq_state;
	int c_put = 0;

	if (drbd_suspended(device) && !((s | clear) & RQ_COMPLETION_SUSP))
		set |= RQ_COMPLETION_SUSP;

	/* apply */

	req->rq_state &= ~clear;
	req->rq_state |= set;

	/* no change? */
	if (req->rq_state == s)
		return;

	/* intent: get references */

	kref_get(&req->kref);

	if (!(s & RQ_LOCAL_PENDING) && (set & RQ_LOCAL_PENDING))
		atomic_inc(&req->completion_ref);

	if (!(s & RQ_NET_PENDING) && (set & RQ_NET_PENDING)) {
		inc_ap_pending(device);
		atomic_inc(&req->completion_ref);
	}

	if (!(s & RQ_NET_QUEUED) && (set & RQ_NET_QUEUED)) {
		atomic_inc(&req->completion_ref);
		set_if_null_req_next(peer_device, req);
	}

	if (!(s & RQ_EXP_BARR_ACK) && (set & RQ_EXP_BARR_ACK))
		kref_get(&req->kref); /* wait for the DONE */

	if (!(s & RQ_NET_SENT) && (set & RQ_NET_SENT)) {
		/* potentially already completed in the ack_receiver thread */
		if (!(s & RQ_NET_DONE)) {
			atomic_add(req->i.size >> 9, &device->ap_in_flight);
			set_if_null_req_not_net_done(peer_device, req);
		}
		if (req->rq_state & RQ_NET_PENDING)
			set_if_null_req_ack_pending(peer_device, req);
	}

	if (!(s & RQ_COMPLETION_SUSP) && (set & RQ_COMPLETION_SUSP))
		atomic_inc(&req->completion_ref);

	/* progress: put references */

	if ((s & RQ_COMPLETION_SUSP) && (clear & RQ_COMPLETION_SUSP))
		++c_put;

	if (!(s & RQ_LOCAL_ABORTED) && (set & RQ_LOCAL_ABORTED)) {
		D_ASSERT(device, req->rq_state & RQ_LOCAL_PENDING);
		++c_put;
	}

	if ((s & RQ_LOCAL_PENDING) && (clear & RQ_LOCAL_PENDING)) {
		if (req->rq_state & RQ_LOCAL_ABORTED)
			kref_put(&req->kref, drbd_req_destroy);
		else
			++c_put;
		list_del_init(&req->req_pending_local);
	}

	if ((s & RQ_NET_PENDING) && (clear & RQ_NET_PENDING)) {
		dec_ap_pending(device);
		++c_put;
		req->acked_jif = jiffies;
		advance_conn_req_ack_pending(peer_device, req);
	}

	if ((s & RQ_NET_QUEUED) && (clear & RQ_NET_QUEUED)) {
		++c_put;
		advance_conn_req_next(peer_device, req);
	}

	if (!(s & RQ_NET_DONE) && (set & RQ_NET_DONE)) {
		if (s & RQ_NET_SENT)
			atomic_sub(req->i.size >> 9, &device->ap_in_flight);
		if (s & RQ_EXP_BARR_ACK)
			kref_put(&req->kref, drbd_req_destroy);
		req->net_done_jif = jiffies;

		/* in ahead/behind mode, or just in case,
		 * before we finally destroy this request,
		 * the caching pointers must not reference it anymore */
		advance_conn_req_next(peer_device, req);
		advance_conn_req_ack_pending(peer_device, req);
		advance_conn_req_not_net_done(peer_device, req);
	}

	/* potentially complete and destroy */

	/* If we made progress, retry conflicting peer requests, if any. */
	if (req->i.waiting)
		wake_up(&device->misc_wait);

	drbd_req_put_completion_ref(req, m, c_put);
	kref_put(&req->kref, drbd_req_destroy);
}

static void drbd_report_io_error(struct drbd_device *device, struct drbd_request *req)
{
        char b[BDEVNAME_SIZE];

	if (!__ratelimit(&drbd_ratelimit_state))
		return;

	drbd_warn(device, "local %s IO error sector %llu+%u on %s\n",
			(req->rq_state & RQ_WRITE) ? "WRITE" : "READ",
			(unsigned long long)req->i.sector,
			req->i.size >> 9,
			bdevname(device->ldev->backing_bdev, b));
}

/* Helper for HANDED_OVER_TO_NETWORK.
 * Is this a protocol A write (neither WRITE_ACK nor RECEIVE_ACK expected)?
 * Is it also still "PENDING"?
 * --> If so, clear PENDING and set NET_OK below.
 * If it is a protocol A write, but not RQ_PENDING anymore, neg-ack was faster
 * (and we must not set RQ_NET_OK) */
static inline bool is_pending_write_protocol_A(struct drbd_request *req)
{
	return (req->rq_state &
		   (RQ_WRITE|RQ_NET_PENDING|RQ_EXP_WRITE_ACK|RQ_EXP_RECEIVE_ACK))
		== (RQ_WRITE|RQ_NET_PENDING);
}

/* obviously this could be coded as many single functions
 * instead of one huge switch,
 * or by putting the code directly in the respective locations
 * (as it has been before).
 *
 * but having it this way
 *  enforces that it is all in this one place, where it is easier to audit,
 *  it makes it obvious that whatever "event" "happens" to a request should
 *  happen "atomically" within the req_lock,
 *  and it enforces that we have to think in a very structured manner
 *  about the "events" that may happen to a request during its life time ...
 */
int __req_mod(struct drbd_request *req, enum drbd_req_event what,
		struct bio_and_error *m)
{
	struct drbd_device *const device = req->device;
	struct drbd_peer_device *const peer_device = first_peer_device(device);
	struct drbd_connection *const connection = peer_device ? peer_device->connection : NULL;
	struct net_conf *nc;
	int p, rv = 0;

	if (m)
		m->bio = NULL;

	switch (what) {
	default:
		drbd_err(device, "LOGIC BUG in %s:%u\n", __FILE__ , __LINE__);
		break;

	/* does not happen...
	 * initialization done in drbd_req_new
	case CREATED:
		break;
		*/

	case TO_BE_SENT: /* via network */
		/* reached via __drbd_make_request
		 * and from w_read_retry_remote */
		D_ASSERT(device, !(req->rq_state & RQ_NET_MASK));
		rcu_read_lock();
		nc = rcu_dereference(connection->net_conf);
		p = nc->wire_protocol;
		rcu_read_unlock();
		req->rq_state |=
			p == DRBD_PROT_C ? RQ_EXP_WRITE_ACK :
			p == DRBD_PROT_B ? RQ_EXP_RECEIVE_ACK : 0;
		mod_rq_state(req, m, 0, RQ_NET_PENDING);
		break;

	case TO_BE_SUBMITTED: /* locally */
		/* reached via __drbd_make_request */
		D_ASSERT(device, !(req->rq_state & RQ_LOCAL_MASK));
		mod_rq_state(req, m, 0, RQ_LOCAL_PENDING);
		break;

	case COMPLETED_OK:
		if (req->rq_state & RQ_WRITE)
			device->writ_cnt += req->i.size >> 9;
		else
			device->read_cnt += req->i.size >> 9;

		mod_rq_state(req, m, RQ_LOCAL_PENDING,
				RQ_LOCAL_COMPLETED|RQ_LOCAL_OK);
		break;

	case ABORT_DISK_IO:
		mod_rq_state(req, m, 0, RQ_LOCAL_ABORTED);
		break;

	case WRITE_COMPLETED_WITH_ERROR:
		drbd_report_io_error(device, req);
		__drbd_chk_io_error(device, DRBD_WRITE_ERROR);
		mod_rq_state(req, m, RQ_LOCAL_PENDING, RQ_LOCAL_COMPLETED);
		break;

	case READ_COMPLETED_WITH_ERROR:
		drbd_set_out_of_sync(device, req->i.sector, req->i.size);
		drbd_report_io_error(device, req);
		__drbd_chk_io_error(device, DRBD_READ_ERROR);
		/* fall through. */
	case READ_AHEAD_COMPLETED_WITH_ERROR:
		/* it is legal to fail read-ahead, no __drbd_chk_io_error in that case. */
		mod_rq_state(req, m, RQ_LOCAL_PENDING, RQ_LOCAL_COMPLETED);
		break;

	case DISCARD_COMPLETED_NOTSUPP:
	case DISCARD_COMPLETED_WITH_ERROR:
		/* I'd rather not detach from local disk just because it
		 * failed a REQ_OP_DISCARD. */
		mod_rq_state(req, m, RQ_LOCAL_PENDING, RQ_LOCAL_COMPLETED);
		break;

	case QUEUE_FOR_NET_READ:
		/* READ, and
		 * no local disk,
		 * or target area marked as invalid,
		 * or just got an io-error. */
		/* from __drbd_make_request
		 * or from bio_endio during read io-error recovery */

		/* So we can verify the handle in the answer packet.
		 * Corresponding drbd_remove_request_interval is in
		 * drbd_req_complete() */
		D_ASSERT(device, drbd_interval_empty(&req->i));
		drbd_insert_interval(&device->read_requests, &req->i);

		set_bit(UNPLUG_REMOTE, &device->flags);

		D_ASSERT(device, req->rq_state & RQ_NET_PENDING);
		D_ASSERT(device, (req->rq_state & RQ_LOCAL_MASK) == 0);
		mod_rq_state(req, m, 0, RQ_NET_QUEUED);
		req->w.cb = w_send_read_req;
		drbd_queue_work(&connection->sender_work,
				&req->w);
		break;

	case QUEUE_FOR_NET_WRITE:
		/* assert something? */
		/* from __drbd_make_request only */

		/* Corresponding drbd_remove_request_interval is in
		 * drbd_req_complete() */
		D_ASSERT(device, drbd_interval_empty(&req->i));
		drbd_insert_interval(&device->write_requests, &req->i);

		/* NOTE
		 * In case the req ended up on the transfer log before being
		 * queued on the worker, it could lead to this request being
		 * missed during cleanup after connection loss.
		 * So we have to do both operations here,
		 * within the same lock that protects the transfer log.
		 *
		 * _req_add_to_epoch(req); this has to be after the
		 * _maybe_start_new_epoch(req); which happened in
		 * __drbd_make_request, because we now may set the bit
		 * again ourselves to close the current epoch.
		 *
		 * Add req to the (now) current epoch (barrier). */

		/* otherwise we may lose an unplug, which may cause some remote
		 * io-scheduler timeout to expire, increasing maximum latency,
		 * hurting performance. */
		set_bit(UNPLUG_REMOTE, &device->flags);

		/* queue work item to send data */
		D_ASSERT(device, req->rq_state & RQ_NET_PENDING);
		mod_rq_state(req, m, 0, RQ_NET_QUEUED|RQ_EXP_BARR_ACK);
		req->w.cb =  w_send_dblock;
		drbd_queue_work(&connection->sender_work,
				&req->w);

		/* close the epoch, in case it outgrew the limit */
		rcu_read_lock();
		nc = rcu_dereference(connection->net_conf);
		p = nc->max_epoch_size;
		rcu_read_unlock();
		if (connection->current_tle_writes >= p)
			start_new_tl_epoch(connection);

		break;

	case QUEUE_FOR_SEND_OOS:
		mod_rq_state(req, m, 0, RQ_NET_QUEUED);
		req->w.cb =  w_send_out_of_sync;
		drbd_queue_work(&connection->sender_work,
				&req->w);
		break;

	case READ_RETRY_REMOTE_CANCELED:
	case SEND_CANCELED:
	case SEND_FAILED:
		/* real cleanup will be done from tl_clear.  just update flags
		 * so it is no longer marked as on the worker queue */
		mod_rq_state(req, m, RQ_NET_QUEUED, 0);
		break;

	case HANDED_OVER_TO_NETWORK:
		/* assert something? */
		if (is_pending_write_protocol_A(req))
			/* this is what is dangerous about protocol A:
			 * pretend it was successfully written on the peer. */
			mod_rq_state(req, m, RQ_NET_QUEUED|RQ_NET_PENDING,
						RQ_NET_SENT|RQ_NET_OK);
		else
			mod_rq_state(req, m, RQ_NET_QUEUED, RQ_NET_SENT);
		/* It is still not yet RQ_NET_DONE until the
		 * corresponding epoch barrier got acked as well,
		 * so we know what to dirty on connection loss. */
		break;

	case OOS_HANDED_TO_NETWORK:
		/* Was not set PENDING, no longer QUEUED, so is now DONE
		 * as far as this connection is concerned. */
		mod_rq_state(req, m, RQ_NET_QUEUED, RQ_NET_DONE);
		break;

	case CONNECTION_LOST_WHILE_PENDING:
		/* transfer log cleanup after connection loss */
		mod_rq_state(req, m,
				RQ_NET_OK|RQ_NET_PENDING|RQ_COMPLETION_SUSP,
				RQ_NET_DONE);
		break;

	case CONFLICT_RESOLVED:
		/* for superseded conflicting writes of multiple primaries,
		 * there is no need to keep anything in the tl, potential
		 * node crashes are covered by the activity log.
		 *
		 * If this request had been marked as RQ_POSTPONED before,
		 * it will actually not be completed, but "restarted",
		 * resubmitted from the retry worker context. */
		D_ASSERT(device, req->rq_state & RQ_NET_PENDING);
		D_ASSERT(device, req->rq_state & RQ_EXP_WRITE_ACK);
		mod_rq_state(req, m, RQ_NET_PENDING, RQ_NET_DONE|RQ_NET_OK);
		break;

	case WRITE_ACKED_BY_PEER_AND_SIS:
		req->rq_state |= RQ_NET_SIS;
	case WRITE_ACKED_BY_PEER:
		/* Normal operation protocol C: successfully written on peer.
		 * During resync, even in protocol != C,
		 * we requested an explicit write ack anyways.
		 * Which means we cannot even assert anything here.
		 * Nothing more to do here.
		 * We want to keep the tl in place for all protocols, to cater
		 * for volatile write-back caches on lower level devices. */
		goto ack_common;
	case RECV_ACKED_BY_PEER:
		D_ASSERT(device, req->rq_state & RQ_EXP_RECEIVE_ACK);
		/* protocol B; pretends to be successfully written on peer.
		 * see also notes above in HANDED_OVER_TO_NETWORK about
		 * protocol != C */
	ack_common:
		mod_rq_state(req, m, RQ_NET_PENDING, RQ_NET_OK);
		break;

	case POSTPONE_WRITE:
		D_ASSERT(device, req->rq_state & RQ_EXP_WRITE_ACK);
		/* If this node has already detected the write conflict, the
		 * worker will be waiting on misc_wait.  Wake it up once this
		 * request has completed locally.
		 */
		D_ASSERT(device, req->rq_state & RQ_NET_PENDING);
		req->rq_state |= RQ_POSTPONED;
		if (req->i.waiting)
			wake_up(&device->misc_wait);
		/* Do not clear RQ_NET_PENDING. This request will make further
		 * progress via restart_conflicting_writes() or
		 * fail_postponed_requests(). Hopefully. */
		break;

	case NEG_ACKED:
		mod_rq_state(req, m, RQ_NET_OK|RQ_NET_PENDING, 0);
		break;

	case FAIL_FROZEN_DISK_IO:
		if (!(req->rq_state & RQ_LOCAL_COMPLETED))
			break;
		mod_rq_state(req, m, RQ_COMPLETION_SUSP, 0);
		break;

	case RESTART_FROZEN_DISK_IO:
		if (!(req->rq_state & RQ_LOCAL_COMPLETED))
			break;

		mod_rq_state(req, m,
				RQ_COMPLETION_SUSP|RQ_LOCAL_COMPLETED,
				RQ_LOCAL_PENDING);

		rv = MR_READ;
		if (bio_data_dir(req->master_bio) == WRITE)
			rv = MR_WRITE;

		get_ldev(device); /* always succeeds in this call path */
		req->w.cb = w_restart_disk_io;
		drbd_queue_work(&connection->sender_work,
				&req->w);
		break;

	case RESEND:
		/* Simply complete (local only) READs. */
		if (!(req->rq_state & RQ_WRITE) && !req->w.cb) {
			mod_rq_state(req, m, RQ_COMPLETION_SUSP, 0);
			break;
		}

		/* If RQ_NET_OK is already set, we got a P_WRITE_ACK or P_RECV_ACK
		   before the connection loss (B&C only); only P_BARRIER_ACK
		   (or the local completion?) was missing when we suspended.
		   Throwing them out of the TL here by pretending we got a BARRIER_ACK.
		   During connection handshake, we ensure that the peer was not rebooted. */
		if (!(req->rq_state & RQ_NET_OK)) {
			/* FIXME could this possibly be a req->dw.cb == w_send_out_of_sync?
			 * in that case we must not set RQ_NET_PENDING. */

			mod_rq_state(req, m, RQ_COMPLETION_SUSP, RQ_NET_QUEUED|RQ_NET_PENDING);
			if (req->w.cb) {
				/* w.cb expected to be w_send_dblock, or w_send_read_req */
				drbd_queue_work(&connection->sender_work,
						&req->w);
				rv = req->rq_state & RQ_WRITE ? MR_WRITE : MR_READ;
			} /* else: FIXME can this happen? */
			break;
		}
		/* else, fall through - to BARRIER_ACKED */

	case BARRIER_ACKED:
		/* barrier ack for READ requests does not make sense */
		if (!(req->rq_state & RQ_WRITE))
			break;

		if (req->rq_state & RQ_NET_PENDING) {
			/* barrier came in before all requests were acked.
			 * this is bad, because if the connection is lost now,
			 * we won't be able to clean them up... */
			drbd_err(device, "FIXME (BARRIER_ACKED but pending)\n");
		}
		/* Allowed to complete requests, even while suspended.
		 * As this is called for all requests within a matching epoch,
		 * we need to filter, and only set RQ_NET_DONE for those that
		 * have actually been on the wire. */
		mod_rq_state(req, m, RQ_COMPLETION_SUSP,
				(req->rq_state & RQ_NET_MASK) ? RQ_NET_DONE : 0);
		break;

	case DATA_RECEIVED:
		D_ASSERT(device, req->rq_state & RQ_NET_PENDING);
		mod_rq_state(req, m, RQ_NET_PENDING, RQ_NET_OK|RQ_NET_DONE);
		break;

	case QUEUE_AS_DRBD_BARRIER:
		start_new_tl_epoch(connection);
		mod_rq_state(req, m, 0, RQ_NET_OK|RQ_NET_DONE);
		break;
	}

	return rv;
}

/* we may do a local read if:
 * - we are consistent (of course),
 * - or we are generally inconsistent,
 *   BUT we are still/already IN SYNC for this area.
 *   since size may be bigger than BM_BLOCK_SIZE,
 *   we may need to check several bits.
 */
static bool drbd_may_do_local_read(struct drbd_device *device, sector_t sector, int size)
{
	unsigned long sbnr, ebnr;
	sector_t esector, nr_sectors;

	if (device->state.disk == D_UP_TO_DATE)
		return true;
	if (device->state.disk != D_INCONSISTENT)
		return false;
	esector = sector + (size >> 9) - 1;
	nr_sectors = drbd_get_capacity(device->this_bdev);
	D_ASSERT(device, sector  < nr_sectors);
	D_ASSERT(device, esector < nr_sectors);

	sbnr = BM_SECT_TO_BIT(sector);
	ebnr = BM_SECT_TO_BIT(esector);

	return drbd_bm_count_bits(device, sbnr, ebnr) == 0;
}

static bool remote_due_to_read_balancing(struct drbd_device *device, sector_t sector,
		enum drbd_read_balancing rbm)
{
	struct backing_dev_info *bdi;
	int stripe_shift;

	switch (rbm) {
	case RB_CONGESTED_REMOTE:
		bdi = device->ldev->backing_bdev->bd_disk->queue->backing_dev_info;
		return bdi_read_congested(bdi);
	case RB_LEAST_PENDING:
		return atomic_read(&device->local_cnt) >
			atomic_read(&device->ap_pending_cnt) + atomic_read(&device->rs_pending_cnt);
	case RB_32K_STRIPING:  /* stripe_shift = 15 */
	case RB_64K_STRIPING:
	case RB_128K_STRIPING:
	case RB_256K_STRIPING:
	case RB_512K_STRIPING:
	case RB_1M_STRIPING:   /* stripe_shift = 20 */
		stripe_shift = (rbm - RB_32K_STRIPING + 15);
		return (sector >> (stripe_shift - 9)) & 1;
	case RB_ROUND_ROBIN:
		return test_and_change_bit(READ_BALANCE_RR, &device->flags);
	case RB_PREFER_REMOTE:
		return true;
	case RB_PREFER_LOCAL:
	default:
		return false;
	}
}

/*
 * complete_conflicting_writes  -  wait for any conflicting write requests
 *
 * The write_requests tree contains all active write requests which we
 * currently know about.  Wait for any requests to complete which conflict with
 * the new one.
 *
 * Only way out: remove the conflicting intervals from the tree.
 */
static void complete_conflicting_writes(struct drbd_request *req)
{
	DEFINE_WAIT(wait);
	struct drbd_device *device = req->device;
	struct drbd_interval *i;
	sector_t sector = req->i.sector;
	int size = req->i.size;

	for (;;) {
		drbd_for_each_overlap(i, &device->write_requests, sector, size) {
			/* Ignore, if already completed to upper layers. */
			if (i->completed)
				continue;
			/* Handle the first found overlap.  After the schedule
			 * we have to restart the tree walk. */
			break;
		}
		if (!i)	/* if any */
			break;

		/* Indicate to wake up device->misc_wait on progress.  */
		prepare_to_wait(&device->misc_wait, &wait, TASK_UNINTERRUPTIBLE);
		i->waiting = true;
		spin_unlock_irq(&device->resource->req_lock);
		schedule();
		spin_lock_irq(&device->resource->req_lock);
	}
	finish_wait(&device->misc_wait, &wait);
}

/* called within req_lock */
static void maybe_pull_ahead(struct drbd_device *device)
{
	struct drbd_connection *connection = first_peer_device(device)->connection;
	struct net_conf *nc;
	bool congested = false;
	enum drbd_on_congestion on_congestion;

	rcu_read_lock();
	nc = rcu_dereference(connection->net_conf);
	on_congestion = nc ? nc->on_congestion : OC_BLOCK;
	rcu_read_unlock();
	if (on_congestion == OC_BLOCK ||
	    connection->agreed_pro_version < 96)
		return;

	if (on_congestion == OC_PULL_AHEAD && device->state.conn == C_AHEAD)
		return; /* nothing to do ... */

	/* If I don't even have good local storage, we can not reasonably try
	 * to pull ahead of the peer. We also need the local reference to make
	 * sure device->act_log is there.
	 */
	if (!get_ldev_if_state(device, D_UP_TO_DATE))
		return;

	if (nc->cong_fill &&
	    atomic_read(&device->ap_in_flight) >= nc->cong_fill) {
		drbd_info(device, "Congestion-fill threshold reached\n");
		congested = true;
	}

	if (device->act_log->used >= nc->cong_extents) {
		drbd_info(device, "Congestion-extents threshold reached\n");
		congested = true;
	}

	if (congested) {
		/* start a new epoch for non-mirrored writes */
		start_new_tl_epoch(first_peer_device(device)->connection);

		if (on_congestion == OC_PULL_AHEAD)
			_drbd_set_state(_NS(device, conn, C_AHEAD), 0, NULL);
		else  /*nc->on_congestion == OC_DISCONNECT */
			_drbd_set_state(_NS(device, conn, C_DISCONNECTING), 0, NULL);
	}
	put_ldev(device);
}

/* If this returns false, and req->private_bio is still set,
 * this should be submitted locally.
 *
 * If it returns false, but req->private_bio is not set,
 * we do not have access to good data :(
 *
 * Otherwise, this destroys req->private_bio, if any,
 * and returns true.
 */
static bool do_remote_read(struct drbd_request *req)
{
	struct drbd_device *device = req->device;
	enum drbd_read_balancing rbm;

	if (req->private_bio) {
		if (!drbd_may_do_local_read(device,
					req->i.sector, req->i.size)) {
			bio_put(req->private_bio);
			req->private_bio = NULL;
			put_ldev(device);
		}
	}

	if (device->state.pdsk != D_UP_TO_DATE)
		return false;

	if (req->private_bio == NULL)
		return true;

	/* TODO: improve read balancing decisions, take into account drbd
	 * protocol, pending requests etc. */

	rcu_read_lock();
	rbm = rcu_dereference(device->ldev->disk_conf)->read_balancing;
	rcu_read_unlock();

	if (rbm == RB_PREFER_LOCAL && req->private_bio)
		return false; /* submit locally */

	if (remote_due_to_read_balancing(device, req->i.sector, rbm)) {
		if (req->private_bio) {
			bio_put(req->private_bio);
			req->private_bio = NULL;
			put_ldev(device);
		}
		return true;
	}

	return false;
}

bool drbd_should_do_remote(union drbd_dev_state s)
{
	return s.pdsk == D_UP_TO_DATE ||
		(s.pdsk >= D_INCONSISTENT &&
		 s.conn >= C_WF_BITMAP_T &&
		 s.conn < C_AHEAD);
	/* Before proto 96 that was >= CONNECTED instead of >= C_WF_BITMAP_T.
	   That is equivalent since before 96 IO was frozen in the C_WF_BITMAP*
	   states. */
}

static bool drbd_should_send_out_of_sync(union drbd_dev_state s)
{
	return s.conn == C_AHEAD || s.conn == C_WF_BITMAP_S;
	/* pdsk = D_INCONSISTENT as a consequence. Protocol 96 check not necessary
	   since we enter state C_AHEAD only if proto >= 96 */
}

/* returns number of connections (== 1, for drbd 8.4)
 * expected to actually write this data,
 * which does NOT include those that we are L_AHEAD for. */
static int drbd_process_write_request(struct drbd_request *req)
{
	struct drbd_device *device = req->device;
	int remote, send_oos;

	remote = drbd_should_do_remote(device->state);
	send_oos = drbd_should_send_out_of_sync(device->state);

	/* Need to replicate writes.  Unless it is an empty flush,
	 * which is better mapped to a DRBD P_BARRIER packet,
	 * also for drbd wire protocol compatibility reasons.
	 * If this was a flush, just start a new epoch.
	 * Unless the current epoch was empty anyways, or we are not currently
	 * replicating, in which case there is no point. */
	if (unlikely(req->i.size == 0)) {
		/* The only size==0 bios we expect are empty flushes. */
		D_ASSERT(device, req->master_bio->bi_opf & REQ_PREFLUSH);
		if (remote)
			_req_mod(req, QUEUE_AS_DRBD_BARRIER);
		return remote;
	}

	if (!remote && !send_oos)
		return 0;

	D_ASSERT(device, !(remote && send_oos));

	if (remote) {
		_req_mod(req, TO_BE_SENT);
		_req_mod(req, QUEUE_FOR_NET_WRITE);
	} else if (drbd_set_out_of_sync(device, req->i.sector, req->i.size))
		_req_mod(req, QUEUE_FOR_SEND_OOS);

	return remote;
}

static void drbd_process_discard_or_zeroes_req(struct drbd_request *req, int flags)
{
	int err = drbd_issue_discard_or_zero_out(req->device,
				req->i.sector, req->i.size >> 9, flags);
	if (err)
		req->private_bio->bi_status = BLK_STS_IOERR;
	bio_endio(req->private_bio);
}

static void
drbd_submit_req_private_bio(struct drbd_request *req)
{
	struct drbd_device *device = req->device;
	struct bio *bio = req->private_bio;
	unsigned int type;

	if (bio_op(bio) != REQ_OP_READ)
		type = DRBD_FAULT_DT_WR;
	else if (bio->bi_opf & REQ_RAHEAD)
		type = DRBD_FAULT_DT_RA;
	else
		type = DRBD_FAULT_DT_RD;

	bio_set_dev(bio, device->ldev->backing_bdev);

	/* State may have changed since we grabbed our reference on the
	 * ->ldev member. Double check, and short-circuit to endio.
	 * In case the last activity log transaction failed to get on
	 * stable storage, and this is a WRITE, we may not even submit
	 * this bio. */
	if (get_ldev(device)) {
		if (drbd_insert_fault(device, type))
			bio_io_error(bio);
		else if (bio_op(bio) == REQ_OP_WRITE_ZEROES)
			drbd_process_discard_or_zeroes_req(req, EE_ZEROOUT |
			    ((bio->bi_opf & REQ_NOUNMAP) ? 0 : EE_TRIM));
		else if (bio_op(bio) == REQ_OP_DISCARD)
			drbd_process_discard_or_zeroes_req(req, EE_TRIM);
		else
			generic_make_request(bio);
		put_ldev(device);
	} else
		bio_io_error(bio);
}

static void drbd_queue_write(struct drbd_device *device, struct drbd_request *req)
{
	spin_lock_irq(&device->resource->req_lock);
	list_add_tail(&req->tl_requests, &device->submit.writes);
	list_add_tail(&req->req_pending_master_completion,
			&device->pending_master_completion[1 /* WRITE */]);
	spin_unlock_irq(&device->resource->req_lock);
	queue_work(device->submit.wq, &device->submit.worker);
	/* do_submit() may sleep internally on al_wait, too */
	wake_up(&device->al_wait);
}

/* returns the new drbd_request pointer, if the caller is expected to
 * drbd_send_and_submit() it (to save latency), or NULL if we queued the
 * request on the submitter thread.
 * Returns ERR_PTR(-ENOMEM) if we cannot allocate a drbd_request.
 */
static struct drbd_request *
drbd_request_prepare(struct drbd_device *device, struct bio *bio, unsigned long start_jif)
{
	const int rw = bio_data_dir(bio);
	struct drbd_request *req;

	/* allocate outside of all locks; */
	req = drbd_req_new(device, bio);
	if (!req) {
		dec_ap_bio(device);
		/* only pass the error to the upper layers.
		 * if user cannot handle io errors, that's not our business. */
		drbd_err(device, "could not kmalloc() req\n");
		bio->bi_status = BLK_STS_RESOURCE;
		bio_endio(bio);
		return ERR_PTR(-ENOMEM);
	}
	req->start_jif = start_jif;

	if (!get_ldev(device)) {
		bio_put(req->private_bio);
		req->private_bio = NULL;
	}

	/* Update disk stats */
	_drbd_start_io_acct(device, req);

	/* process discards always from our submitter thread */
	if (bio_op(bio) == REQ_OP_WRITE_ZEROES ||
	    bio_op(bio) == REQ_OP_DISCARD)
		goto queue_for_submitter_thread;

	if (rw == WRITE && req->private_bio && req->i.size
	&& !test_bit(AL_SUSPENDED, &device->flags)) {
		if (!drbd_al_begin_io_fastpath(device, &req->i))
			goto queue_for_submitter_thread;
		req->rq_state |= RQ_IN_ACT_LOG;
		req->in_actlog_jif = jiffies;
	}
	return req;

 queue_for_submitter_thread:
	atomic_inc(&device->ap_actlog_cnt);
	drbd_queue_write(device, req);
	return NULL;
}

/* Require at least one path to current data.
 * We don't want to allow writes on C_STANDALONE D_INCONSISTENT:
 * We would not allow to read what was written,
 * we would not have bumped the data generation uuids,
 * we would cause data divergence for all the wrong reasons.
 *
 * If we don't see at least one D_UP_TO_DATE, we will fail this request,
 * which either returns EIO, or, if OND_SUSPEND_IO is set, suspends IO,
 * and queues for retry later.
 */
static bool may_do_writes(struct drbd_device *device)
{
	const union drbd_dev_state s = device->state;
	return s.disk == D_UP_TO_DATE || s.pdsk == D_UP_TO_DATE;
}

struct drbd_plug_cb {
	struct blk_plug_cb cb;
	struct drbd_request *most_recent_req;
	/* do we need more? */
};

static void drbd_unplug(struct blk_plug_cb *cb, bool from_schedule)
{
	struct drbd_plug_cb *plug = container_of(cb, struct drbd_plug_cb, cb);
	struct drbd_resource *resource = plug->cb.data;
	struct drbd_request *req = plug->most_recent_req;

	kfree(cb);
	if (!req)
		return;

	spin_lock_irq(&resource->req_lock);
	/* In case the sender did not process it yet, raise the flag to
	 * have it followed with P_UNPLUG_REMOTE just after. */
	req->rq_state |= RQ_UNPLUG;
	/* but also queue a generic unplug */
	drbd_queue_unplug(req->device);
	kref_put(&req->kref, drbd_req_destroy);
	spin_unlock_irq(&resource->req_lock);
}

static struct drbd_plug_cb* drbd_check_plugged(struct drbd_resource *resource)
{
	/* A lot of text to say
	 * return (struct drbd_plug_cb*)blk_check_plugged(); */
	struct drbd_plug_cb *plug;
	struct blk_plug_cb *cb = blk_check_plugged(drbd_unplug, resource, sizeof(*plug));

	if (cb)
		plug = container_of(cb, struct drbd_plug_cb, cb);
	else
		plug = NULL;
	return plug;
}

static void drbd_update_plug(struct drbd_plug_cb *plug, struct drbd_request *req)
{
	struct drbd_request *tmp = plug->most_recent_req;
	/* Will be sent to some peer.
	 * Remember to tag it with UNPLUG_REMOTE on unplug */
	kref_get(&req->kref);
	plug->most_recent_req = req;
	if (tmp)
		kref_put(&tmp->kref, drbd_req_destroy);
}

static void drbd_send_and_submit(struct drbd_device *device, struct drbd_request *req)
{
	struct drbd_resource *resource = device->resource;
	const int rw = bio_data_dir(req->master_bio);
	struct bio_and_error m = { NULL, };
	bool no_remote = false;
	bool submit_private_bio = false;

	spin_lock_irq(&resource->req_lock);
	if (rw == WRITE) {
		/* This may temporarily give up the req_lock,
		 * but will re-aquire it before it returns here.
		 * Needs to be before the check on drbd_suspended() */
		complete_conflicting_writes(req);
		/* no more giving up req_lock from now on! */

		/* check for congestion, and potentially stop sending
		 * full data updates, but start sending "dirty bits" only. */
		maybe_pull_ahead(device);
	}


	if (drbd_suspended(device)) {
		/* push back and retry: */
		req->rq_state |= RQ_POSTPONED;
		if (req->private_bio) {
			bio_put(req->private_bio);
			req->private_bio = NULL;
			put_ldev(device);
		}
		goto out;
	}

	/* We fail READ early, if we can not serve it.
	 * We must do this before req is registered on any lists.
	 * Otherwise, drbd_req_complete() will queue failed READ for retry. */
	if (rw != WRITE) {
		if (!do_remote_read(req) && !req->private_bio)
			goto nodata;
	}

	/* which transfer log epoch does this belong to? */
	req->epoch = atomic_read(&first_peer_device(device)->connection->current_tle_nr);

	/* no point in adding empty flushes to the transfer log,
	 * they are mapped to drbd barriers already. */
	if (likely(req->i.size!=0)) {
		if (rw == WRITE)
			first_peer_device(device)->connection->current_tle_writes++;

		list_add_tail(&req->tl_requests, &first_peer_device(device)->connection->transfer_log);
	}

	if (rw == WRITE) {
		if (req->private_bio && !may_do_writes(device)) {
			bio_put(req->private_bio);
			req->private_bio = NULL;
			put_ldev(device);
			goto nodata;
		}
		if (!drbd_process_write_request(req))
			no_remote = true;
	} else {
		/* We either have a private_bio, or we can read from remote.
		 * Otherwise we had done the goto nodata above. */
		if (req->private_bio == NULL) {
			_req_mod(req, TO_BE_SENT);
			_req_mod(req, QUEUE_FOR_NET_READ);
		} else
			no_remote = true;
	}

	if (no_remote == false) {
		struct drbd_plug_cb *plug = drbd_check_plugged(resource);
		if (plug)
			drbd_update_plug(plug, req);
	}

	/* If it took the fast path in drbd_request_prepare, add it here.
	 * The slow path has added it already. */
	if (list_empty(&req->req_pending_master_completion))
		list_add_tail(&req->req_pending_master_completion,
			&device->pending_master_completion[rw == WRITE]);
	if (req->private_bio) {
		/* needs to be marked within the same spinlock */
		req->pre_submit_jif = jiffies;
		list_add_tail(&req->req_pending_local,
			&device->pending_completion[rw == WRITE]);
		_req_mod(req, TO_BE_SUBMITTED);
		/* but we need to give up the spinlock to submit */
		submit_private_bio = true;
	} else if (no_remote) {
nodata:
		if (__ratelimit(&drbd_ratelimit_state))
			drbd_err(device, "IO ERROR: neither local nor remote data, sector %llu+%u\n",
					(unsigned long long)req->i.sector, req->i.size >> 9);
		/* A write may have been queued for send_oos, however.
		 * So we can not simply free it, we must go through drbd_req_put_completion_ref() */
	}

out:
	drbd_req_put_completion_ref(req, &m, 1);
	spin_unlock_irq(&resource->req_lock);

	/* Even though above is a kref_put(), this is safe.
	 * As long as we still need to submit our private bio,
	 * we hold a completion ref, and the request cannot disappear.
	 * If however this request did not even have a private bio to submit
	 * (e.g. remote read), req may already be invalid now.
	 * That's why we cannot check on req->private_bio. */
	if (submit_private_bio)
		drbd_submit_req_private_bio(req);
	if (m.bio)
		complete_master_bio(device, &m);
}

void __drbd_make_request(struct drbd_device *device, struct bio *bio, unsigned long start_jif)
{
	struct drbd_request *req = drbd_request_prepare(device, bio, start_jif);
	if (IS_ERR_OR_NULL(req))
		return;
	drbd_send_and_submit(device, req);
}

static void submit_fast_path(struct drbd_device *device, struct list_head *incoming)
{
	struct blk_plug plug;
	struct drbd_request *req, *tmp;

	blk_start_plug(&plug);
	list_for_each_entry_safe(req, tmp, incoming, tl_requests) {
		const int rw = bio_data_dir(req->master_bio);

		if (rw == WRITE /* rw != WRITE should not even end up here! */
		&& req->private_bio && req->i.size
		&& !test_bit(AL_SUSPENDED, &device->flags)) {
			if (!drbd_al_begin_io_fastpath(device, &req->i))
				continue;

			req->rq_state |= RQ_IN_ACT_LOG;
			req->in_actlog_jif = jiffies;
			atomic_dec(&device->ap_actlog_cnt);
		}

		list_del_init(&req->tl_requests);
		drbd_send_and_submit(device, req);
	}
	blk_finish_plug(&plug);
}

static bool prepare_al_transaction_nonblock(struct drbd_device *device,
					    struct list_head *incoming,
					    struct list_head *pending,
					    struct list_head *later)
{
	struct drbd_request *req;
	int wake = 0;
	int err;

	spin_lock_irq(&device->al_lock);
	while ((req = list_first_entry_or_null(incoming, struct drbd_request, tl_requests))) {
		err = drbd_al_begin_io_nonblock(device, &req->i);
		if (err == -ENOBUFS)
			break;
		if (err == -EBUSY)
			wake = 1;
		if (err)
			list_move_tail(&req->tl_requests, later);
		else
			list_move_tail(&req->tl_requests, pending);
	}
	spin_unlock_irq(&device->al_lock);
	if (wake)
		wake_up(&device->al_wait);
	return !list_empty(pending);
}

static void send_and_submit_pending(struct drbd_device *device, struct list_head *pending)
{
	struct blk_plug plug;
	struct drbd_request *req;

	blk_start_plug(&plug);
	while ((req = list_first_entry_or_null(pending, struct drbd_request, tl_requests))) {
		req->rq_state |= RQ_IN_ACT_LOG;
		req->in_actlog_jif = jiffies;
		atomic_dec(&device->ap_actlog_cnt);
		list_del_init(&req->tl_requests);
		drbd_send_and_submit(device, req);
	}
	blk_finish_plug(&plug);
}

void do_submit(struct work_struct *ws)
{
	struct drbd_device *device = container_of(ws, struct drbd_device, submit.worker);
	LIST_HEAD(incoming);	/* from drbd_make_request() */
	LIST_HEAD(pending);	/* to be submitted after next AL-transaction commit */
	LIST_HEAD(busy);	/* blocked by resync requests */

	/* grab new incoming requests */
	spin_lock_irq(&device->resource->req_lock);
	list_splice_tail_init(&device->submit.writes, &incoming);
	spin_unlock_irq(&device->resource->req_lock);

	for (;;) {
		DEFINE_WAIT(wait);

		/* move used-to-be-busy back to front of incoming */
		list_splice_init(&busy, &incoming);
		submit_fast_path(device, &incoming);
		if (list_empty(&incoming))
			break;

		for (;;) {
			prepare_to_wait(&device->al_wait, &wait, TASK_UNINTERRUPTIBLE);

			list_splice_init(&busy, &incoming);
			prepare_al_transaction_nonblock(device, &incoming, &pending, &busy);
			if (!list_empty(&pending))
				break;

			schedule();

			/* If all currently "hot" activity log extents are kept busy by
			 * incoming requests, we still must not totally starve new
			 * requests to "cold" extents.
			 * Something left on &incoming means there had not been
			 * enough update slots available, and the activity log
			 * has been marked as "starving".
			 *
			 * Try again now, without looking for new requests,
			 * effectively blocking all new requests until we made
			 * at least _some_ progress with what we currently have.
			 */
			if (!list_empty(&incoming))
				continue;

			/* Nothing moved to pending, but nothing left
			 * on incoming: all moved to busy!
			 * Grab new and iterate. */
			spin_lock_irq(&device->resource->req_lock);
			list_splice_tail_init(&device->submit.writes, &incoming);
			spin_unlock_irq(&device->resource->req_lock);
		}
		finish_wait(&device->al_wait, &wait);

		/* If the transaction was full, before all incoming requests
		 * had been processed, skip ahead to commit, and iterate
		 * without splicing in more incoming requests from upper layers.
		 *
		 * Else, if all incoming have been processed,
		 * they have become either "pending" (to be submitted after
		 * next transaction commit) or "busy" (blocked by resync).
		 *
		 * Maybe more was queued, while we prepared the transaction?
		 * Try to stuff those into this transaction as well.
		 * Be strictly non-blocking here,
		 * we already have something to commit.
		 *
		 * Commit if we don't make any more progres.
		 */

		while (list_empty(&incoming)) {
			LIST_HEAD(more_pending);
			LIST_HEAD(more_incoming);
			bool made_progress;

			/* It is ok to look outside the lock,
			 * it's only an optimization anyways */
			if (list_empty(&device->submit.writes))
				break;

			spin_lock_irq(&device->resource->req_lock);
			list_splice_tail_init(&device->submit.writes, &more_incoming);
			spin_unlock_irq(&device->resource->req_lock);

			if (list_empty(&more_incoming))
				break;

			made_progress = prepare_al_transaction_nonblock(device, &more_incoming, &more_pending, &busy);

			list_splice_tail_init(&more_pending, &pending);
			list_splice_tail_init(&more_incoming, &incoming);
			if (!made_progress)
				break;
		}

		drbd_al_begin_io_commit(device);
		send_and_submit_pending(device, &pending);
	}
}

blk_qc_t drbd_make_request(struct request_queue *q, struct bio *bio)
{
	struct drbd_device *device = (struct drbd_device *) q->queuedata;
	unsigned long start_jif;

	blk_queue_split(q, &bio);

	start_jif = jiffies;

	/*
	 * what we "blindly" assume:
	 */
	D_ASSERT(device, IS_ALIGNED(bio->bi_iter.bi_size, 512));

	inc_ap_bio(device);
	__drbd_make_request(device, bio, start_jif);
	return BLK_QC_T_NONE;
}

static bool net_timeout_reached(struct drbd_request *net_req,
		struct drbd_connection *connection,
		unsigned long now, unsigned long ent,
		unsigned int ko_count, unsigned int timeout)
{
	struct drbd_device *device = net_req->device;

	if (!time_after(now, net_req->pre_send_jif + ent))
		return false;

	if (time_in_range(now, connection->last_reconnect_jif, connection->last_reconnect_jif + ent))
		return false;

	if (net_req->rq_state & RQ_NET_PENDING) {
		drbd_warn(device, "Remote failed to finish a request within %ums > ko-count (%u) * timeout (%u * 0.1s)\n",
			jiffies_to_msecs(now - net_req->pre_send_jif), ko_count, timeout);
		return true;
	}

	/* We received an ACK already (or are using protocol A),
	 * but are waiting for the epoch closing barrier ack.
	 * Check if we sent the barrier already.  We should not blame the peer
	 * for being unresponsive, if we did not even ask it yet. */
	if (net_req->epoch == connection->send.current_epoch_nr) {
		drbd_warn(device,
			"We did not send a P_BARRIER for %ums > ko-count (%u) * timeout (%u * 0.1s); drbd kernel thread blocked?\n",
			jiffies_to_msecs(now - net_req->pre_send_jif), ko_count, timeout);
		return false;
	}

	/* Worst case: we may have been blocked for whatever reason, then
	 * suddenly are able to send a lot of requests (and epoch separating
	 * barriers) in quick succession.
	 * The timestamp of the net_req may be much too old and not correspond
	 * to the sending time of the relevant unack'ed barrier packet, so
	 * would trigger a spurious timeout.  The latest barrier packet may
	 * have a too recent timestamp to trigger the timeout, potentially miss
	 * a timeout.  Right now we don't have a place to conveniently store
	 * these timestamps.
	 * But in this particular situation, the application requests are still
	 * completed to upper layers, DRBD should still "feel" responsive.
	 * No need yet to kill this connection, it may still recover.
	 * If not, eventually we will have queued enough into the network for
	 * us to block. From that point of view, the timestamp of the last sent
	 * barrier packet is relevant enough.
	 */
	if (time_after(now, connection->send.last_sent_barrier_jif + ent)) {
		drbd_warn(device, "Remote failed to answer a P_BARRIER (sent at %lu jif; now=%lu jif) within %ums > ko-count (%u) * timeout (%u * 0.1s)\n",
			connection->send.last_sent_barrier_jif, now,
			jiffies_to_msecs(now - connection->send.last_sent_barrier_jif), ko_count, timeout);
		return true;
	}
	return false;
}

/* A request is considered timed out, if
 * - we have some effective timeout from the configuration,
 *   with some state restrictions applied,
 * - the oldest request is waiting for a response from the network
 *   resp. the local disk,
 * - the oldest request is in fact older than the effective timeout,
 * - the connection was established (resp. disk was attached)
 *   for longer than the timeout already.
 * Note that for 32bit jiffies and very stable connections/disks,
 * we may have a wrap around, which is catched by
 *   !time_in_range(now, last_..._jif, last_..._jif + timeout).
 *
 * Side effect: once per 32bit wrap-around interval, which means every
 * ~198 days with 250 HZ, we have a window where the timeout would need
 * to expire twice (worst case) to become effective. Good enough.
 */

void request_timer_fn(struct timer_list *t)
{
	struct drbd_device *device = from_timer(device, t, request_timer);
	struct drbd_connection *connection = first_peer_device(device)->connection;
	struct drbd_request *req_read, *req_write, *req_peer; /* oldest request */
	struct net_conf *nc;
	unsigned long oldest_submit_jif;
	unsigned long ent = 0, dt = 0, et, nt; /* effective timeout = ko_count * timeout */
	unsigned long now;
	unsigned int ko_count = 0, timeout = 0;

	rcu_read_lock();
	nc = rcu_dereference(connection->net_conf);
	if (nc && device->state.conn >= C_WF_REPORT_PARAMS) {
		ko_count = nc->ko_count;
		timeout = nc->timeout;
	}

	if (get_ldev(device)) { /* implicit state.disk >= D_INCONSISTENT */
		dt = rcu_dereference(device->ldev->disk_conf)->disk_timeout * HZ / 10;
		put_ldev(device);
	}
	rcu_read_unlock();


	ent = timeout * HZ/10 * ko_count;
	et = min_not_zero(dt, ent);

	if (!et)
		return; /* Recurring timer stopped */

	now = jiffies;
	nt = now + et;

	spin_lock_irq(&device->resource->req_lock);
	req_read = list_first_entry_or_null(&device->pending_completion[0], struct drbd_request, req_pending_local);
	req_write = list_first_entry_or_null(&device->pending_completion[1], struct drbd_request, req_pending_local);

	/* maybe the oldest request waiting for the peer is in fact still
	 * blocking in tcp sendmsg.  That's ok, though, that's handled via the
	 * socket send timeout, requesting a ping, and bumping ko-count in
	 * we_should_drop_the_connection().
	 */

	/* check the oldest request we did successfully sent,
	 * but which is still waiting for an ACK. */
	req_peer = connection->req_ack_pending;

	/* if we don't have such request (e.g. protocoll A)
	 * check the oldest requests which is still waiting on its epoch
	 * closing barrier ack. */
	if (!req_peer)
		req_peer = connection->req_not_net_done;

	/* evaluate the oldest peer request only in one timer! */
	if (req_peer && req_peer->device != device)
		req_peer = NULL;

	/* do we have something to evaluate? */
	if (req_peer == NULL && req_write == NULL && req_read == NULL)
		goto out;

	oldest_submit_jif =
		(req_write && req_read)
		? ( time_before(req_write->pre_submit_jif, req_read->pre_submit_jif)
		  ? req_write->pre_submit_jif : req_read->pre_submit_jif )
		: req_write ? req_write->pre_submit_jif
		: req_read ? req_read->pre_submit_jif : now;

	if (ent && req_peer && net_timeout_reached(req_peer, connection, now, ent, ko_count, timeout))
		_conn_request_state(connection, NS(conn, C_TIMEOUT), CS_VERBOSE | CS_HARD);

	if (dt && oldest_submit_jif != now &&
		 time_after(now, oldest_submit_jif + dt) &&
		!time_in_range(now, device->last_reattach_jif, device->last_reattach_jif + dt)) {
		drbd_warn(device, "Local backing device failed to meet the disk-timeout\n");
		__drbd_chk_io_error(device, DRBD_FORCE_DETACH);
	}

	/* Reschedule timer for the nearest not already expired timeout.
	 * Fallback to now + min(effective network timeout, disk timeout). */
	ent = (ent && req_peer && time_before(now, req_peer->pre_send_jif + ent))
		? req_peer->pre_send_jif + ent : now + et;
	dt = (dt && oldest_submit_jif != now && time_before(now, oldest_submit_jif + dt))
		? oldest_submit_jif + dt : now + et;
	nt = time_before(ent, dt) ? ent : dt;
out:
	spin_unlock_irq(&device->resource->req_lock);
	mod_timer(&device->request_timer, nt);
}