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1362729b16
Simplify the way fscache cache objects retain their cookie. The way I implemented the cookie storage handling made synchronisation a pain (ie. the object state machine can't rely on the cookie actually still being there). Instead of the the object being detached from the cookie and the cookie being freed in __fscache_relinquish_cookie(), we defer both operations: (*) The detachment of the object from the list in the cookie now takes place in fscache_drop_object() and is thus governed by the object state machine (fscache_detach_from_cookie() has been removed). (*) The release of the cookie is now in fscache_object_destroy() - which is called by the cache backend just before it frees the object. This means that the fscache_cookie struct is now available to the cache all the way through from ->alloc_object() to ->drop_object() and ->put_object() - meaning that it's no longer necessary to take object->lock to guarantee access. However, __fscache_relinquish_cookie() doesn't wait for the object to go all the way through to destruction before letting the netfs proceed. That would massively slow down the netfs. Since __fscache_relinquish_cookie() leaves the cookie around, in must therefore break all attachments to the netfs - which includes ->def, ->netfs_data and any outstanding page read/writes. To handle this, struct fscache_cookie now has an n_active counter: (1) This starts off initialised to 1. (2) Any time the cache needs to get at the netfs data, it calls fscache_use_cookie() to increment it - if it is not zero. If it was zero, then access is not permitted. (3) When the cache has finished with the data, it calls fscache_unuse_cookie() to decrement it. This does a wake-up on it if it reaches 0. (4) __fscache_relinquish_cookie() decrements n_active and then waits for it to reach 0. The initialisation to 1 in step (1) ensures that we only get wake ups when we're trying to get rid of the cookie. This leaves __fscache_relinquish_cookie() a lot simpler. *** This fixes a problem in the current code whereby if fscache_invalidate() is followed sufficiently quickly by fscache_relinquish_cookie() then it is possible for __fscache_relinquish_cookie() to have detached the cookie from the object and cleared the pointer before a thread is dispatched to process the invalidation state in the object state machine. Since the pending write clearance was deferred to the invalidation state to make it asynchronous, we need to either wait in relinquishment for the stores tree to be cleared in the invalidation state or we need to handle the clearance in relinquishment. Further, if the relinquishment code does clear the tree, then the invalidation state need to make the clearance contingent on still having the cookie to hand (since that's where the tree is rooted) and we have to prevent the cookie from disappearing for the duration. This can lead to an oops like the following: BUG: unable to handle kernel NULL pointer dereference at 000000000000000c ... RIP: 0010:[<ffffffff8151023e>] _spin_lock+0xe/0x30 ... CR2: 000000000000000c ... ... Process kslowd002 (...) .... Call Trace: [<ffffffffa01c3278>] fscache_invalidate_writes+0x38/0xd0 [fscache] [<ffffffff810096f0>] ? __switch_to+0xd0/0x320 [<ffffffff8105e759>] ? find_busiest_queue+0x69/0x150 [<ffffffff8110ddd4>] ? slow_work_enqueue+0x104/0x180 [<ffffffffa01c1303>] fscache_object_slow_work_execute+0x5e3/0x9d0 [fscache] [<ffffffff81096b67>] ? bit_waitqueue+0x17/0xd0 [<ffffffff8110e233>] slow_work_execute+0x233/0x310 [<ffffffff8110e515>] slow_work_thread+0x205/0x360 [<ffffffff81096ca0>] ? autoremove_wake_function+0x0/0x40 [<ffffffff8110e310>] ? slow_work_thread+0x0/0x360 [<ffffffff81096936>] kthread+0x96/0xa0 [<ffffffff8100c0ca>] child_rip+0xa/0x20 [<ffffffff810968a0>] ? kthread+0x0/0xa0 [<ffffffff8100c0c0>] ? child_rip+0x0/0x20 The parameter to fscache_invalidate_writes() was object->cookie which is NULL. Signed-off-by: David Howells <dhowells@redhat.com> Tested-By: Milosz Tanski <milosz@adfin.com> Acked-by: Jeff Layton <jlayton@redhat.com>
146 lines
4.3 KiB
C
146 lines
4.3 KiB
C
/* Filesystem index definition
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*
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* Copyright (C) 2004-2007 Red Hat, Inc. All Rights Reserved.
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* Written by David Howells (dhowells@redhat.com)
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*
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* This program is free software; you can redistribute it and/or
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* modify it under the terms of the GNU General Public License
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* as published by the Free Software Foundation; either version
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* 2 of the License, or (at your option) any later version.
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*/
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#define FSCACHE_DEBUG_LEVEL CACHE
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#include <linux/module.h>
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#include "internal.h"
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static uint16_t fscache_fsdef_netfs_get_key(const void *cookie_netfs_data,
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void *buffer, uint16_t bufmax);
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static uint16_t fscache_fsdef_netfs_get_aux(const void *cookie_netfs_data,
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void *buffer, uint16_t bufmax);
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static
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enum fscache_checkaux fscache_fsdef_netfs_check_aux(void *cookie_netfs_data,
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const void *data,
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uint16_t datalen);
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/*
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* The root index is owned by FS-Cache itself.
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*
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* When a netfs requests caching facilities, FS-Cache will, if one doesn't
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* already exist, create an entry in the root index with the key being the name
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* of the netfs ("AFS" for example), and the auxiliary data holding the index
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* structure version supplied by the netfs:
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*
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* FSDEF
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* |
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* +-----------+
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* | |
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* NFS AFS
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* [v=1] [v=1]
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*
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* If an entry with the appropriate name does already exist, the version is
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* compared. If the version is different, the entire subtree from that entry
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* will be discarded and a new entry created.
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*
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* The new entry will be an index, and a cookie referring to it will be passed
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* to the netfs. This is then the root handle by which the netfs accesses the
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* cache. It can create whatever objects it likes in that index, including
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* further indices.
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*/
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static struct fscache_cookie_def fscache_fsdef_index_def = {
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.name = ".FS-Cache",
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.type = FSCACHE_COOKIE_TYPE_INDEX,
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};
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struct fscache_cookie fscache_fsdef_index = {
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.usage = ATOMIC_INIT(1),
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.n_active = ATOMIC_INIT(1),
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.lock = __SPIN_LOCK_UNLOCKED(fscache_fsdef_index.lock),
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.backing_objects = HLIST_HEAD_INIT,
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.def = &fscache_fsdef_index_def,
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};
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EXPORT_SYMBOL(fscache_fsdef_index);
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/*
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* Definition of an entry in the root index. Each entry is an index, keyed to
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* a specific netfs and only applicable to a particular version of the index
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* structure used by that netfs.
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*/
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struct fscache_cookie_def fscache_fsdef_netfs_def = {
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.name = "FSDEF.netfs",
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.type = FSCACHE_COOKIE_TYPE_INDEX,
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.get_key = fscache_fsdef_netfs_get_key,
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.get_aux = fscache_fsdef_netfs_get_aux,
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.check_aux = fscache_fsdef_netfs_check_aux,
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};
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/*
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* get the key data for an FSDEF index record - this is the name of the netfs
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* for which this entry is created
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*/
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static uint16_t fscache_fsdef_netfs_get_key(const void *cookie_netfs_data,
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void *buffer, uint16_t bufmax)
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{
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const struct fscache_netfs *netfs = cookie_netfs_data;
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unsigned klen;
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_enter("{%s.%u},", netfs->name, netfs->version);
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klen = strlen(netfs->name);
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if (klen > bufmax)
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return 0;
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memcpy(buffer, netfs->name, klen);
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return klen;
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}
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/*
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* get the auxiliary data for an FSDEF index record - this is the index
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* structure version number of the netfs for which this version is created
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*/
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static uint16_t fscache_fsdef_netfs_get_aux(const void *cookie_netfs_data,
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void *buffer, uint16_t bufmax)
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{
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const struct fscache_netfs *netfs = cookie_netfs_data;
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unsigned dlen;
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_enter("{%s.%u},", netfs->name, netfs->version);
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dlen = sizeof(uint32_t);
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if (dlen > bufmax)
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return 0;
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memcpy(buffer, &netfs->version, dlen);
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return dlen;
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}
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/*
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* check that the index structure version number stored in the auxiliary data
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* matches the one the netfs gave us
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*/
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static enum fscache_checkaux fscache_fsdef_netfs_check_aux(
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void *cookie_netfs_data,
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const void *data,
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uint16_t datalen)
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{
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struct fscache_netfs *netfs = cookie_netfs_data;
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uint32_t version;
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_enter("{%s},,%hu", netfs->name, datalen);
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if (datalen != sizeof(version)) {
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_leave(" = OBSOLETE [dl=%d v=%zu]", datalen, sizeof(version));
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return FSCACHE_CHECKAUX_OBSOLETE;
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}
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memcpy(&version, data, sizeof(version));
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if (version != netfs->version) {
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_leave(" = OBSOLETE [ver=%x net=%x]", version, netfs->version);
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return FSCACHE_CHECKAUX_OBSOLETE;
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}
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_leave(" = OKAY");
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return FSCACHE_CHECKAUX_OKAY;
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}
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