2
0
mirror of https://github.com/edk2-porting/linux-next.git synced 2024-12-21 03:33:59 +08:00
linux-next/fs/btrfs/locking.c
David Sterba d4e253bbbc btrfs: document extent buffer locking
Signed-off-by: David Sterba <dsterba@suse.com>
2019-11-18 17:51:50 +01:00

526 lines
15 KiB
C

// SPDX-License-Identifier: GPL-2.0
/*
* Copyright (C) 2008 Oracle. All rights reserved.
*/
#include <linux/sched.h>
#include <linux/pagemap.h>
#include <linux/spinlock.h>
#include <linux/page-flags.h>
#include <asm/bug.h>
#include "misc.h"
#include "ctree.h"
#include "extent_io.h"
#include "locking.h"
/*
* Extent buffer locking
* =====================
*
* The locks use a custom scheme that allows to do more operations than are
* available fromt current locking primitives. The building blocks are still
* rwlock and wait queues.
*
* Required semantics:
*
* - reader/writer exclusion
* - writer/writer exclusion
* - reader/reader sharing
* - spinning lock semantics
* - blocking lock semantics
* - try-lock semantics for readers and writers
* - one level nesting, allowing read lock to be taken by the same thread that
* already has write lock
*
* The extent buffer locks (also called tree locks) manage access to eb data
* related to the storage in the b-tree (keys, items, but not the individual
* members of eb).
* We want concurrency of many readers and safe updates. The underlying locking
* is done by read-write spinlock and the blocking part is implemented using
* counters and wait queues.
*
* spinning semantics - the low-level rwlock is held so all other threads that
* want to take it are spinning on it.
*
* blocking semantics - the low-level rwlock is not held but the counter
* denotes how many times the blocking lock was held;
* sleeping is possible
*
* Write lock always allows only one thread to access the data.
*
*
* Debugging
* ---------
*
* There are additional state counters that are asserted in various contexts,
* removed from non-debug build to reduce extent_buffer size and for
* performance reasons.
*
*
* Lock nesting
* ------------
*
* A write operation on a tree might indirectly start a look up on the same
* tree. This can happen when btrfs_cow_block locks the tree and needs to
* lookup free extents.
*
* btrfs_cow_block
* ..
* alloc_tree_block_no_bg_flush
* btrfs_alloc_tree_block
* btrfs_reserve_extent
* ..
* load_free_space_cache
* ..
* btrfs_lookup_file_extent
* btrfs_search_slot
*
*
* Locking pattern - spinning
* --------------------------
*
* The simple locking scenario, the +--+ denotes the spinning section.
*
* +- btrfs_tree_lock
* | - extent_buffer::rwlock is held
* | - no heavy operations should happen, eg. IO, memory allocations, large
* | structure traversals
* +- btrfs_tree_unock
*
*
* Locking pattern - blocking
* --------------------------
*
* The blocking write uses the following scheme. The +--+ denotes the spinning
* section.
*
* +- btrfs_tree_lock
* |
* +- btrfs_set_lock_blocking_write
*
* - allowed: IO, memory allocations, etc.
*
* -- btrfs_tree_unlock - note, no explicit unblocking necessary
*
*
* Blocking read is similar.
*
* +- btrfs_tree_read_lock
* |
* +- btrfs_set_lock_blocking_read
*
* - heavy operations allowed
*
* +- btrfs_tree_read_unlock_blocking
* |
* +- btrfs_tree_read_unlock
*
*/
#ifdef CONFIG_BTRFS_DEBUG
static inline void btrfs_assert_spinning_writers_get(struct extent_buffer *eb)
{
WARN_ON(eb->spinning_writers);
eb->spinning_writers++;
}
static inline void btrfs_assert_spinning_writers_put(struct extent_buffer *eb)
{
WARN_ON(eb->spinning_writers != 1);
eb->spinning_writers--;
}
static inline void btrfs_assert_no_spinning_writers(struct extent_buffer *eb)
{
WARN_ON(eb->spinning_writers);
}
static inline void btrfs_assert_spinning_readers_get(struct extent_buffer *eb)
{
atomic_inc(&eb->spinning_readers);
}
static inline void btrfs_assert_spinning_readers_put(struct extent_buffer *eb)
{
WARN_ON(atomic_read(&eb->spinning_readers) == 0);
atomic_dec(&eb->spinning_readers);
}
static inline void btrfs_assert_tree_read_locks_get(struct extent_buffer *eb)
{
atomic_inc(&eb->read_locks);
}
static inline void btrfs_assert_tree_read_locks_put(struct extent_buffer *eb)
{
atomic_dec(&eb->read_locks);
}
static inline void btrfs_assert_tree_read_locked(struct extent_buffer *eb)
{
BUG_ON(!atomic_read(&eb->read_locks));
}
static inline void btrfs_assert_tree_write_locks_get(struct extent_buffer *eb)
{
eb->write_locks++;
}
static inline void btrfs_assert_tree_write_locks_put(struct extent_buffer *eb)
{
eb->write_locks--;
}
#else
static void btrfs_assert_spinning_writers_get(struct extent_buffer *eb) { }
static void btrfs_assert_spinning_writers_put(struct extent_buffer *eb) { }
static void btrfs_assert_no_spinning_writers(struct extent_buffer *eb) { }
static void btrfs_assert_spinning_readers_put(struct extent_buffer *eb) { }
static void btrfs_assert_spinning_readers_get(struct extent_buffer *eb) { }
static void btrfs_assert_tree_read_locked(struct extent_buffer *eb) { }
static void btrfs_assert_tree_read_locks_get(struct extent_buffer *eb) { }
static void btrfs_assert_tree_read_locks_put(struct extent_buffer *eb) { }
static void btrfs_assert_tree_write_locks_get(struct extent_buffer *eb) { }
static void btrfs_assert_tree_write_locks_put(struct extent_buffer *eb) { }
#endif
/*
* Mark already held read lock as blocking. Can be nested in write lock by the
* same thread.
*
* Use when there are potentially long operations ahead so other thread waiting
* on the lock will not actively spin but sleep instead.
*
* The rwlock is released and blocking reader counter is increased.
*/
void btrfs_set_lock_blocking_read(struct extent_buffer *eb)
{
trace_btrfs_set_lock_blocking_read(eb);
/*
* No lock is required. The lock owner may change if we have a read
* lock, but it won't change to or away from us. If we have the write
* lock, we are the owner and it'll never change.
*/
if (eb->lock_nested && current->pid == eb->lock_owner)
return;
btrfs_assert_tree_read_locked(eb);
atomic_inc(&eb->blocking_readers);
btrfs_assert_spinning_readers_put(eb);
read_unlock(&eb->lock);
}
/*
* Mark already held write lock as blocking.
*
* Use when there are potentially long operations ahead so other threads
* waiting on the lock will not actively spin but sleep instead.
*
* The rwlock is released and blocking writers is set.
*/
void btrfs_set_lock_blocking_write(struct extent_buffer *eb)
{
trace_btrfs_set_lock_blocking_write(eb);
/*
* No lock is required. The lock owner may change if we have a read
* lock, but it won't change to or away from us. If we have the write
* lock, we are the owner and it'll never change.
*/
if (eb->lock_nested && current->pid == eb->lock_owner)
return;
if (eb->blocking_writers == 0) {
btrfs_assert_spinning_writers_put(eb);
btrfs_assert_tree_locked(eb);
WRITE_ONCE(eb->blocking_writers, 1);
write_unlock(&eb->lock);
}
}
/*
* Lock the extent buffer for read. Wait for any writers (spinning or blocking).
* Can be nested in write lock by the same thread.
*
* Use when the locked section does only lightweight actions and busy waiting
* would be cheaper than making other threads do the wait/wake loop.
*
* The rwlock is held upon exit.
*/
void btrfs_tree_read_lock(struct extent_buffer *eb)
{
u64 start_ns = 0;
if (trace_btrfs_tree_read_lock_enabled())
start_ns = ktime_get_ns();
again:
read_lock(&eb->lock);
BUG_ON(eb->blocking_writers == 0 &&
current->pid == eb->lock_owner);
if (eb->blocking_writers) {
if (current->pid == eb->lock_owner) {
/*
* This extent is already write-locked by our thread.
* We allow an additional read lock to be added because
* it's for the same thread. btrfs_find_all_roots()
* depends on this as it may be called on a partly
* (write-)locked tree.
*/
BUG_ON(eb->lock_nested);
eb->lock_nested = true;
read_unlock(&eb->lock);
trace_btrfs_tree_read_lock(eb, start_ns);
return;
}
read_unlock(&eb->lock);
wait_event(eb->write_lock_wq,
READ_ONCE(eb->blocking_writers) == 0);
goto again;
}
btrfs_assert_tree_read_locks_get(eb);
btrfs_assert_spinning_readers_get(eb);
trace_btrfs_tree_read_lock(eb, start_ns);
}
/*
* Lock extent buffer for read, optimistically expecting that there are no
* contending blocking writers. If there are, don't wait.
*
* Return 1 if the rwlock has been taken, 0 otherwise
*/
int btrfs_tree_read_lock_atomic(struct extent_buffer *eb)
{
if (READ_ONCE(eb->blocking_writers))
return 0;
read_lock(&eb->lock);
/* Refetch value after lock */
if (READ_ONCE(eb->blocking_writers)) {
read_unlock(&eb->lock);
return 0;
}
btrfs_assert_tree_read_locks_get(eb);
btrfs_assert_spinning_readers_get(eb);
trace_btrfs_tree_read_lock_atomic(eb);
return 1;
}
/*
* Try-lock for read. Don't block or wait for contending writers.
*
* Retrun 1 if the rwlock has been taken, 0 otherwise
*/
int btrfs_try_tree_read_lock(struct extent_buffer *eb)
{
if (READ_ONCE(eb->blocking_writers))
return 0;
if (!read_trylock(&eb->lock))
return 0;
/* Refetch value after lock */
if (READ_ONCE(eb->blocking_writers)) {
read_unlock(&eb->lock);
return 0;
}
btrfs_assert_tree_read_locks_get(eb);
btrfs_assert_spinning_readers_get(eb);
trace_btrfs_try_tree_read_lock(eb);
return 1;
}
/*
* Try-lock for write. May block until the lock is uncontended, but does not
* wait until it is free.
*
* Retrun 1 if the rwlock has been taken, 0 otherwise
*/
int btrfs_try_tree_write_lock(struct extent_buffer *eb)
{
if (READ_ONCE(eb->blocking_writers) || atomic_read(&eb->blocking_readers))
return 0;
write_lock(&eb->lock);
/* Refetch value after lock */
if (READ_ONCE(eb->blocking_writers) || atomic_read(&eb->blocking_readers)) {
write_unlock(&eb->lock);
return 0;
}
btrfs_assert_tree_write_locks_get(eb);
btrfs_assert_spinning_writers_get(eb);
eb->lock_owner = current->pid;
trace_btrfs_try_tree_write_lock(eb);
return 1;
}
/*
* Release read lock. Must be used only if the lock is in spinning mode. If
* the read lock is nested, must pair with read lock before the write unlock.
*
* The rwlock is not held upon exit.
*/
void btrfs_tree_read_unlock(struct extent_buffer *eb)
{
trace_btrfs_tree_read_unlock(eb);
/*
* if we're nested, we have the write lock. No new locking
* is needed as long as we are the lock owner.
* The write unlock will do a barrier for us, and the lock_nested
* field only matters to the lock owner.
*/
if (eb->lock_nested && current->pid == eb->lock_owner) {
eb->lock_nested = false;
return;
}
btrfs_assert_tree_read_locked(eb);
btrfs_assert_spinning_readers_put(eb);
btrfs_assert_tree_read_locks_put(eb);
read_unlock(&eb->lock);
}
/*
* Release read lock, previously set to blocking by a pairing call to
* btrfs_set_lock_blocking_read(). Can be nested in write lock by the same
* thread.
*
* State of rwlock is unchanged, last reader wakes waiting threads.
*/
void btrfs_tree_read_unlock_blocking(struct extent_buffer *eb)
{
trace_btrfs_tree_read_unlock_blocking(eb);
/*
* if we're nested, we have the write lock. No new locking
* is needed as long as we are the lock owner.
* The write unlock will do a barrier for us, and the lock_nested
* field only matters to the lock owner.
*/
if (eb->lock_nested && current->pid == eb->lock_owner) {
eb->lock_nested = false;
return;
}
btrfs_assert_tree_read_locked(eb);
WARN_ON(atomic_read(&eb->blocking_readers) == 0);
/* atomic_dec_and_test implies a barrier */
if (atomic_dec_and_test(&eb->blocking_readers))
cond_wake_up_nomb(&eb->read_lock_wq);
btrfs_assert_tree_read_locks_put(eb);
}
/*
* Lock for write. Wait for all blocking and spinning readers and writers. This
* starts context where reader lock could be nested by the same thread.
*
* The rwlock is held for write upon exit.
*/
void btrfs_tree_lock(struct extent_buffer *eb)
{
u64 start_ns = 0;
if (trace_btrfs_tree_lock_enabled())
start_ns = ktime_get_ns();
WARN_ON(eb->lock_owner == current->pid);
again:
wait_event(eb->read_lock_wq, atomic_read(&eb->blocking_readers) == 0);
wait_event(eb->write_lock_wq, READ_ONCE(eb->blocking_writers) == 0);
write_lock(&eb->lock);
/* Refetch value after lock */
if (atomic_read(&eb->blocking_readers) ||
READ_ONCE(eb->blocking_writers)) {
write_unlock(&eb->lock);
goto again;
}
btrfs_assert_spinning_writers_get(eb);
btrfs_assert_tree_write_locks_get(eb);
eb->lock_owner = current->pid;
trace_btrfs_tree_lock(eb, start_ns);
}
/*
* Release the write lock, either blocking or spinning (ie. there's no need
* for an explicit blocking unlock, like btrfs_tree_read_unlock_blocking).
* This also ends the context for nesting, the read lock must have been
* released already.
*
* Tasks blocked and waiting are woken, rwlock is not held upon exit.
*/
void btrfs_tree_unlock(struct extent_buffer *eb)
{
/*
* This is read both locked and unlocked but always by the same thread
* that already owns the lock so we don't need to use READ_ONCE
*/
int blockers = eb->blocking_writers;
BUG_ON(blockers > 1);
btrfs_assert_tree_locked(eb);
trace_btrfs_tree_unlock(eb);
eb->lock_owner = 0;
btrfs_assert_tree_write_locks_put(eb);
if (blockers) {
btrfs_assert_no_spinning_writers(eb);
/* Unlocked write */
WRITE_ONCE(eb->blocking_writers, 0);
/*
* We need to order modifying blocking_writers above with
* actually waking up the sleepers to ensure they see the
* updated value of blocking_writers
*/
cond_wake_up(&eb->write_lock_wq);
} else {
btrfs_assert_spinning_writers_put(eb);
write_unlock(&eb->lock);
}
}
/*
* Set all locked nodes in the path to blocking locks. This should be done
* before scheduling
*/
void btrfs_set_path_blocking(struct btrfs_path *p)
{
int i;
for (i = 0; i < BTRFS_MAX_LEVEL; i++) {
if (!p->nodes[i] || !p->locks[i])
continue;
/*
* If we currently have a spinning reader or writer lock this
* will bump the count of blocking holders and drop the
* spinlock.
*/
if (p->locks[i] == BTRFS_READ_LOCK) {
btrfs_set_lock_blocking_read(p->nodes[i]);
p->locks[i] = BTRFS_READ_LOCK_BLOCKING;
} else if (p->locks[i] == BTRFS_WRITE_LOCK) {
btrfs_set_lock_blocking_write(p->nodes[i]);
p->locks[i] = BTRFS_WRITE_LOCK_BLOCKING;
}
}
}
/*
* This releases any locks held in the path starting at level and going all the
* way up to the root.
*
* btrfs_search_slot will keep the lock held on higher nodes in a few corner
* cases, such as COW of the block at slot zero in the node. This ignores
* those rules, and it should only be called when there are no more updates to
* be done higher up in the tree.
*/
void btrfs_unlock_up_safe(struct btrfs_path *path, int level)
{
int i;
if (path->keep_locks)
return;
for (i = level; i < BTRFS_MAX_LEVEL; i++) {
if (!path->nodes[i])
continue;
if (!path->locks[i])
continue;
btrfs_tree_unlock_rw(path->nodes[i], path->locks[i]);
path->locks[i] = 0;
}
}