diff options
Diffstat (limited to 'fs/btrfs/locking.c')
-rw-r--r-- | fs/btrfs/locking.c | 693 |
1 files changed, 286 insertions, 407 deletions
diff --git a/fs/btrfs/locking.c b/fs/btrfs/locking.c index 571c4826c428..0eab3cb274a1 100644 --- a/fs/btrfs/locking.c +++ b/fs/btrfs/locking.c @@ -14,488 +14,203 @@ #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 - * + * Lockdep class keys for extent_buffer->lock's in this root. For a given + * eb, the lockdep key is determined by the btrfs_root it belongs to and + * the level the eb occupies in the tree. + * + * Different roots are used for different purposes and may nest inside each + * other and they require separate keysets. As lockdep keys should be + * static, assign keysets according to the purpose of the root as indicated + * by btrfs_root->root_key.objectid. This ensures that all special purpose + * roots have separate keysets. + * + * Lock-nesting across peer nodes is always done with the immediate parent + * node locked thus preventing deadlock. As lockdep doesn't know this, use + * subclass to avoid triggering lockdep warning in such cases. + * + * The key is set by the readpage_end_io_hook after the buffer has passed + * csum validation but before the pages are unlocked. It is also set by + * btrfs_init_new_buffer on freshly allocated blocks. + * + * We also add a check to make sure the highest level of the tree is the + * same as our lockdep setup here. If BTRFS_MAX_LEVEL changes, this code + * needs update as well. */ +#ifdef CONFIG_DEBUG_LOCK_ALLOC +#if BTRFS_MAX_LEVEL != 8 +#error +#endif -#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) +#define DEFINE_LEVEL(stem, level) \ + .names[level] = "btrfs-" stem "-0" #level, + +#define DEFINE_NAME(stem) \ + DEFINE_LEVEL(stem, 0) \ + DEFINE_LEVEL(stem, 1) \ + DEFINE_LEVEL(stem, 2) \ + DEFINE_LEVEL(stem, 3) \ + DEFINE_LEVEL(stem, 4) \ + DEFINE_LEVEL(stem, 5) \ + DEFINE_LEVEL(stem, 6) \ + DEFINE_LEVEL(stem, 7) + +static struct btrfs_lockdep_keyset { + u64 id; /* root objectid */ + /* Longest entry: btrfs-free-space-00 */ + char names[BTRFS_MAX_LEVEL][20]; + struct lock_class_key keys[BTRFS_MAX_LEVEL]; +} btrfs_lockdep_keysets[] = { + { .id = BTRFS_ROOT_TREE_OBJECTID, DEFINE_NAME("root") }, + { .id = BTRFS_EXTENT_TREE_OBJECTID, DEFINE_NAME("extent") }, + { .id = BTRFS_CHUNK_TREE_OBJECTID, DEFINE_NAME("chunk") }, + { .id = BTRFS_DEV_TREE_OBJECTID, DEFINE_NAME("dev") }, + { .id = BTRFS_CSUM_TREE_OBJECTID, DEFINE_NAME("csum") }, + { .id = BTRFS_QUOTA_TREE_OBJECTID, DEFINE_NAME("quota") }, + { .id = BTRFS_TREE_LOG_OBJECTID, DEFINE_NAME("log") }, + { .id = BTRFS_TREE_RELOC_OBJECTID, DEFINE_NAME("treloc") }, + { .id = BTRFS_DATA_RELOC_TREE_OBJECTID, DEFINE_NAME("dreloc") }, + { .id = BTRFS_UUID_TREE_OBJECTID, DEFINE_NAME("uuid") }, + { .id = BTRFS_FREE_SPACE_TREE_OBJECTID, DEFINE_NAME("free-space") }, + { .id = 0, DEFINE_NAME("tree") }, +}; + +#undef DEFINE_LEVEL +#undef DEFINE_NAME + +void btrfs_set_buffer_lockdep_class(u64 objectid, struct extent_buffer *eb, int level) { - atomic_inc(&eb->read_locks); -} + struct btrfs_lockdep_keyset *ks; -static inline void btrfs_assert_tree_read_locks_put(struct extent_buffer *eb) -{ - atomic_dec(&eb->read_locks); -} + BUG_ON(level >= ARRAY_SIZE(ks->keys)); -static inline void btrfs_assert_tree_read_locked(struct extent_buffer *eb) -{ - BUG_ON(!atomic_read(&eb->read_locks)); -} + /* Find the matching keyset, id 0 is the default entry */ + for (ks = btrfs_lockdep_keysets; ks->id; ks++) + if (ks->id == objectid) + break; -static inline void btrfs_assert_tree_write_locks_get(struct extent_buffer *eb) -{ - eb->write_locks++; + lockdep_set_class_and_name(&eb->lock, &ks->keys[level], ks->names[level]); } -static inline void btrfs_assert_tree_write_locks_put(struct extent_buffer *eb) +void btrfs_maybe_reset_lockdep_class(struct btrfs_root *root, struct extent_buffer *eb) { - eb->write_locks--; + if (test_bit(BTRFS_ROOT_RESET_LOCKDEP_CLASS, &root->state)) + btrfs_set_buffer_lockdep_class(root->root_key.objectid, + eb, btrfs_header_level(eb)); } -#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. + * Extent buffer locking + * ===================== * - * Use when there are potentially long operations ahead so other thread waiting - * on the lock will not actively spin but sleep instead. + * We use a rw_semaphore for tree locking, and the semantics are exactly the + * same: * - * 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. + * - reader/writer exclusion + * - writer/writer exclusion + * - reader/reader sharing + * - try-lock semantics for readers and writers * - * The rwlock is released and blocking writers is set. + * The rwsem implementation does opportunistic spinning which reduces number of + * times the locking task needs to sleep. */ -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. + * __btrfs_tree_read_lock - lock extent buffer for read + * @eb: the eb to be locked + * @nest: the nesting level to be used for lockdep * - * 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. + * This takes the read lock on the extent buffer, using the specified nesting + * level for lockdep purposes. */ -void btrfs_tree_read_lock(struct extent_buffer *eb) +void __btrfs_tree_read_lock(struct extent_buffer *eb, enum btrfs_lock_nesting nest) { 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); + + down_read_nested(&eb->lock, nest); 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) +void btrfs_tree_read_lock(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; + __btrfs_tree_read_lock(eb, BTRFS_NESTING_NORMAL); } /* - * Try-lock for read. Don't block or wait for contending writers. + * Try-lock for read. * - * Retrun 1 if the rwlock has been taken, 0 otherwise + * Return 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; + if (down_read_trylock(&eb->lock)) { + trace_btrfs_try_tree_read_lock(eb); + return 1; } - btrfs_assert_tree_read_locks_get(eb); - btrfs_assert_spinning_readers_get(eb); - trace_btrfs_try_tree_read_lock(eb); - return 1; + return 0; } /* - * Try-lock for write. May block until the lock is uncontended, but does not - * wait until it is free. + * Try-lock for write. * - * Retrun 1 if the rwlock has been taken, 0 otherwise + * Return 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; + if (down_write_trylock(&eb->lock)) { + eb->lock_owner = current->pid; + trace_btrfs_try_tree_write_lock(eb); + return 1; } - 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; + return 0; } /* - * 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. + * Release read lock. */ 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); + up_read(&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. + * __btrfs_tree_lock - lock eb for write + * @eb: the eb to lock + * @nest: the nesting to use for the lock * - * State of rwlock is unchanged, last reader wakes waiting threads. + * Returns with the eb->lock write locked. */ -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) +void __btrfs_tree_lock(struct extent_buffer *eb, enum btrfs_lock_nesting nest) + __acquires(&eb->lock) { 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); + down_write_nested(&eb->lock, nest); 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) +void btrfs_tree_lock(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); - } + __btrfs_tree_lock(eb, BTRFS_NESTING_NORMAL); } /* - * Set all locked nodes in the path to blocking locks. This should be done - * before scheduling + * Release the write lock. */ -void btrfs_set_path_blocking(struct btrfs_path *p) +void btrfs_tree_unlock(struct extent_buffer *eb) { - 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; - } - } + trace_btrfs_tree_unlock(eb); + eb->lock_owner = 0; + up_write(&eb->lock); } /* @@ -523,3 +238,167 @@ void btrfs_unlock_up_safe(struct btrfs_path *path, int level) path->locks[i] = 0; } } + +/* + * Loop around taking references on and locking the root node of the tree until + * we end up with a lock on the root node. + * + * Return: root extent buffer with write lock held + */ +struct extent_buffer *btrfs_lock_root_node(struct btrfs_root *root) +{ + struct extent_buffer *eb; + + while (1) { + eb = btrfs_root_node(root); + + btrfs_maybe_reset_lockdep_class(root, eb); + btrfs_tree_lock(eb); + if (eb == root->node) + break; + btrfs_tree_unlock(eb); + free_extent_buffer(eb); + } + return eb; +} + +/* + * Loop around taking references on and locking the root node of the tree until + * we end up with a lock on the root node. + * + * Return: root extent buffer with read lock held + */ +struct extent_buffer *btrfs_read_lock_root_node(struct btrfs_root *root) +{ + struct extent_buffer *eb; + + while (1) { + eb = btrfs_root_node(root); + + btrfs_maybe_reset_lockdep_class(root, eb); + btrfs_tree_read_lock(eb); + if (eb == root->node) + break; + btrfs_tree_read_unlock(eb); + free_extent_buffer(eb); + } + return eb; +} + +/* + * Loop around taking references on and locking the root node of the tree in + * nowait mode until we end up with a lock on the root node or returning to + * avoid blocking. + * + * Return: root extent buffer with read lock held or -EAGAIN. + */ +struct extent_buffer *btrfs_try_read_lock_root_node(struct btrfs_root *root) +{ + struct extent_buffer *eb; + + while (1) { + eb = btrfs_root_node(root); + if (!btrfs_try_tree_read_lock(eb)) { + free_extent_buffer(eb); + return ERR_PTR(-EAGAIN); + } + if (eb == root->node) + break; + btrfs_tree_read_unlock(eb); + free_extent_buffer(eb); + } + return eb; +} + +/* + * DREW locks + * ========== + * + * DREW stands for double-reader-writer-exclusion lock. It's used in situation + * where you want to provide A-B exclusion but not AA or BB. + * + * Currently implementation gives more priority to reader. If a reader and a + * writer both race to acquire their respective sides of the lock the writer + * would yield its lock as soon as it detects a concurrent reader. Additionally + * if there are pending readers no new writers would be allowed to come in and + * acquire the lock. + */ + +int btrfs_drew_lock_init(struct btrfs_drew_lock *lock) +{ + int ret; + + ret = percpu_counter_init(&lock->writers, 0, GFP_KERNEL); + if (ret) + return ret; + + atomic_set(&lock->readers, 0); + init_waitqueue_head(&lock->pending_readers); + init_waitqueue_head(&lock->pending_writers); + + return 0; +} + +void btrfs_drew_lock_destroy(struct btrfs_drew_lock *lock) +{ + percpu_counter_destroy(&lock->writers); +} + +/* Return true if acquisition is successful, false otherwise */ +bool btrfs_drew_try_write_lock(struct btrfs_drew_lock *lock) +{ + if (atomic_read(&lock->readers)) + return false; + + percpu_counter_inc(&lock->writers); + + /* Ensure writers count is updated before we check for pending readers */ + smp_mb(); + if (atomic_read(&lock->readers)) { + btrfs_drew_write_unlock(lock); + return false; + } + + return true; +} + +void btrfs_drew_write_lock(struct btrfs_drew_lock *lock) +{ + while (true) { + if (btrfs_drew_try_write_lock(lock)) + return; + wait_event(lock->pending_writers, !atomic_read(&lock->readers)); + } +} + +void btrfs_drew_write_unlock(struct btrfs_drew_lock *lock) +{ + percpu_counter_dec(&lock->writers); + cond_wake_up(&lock->pending_readers); +} + +void btrfs_drew_read_lock(struct btrfs_drew_lock *lock) +{ + atomic_inc(&lock->readers); + + /* + * Ensure the pending reader count is perceieved BEFORE this reader + * goes to sleep in case of active writers. This guarantees new writers + * won't be allowed and that the current reader will be woken up when + * the last active writer finishes its jobs. + */ + smp_mb__after_atomic(); + + wait_event(lock->pending_readers, + percpu_counter_sum(&lock->writers) == 0); +} + +void btrfs_drew_read_unlock(struct btrfs_drew_lock *lock) +{ + /* + * atomic_dec_and_test implies a full barrier, so woken up writers + * are guaranteed to see the decrement + */ + if (atomic_dec_and_test(&lock->readers)) + wake_up(&lock->pending_writers); +} |