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Diffstat (limited to '')
| -rw-r--r-- | fs/btrfs/locking.c | 674 |
1 files changed, 674 insertions, 0 deletions
diff --git a/fs/btrfs/locking.c b/fs/btrfs/locking.c new file mode 100644 index 000000000..66e02ebdd --- /dev/null +++ b/fs/btrfs/locking.c @@ -0,0 +1,674 @@ +// 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 recursion + * -------------- + * + * 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_recursed && 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_recursed && 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, enum btrfs_lock_nesting nest, + bool recurse) +{ + 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. + */ + WARN_ON(!recurse); + BUG_ON(eb->lock_recursed); + eb->lock_recursed = 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); +} + +void btrfs_tree_read_lock(struct extent_buffer *eb) +{ + __btrfs_tree_read_lock(eb, BTRFS_NESTING_NORMAL, false); +} + +/* + * 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_recursed + * field only matters to the lock owner. + */ + if (eb->lock_recursed && current->pid == eb->lock_owner) { + eb->lock_recursed = 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_recursed + * field only matters to the lock owner. + */ + if (eb->lock_recursed && current->pid == eb->lock_owner) { + eb->lock_recursed = 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, 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); + eb->lock_owner = current->pid; + trace_btrfs_tree_lock(eb, start_ns); +} + +void btrfs_tree_lock(struct extent_buffer *eb) +{ + __btrfs_tree_lock(eb, BTRFS_NESTING_NORMAL); +} + +/* + * 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; + } +} + +/* + * 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_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, + bool recurse) +{ + struct extent_buffer *eb; + + while (1) { + eb = btrfs_root_node(root); + __btrfs_tree_read_lock(eb, BTRFS_NESTING_NORMAL, recurse); + 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); +} |