// SPDX-License-Identifier: GPL-2.0 /* * Copyright (C) 2008 Oracle. All rights reserved. */ #include #include #include #include #include #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; } }