| // 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 |
| * ===================== |
| * |
| * We use a rw_semaphore for tree locking, and the semantics are exactly the |
| * same: |
| * |
| * - reader/writer exclusion |
| * - writer/writer exclusion |
| * - reader/reader sharing |
| * - try-lock semantics for readers and writers |
| * |
| * The rwsem implementation does opportunistic spinning which reduces number of |
| * times the locking task needs to sleep. |
| */ |
| |
| /* |
| * __btrfs_tree_read_lock - lock extent buffer for read |
| * @eb: the eb to be locked |
| * @nest: the nesting level to be used for lockdep |
| * |
| * 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, enum btrfs_lock_nesting nest) |
| { |
| u64 start_ns = 0; |
| |
| if (trace_btrfs_tree_read_lock_enabled()) |
| start_ns = ktime_get_ns(); |
| |
| down_read_nested(&eb->lock, nest); |
| 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); |
| } |
| |
| /* |
| * Try-lock for read. |
| * |
| * Return 1 if the rwlock has been taken, 0 otherwise |
| */ |
| int btrfs_try_tree_read_lock(struct extent_buffer *eb) |
| { |
| if (down_read_trylock(&eb->lock)) { |
| trace_btrfs_try_tree_read_lock(eb); |
| return 1; |
| } |
| return 0; |
| } |
| |
| /* |
| * Try-lock for write. |
| * |
| * Return 1 if the rwlock has been taken, 0 otherwise |
| */ |
| int btrfs_try_tree_write_lock(struct extent_buffer *eb) |
| { |
| if (down_write_trylock(&eb->lock)) { |
| eb->lock_owner = current->pid; |
| trace_btrfs_try_tree_write_lock(eb); |
| return 1; |
| } |
| return 0; |
| } |
| |
| /* |
| * Release read lock. |
| */ |
| void btrfs_tree_read_unlock(struct extent_buffer *eb) |
| { |
| trace_btrfs_tree_read_unlock(eb); |
| up_read(&eb->lock); |
| } |
| |
| /* |
| * __btrfs_tree_lock - lock eb for write |
| * @eb: the eb to lock |
| * @nest: the nesting to use for the lock |
| * |
| * Returns with the eb->lock write locked. |
| */ |
| 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(); |
| |
| down_write_nested(&eb->lock, nest); |
| 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. |
| */ |
| void btrfs_tree_unlock(struct extent_buffer *eb) |
| { |
| trace_btrfs_tree_unlock(eb); |
| eb->lock_owner = 0; |
| up_write(&eb->lock); |
| } |
| |
| /* |
| * 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) |
| { |
| struct extent_buffer *eb; |
| |
| while (1) { |
| eb = btrfs_root_node(root); |
| btrfs_tree_read_lock(eb); |
| 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); |
| } |