292 lines
6.6 KiB
C
292 lines
6.6 KiB
C
// SPDX-License-Identifier: GPL-2.0
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/*
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* Copyright (C) 2008 Oracle. All rights reserved.
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*/
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#include <linux/sched.h>
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#include <linux/pagemap.h>
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#include <linux/spinlock.h>
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#include <linux/page-flags.h>
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#include <asm/bug.h>
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#include "misc.h"
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#include "ctree.h"
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#include "extent_io.h"
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#include "locking.h"
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/*
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* Extent buffer locking
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* =====================
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*
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* We use a rw_semaphore for tree locking, and the semantics are exactly the
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* same:
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*
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* - reader/writer exclusion
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* - writer/writer exclusion
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* - reader/reader sharing
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* - try-lock semantics for readers and writers
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*
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* The rwsem implementation does opportunistic spinning which reduces number of
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* times the locking task needs to sleep.
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*/
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/*
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* __btrfs_tree_read_lock - lock extent buffer for read
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* @eb: the eb to be locked
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* @nest: the nesting level to be used for lockdep
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*
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* This takes the read lock on the extent buffer, using the specified nesting
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* level for lockdep purposes.
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*/
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void __btrfs_tree_read_lock(struct extent_buffer *eb, enum btrfs_lock_nesting nest)
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{
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u64 start_ns = 0;
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if (trace_btrfs_tree_read_lock_enabled())
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start_ns = ktime_get_ns();
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down_read_nested(&eb->lock, nest);
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eb->lock_owner = current->pid;
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trace_btrfs_tree_read_lock(eb, start_ns);
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}
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void btrfs_tree_read_lock(struct extent_buffer *eb)
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{
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__btrfs_tree_read_lock(eb, BTRFS_NESTING_NORMAL);
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}
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/*
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* Try-lock for read.
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*
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* Return 1 if the rwlock has been taken, 0 otherwise
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*/
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int btrfs_try_tree_read_lock(struct extent_buffer *eb)
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{
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if (down_read_trylock(&eb->lock)) {
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eb->lock_owner = current->pid;
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trace_btrfs_try_tree_read_lock(eb);
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return 1;
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}
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return 0;
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}
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/*
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* Try-lock for write.
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*
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* Return 1 if the rwlock has been taken, 0 otherwise
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*/
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int btrfs_try_tree_write_lock(struct extent_buffer *eb)
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{
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if (down_write_trylock(&eb->lock)) {
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eb->lock_owner = current->pid;
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trace_btrfs_try_tree_write_lock(eb);
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return 1;
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}
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return 0;
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}
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/*
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* Release read lock.
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*/
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void btrfs_tree_read_unlock(struct extent_buffer *eb)
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{
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trace_btrfs_tree_read_unlock(eb);
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eb->lock_owner = 0;
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up_read(&eb->lock);
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}
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/*
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* __btrfs_tree_lock - lock eb for write
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* @eb: the eb to lock
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* @nest: the nesting to use for the lock
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*
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* Returns with the eb->lock write locked.
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*/
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void __btrfs_tree_lock(struct extent_buffer *eb, enum btrfs_lock_nesting nest)
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__acquires(&eb->lock)
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{
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u64 start_ns = 0;
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if (trace_btrfs_tree_lock_enabled())
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start_ns = ktime_get_ns();
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down_write_nested(&eb->lock, nest);
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eb->lock_owner = current->pid;
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trace_btrfs_tree_lock(eb, start_ns);
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}
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void btrfs_tree_lock(struct extent_buffer *eb)
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{
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__btrfs_tree_lock(eb, BTRFS_NESTING_NORMAL);
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}
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/*
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* Release the write lock.
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*/
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void btrfs_tree_unlock(struct extent_buffer *eb)
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{
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trace_btrfs_tree_unlock(eb);
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eb->lock_owner = 0;
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up_write(&eb->lock);
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}
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/*
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* This releases any locks held in the path starting at level and going all the
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* way up to the root.
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*
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* btrfs_search_slot will keep the lock held on higher nodes in a few corner
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* cases, such as COW of the block at slot zero in the node. This ignores
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* those rules, and it should only be called when there are no more updates to
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* be done higher up in the tree.
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*/
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void btrfs_unlock_up_safe(struct btrfs_path *path, int level)
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{
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int i;
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if (path->keep_locks)
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return;
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for (i = level; i < BTRFS_MAX_LEVEL; i++) {
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if (!path->nodes[i])
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continue;
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if (!path->locks[i])
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continue;
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btrfs_tree_unlock_rw(path->nodes[i], path->locks[i]);
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path->locks[i] = 0;
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}
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}
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/*
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* Loop around taking references on and locking the root node of the tree until
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* we end up with a lock on the root node.
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*
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* Return: root extent buffer with write lock held
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*/
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struct extent_buffer *btrfs_lock_root_node(struct btrfs_root *root)
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{
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struct extent_buffer *eb;
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while (1) {
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eb = btrfs_root_node(root);
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btrfs_tree_lock(eb);
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if (eb == root->node)
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break;
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btrfs_tree_unlock(eb);
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free_extent_buffer(eb);
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}
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return eb;
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}
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/*
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* Loop around taking references on and locking the root node of the tree until
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* we end up with a lock on the root node.
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*
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* Return: root extent buffer with read lock held
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*/
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struct extent_buffer *btrfs_read_lock_root_node(struct btrfs_root *root)
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{
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struct extent_buffer *eb;
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while (1) {
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eb = btrfs_root_node(root);
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btrfs_tree_read_lock(eb);
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if (eb == root->node)
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break;
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btrfs_tree_read_unlock(eb);
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free_extent_buffer(eb);
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}
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return eb;
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}
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/*
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* DREW locks
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* ==========
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*
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* DREW stands for double-reader-writer-exclusion lock. It's used in situation
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* where you want to provide A-B exclusion but not AA or BB.
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*
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* Currently implementation gives more priority to reader. If a reader and a
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* writer both race to acquire their respective sides of the lock the writer
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* would yield its lock as soon as it detects a concurrent reader. Additionally
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* if there are pending readers no new writers would be allowed to come in and
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* acquire the lock.
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*/
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int btrfs_drew_lock_init(struct btrfs_drew_lock *lock)
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{
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int ret;
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ret = percpu_counter_init(&lock->writers, 0, GFP_KERNEL);
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if (ret)
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return ret;
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atomic_set(&lock->readers, 0);
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init_waitqueue_head(&lock->pending_readers);
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init_waitqueue_head(&lock->pending_writers);
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return 0;
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}
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void btrfs_drew_lock_destroy(struct btrfs_drew_lock *lock)
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{
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percpu_counter_destroy(&lock->writers);
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}
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/* Return true if acquisition is successful, false otherwise */
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bool btrfs_drew_try_write_lock(struct btrfs_drew_lock *lock)
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{
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if (atomic_read(&lock->readers))
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return false;
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percpu_counter_inc(&lock->writers);
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/* Ensure writers count is updated before we check for pending readers */
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smp_mb();
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if (atomic_read(&lock->readers)) {
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btrfs_drew_write_unlock(lock);
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return false;
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}
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return true;
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}
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void btrfs_drew_write_lock(struct btrfs_drew_lock *lock)
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{
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while (true) {
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if (btrfs_drew_try_write_lock(lock))
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return;
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wait_event(lock->pending_writers, !atomic_read(&lock->readers));
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}
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}
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void btrfs_drew_write_unlock(struct btrfs_drew_lock *lock)
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{
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percpu_counter_dec(&lock->writers);
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cond_wake_up(&lock->pending_readers);
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}
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void btrfs_drew_read_lock(struct btrfs_drew_lock *lock)
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{
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atomic_inc(&lock->readers);
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/*
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* Ensure the pending reader count is perceieved BEFORE this reader
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* goes to sleep in case of active writers. This guarantees new writers
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* won't be allowed and that the current reader will be woken up when
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* the last active writer finishes its jobs.
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*/
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smp_mb__after_atomic();
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wait_event(lock->pending_readers,
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percpu_counter_sum(&lock->writers) == 0);
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}
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void btrfs_drew_read_unlock(struct btrfs_drew_lock *lock)
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{
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/*
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* atomic_dec_and_test implies a full barrier, so woken up writers
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* are guaranteed to see the decrement
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*/
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if (atomic_dec_and_test(&lock->readers))
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wake_up(&lock->pending_writers);
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}
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