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![Nikolay Borisov](/assets/img/avatar_default.png)
A (D)ouble (R)eader (W)riter (E)xclustion lock is a locking primitive that allows to have multiple readers or multiple writers but not multiple readers and writers holding it concurrently. The code is factored out from the existing open-coded locking scheme used to exclude pending snapshots from nocow writers and vice-versa. Current implementation actually favors Readers (that is snapshot creaters) to writers (nocow writers of the filesystem). The API provides lock/unlock/trylock for reads and writes. Formal specification for TLA+ provided by Valentin Schneider is at https://lore.kernel.org/linux-btrfs/2dcaf81c-f0d3-409e-cb29-733d8b3b4cc9@arm.com/ Signed-off-by: Nikolay Borisov <nborisov@suse.com> Reviewed-by: David Sterba <dsterba@suse.com> Signed-off-by: David Sterba <dsterba@suse.com>
660 lines
18 KiB
C
660 lines
18 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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* The locks use a custom scheme that allows to do more operations than are
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* available fromt current locking primitives. The building blocks are still
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* rwlock and wait queues.
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*
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* Required semantics:
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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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* - spinning lock semantics
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* - blocking lock semantics
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* - try-lock semantics for readers and writers
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* - one level nesting, allowing read lock to be taken by the same thread that
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* already has write lock
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*
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* The extent buffer locks (also called tree locks) manage access to eb data
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* related to the storage in the b-tree (keys, items, but not the individual
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* members of eb).
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* We want concurrency of many readers and safe updates. The underlying locking
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* is done by read-write spinlock and the blocking part is implemented using
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* counters and wait queues.
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*
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* spinning semantics - the low-level rwlock is held so all other threads that
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* want to take it are spinning on it.
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*
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* blocking semantics - the low-level rwlock is not held but the counter
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* denotes how many times the blocking lock was held;
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* sleeping is possible
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*
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* Write lock always allows only one thread to access the data.
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*
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*
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* Debugging
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* ---------
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*
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* There are additional state counters that are asserted in various contexts,
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* removed from non-debug build to reduce extent_buffer size and for
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* performance reasons.
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*
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*
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* Lock nesting
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* ------------
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*
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* A write operation on a tree might indirectly start a look up on the same
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* tree. This can happen when btrfs_cow_block locks the tree and needs to
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* lookup free extents.
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*
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* btrfs_cow_block
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* ..
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* alloc_tree_block_no_bg_flush
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* btrfs_alloc_tree_block
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* btrfs_reserve_extent
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* ..
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* load_free_space_cache
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* ..
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* btrfs_lookup_file_extent
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* btrfs_search_slot
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*
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*
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* Locking pattern - spinning
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* --------------------------
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*
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* The simple locking scenario, the +--+ denotes the spinning section.
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*
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* +- btrfs_tree_lock
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* | - extent_buffer::rwlock is held
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* | - no heavy operations should happen, eg. IO, memory allocations, large
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* | structure traversals
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* +- btrfs_tree_unock
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*
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*
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* Locking pattern - blocking
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* --------------------------
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*
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* The blocking write uses the following scheme. The +--+ denotes the spinning
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* section.
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*
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* +- btrfs_tree_lock
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* |
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* +- btrfs_set_lock_blocking_write
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*
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* - allowed: IO, memory allocations, etc.
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*
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* -- btrfs_tree_unlock - note, no explicit unblocking necessary
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*
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*
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* Blocking read is similar.
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*
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* +- btrfs_tree_read_lock
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* |
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* +- btrfs_set_lock_blocking_read
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*
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* - heavy operations allowed
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*
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* +- btrfs_tree_read_unlock_blocking
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* |
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* +- btrfs_tree_read_unlock
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*
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*/
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#ifdef CONFIG_BTRFS_DEBUG
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static inline void btrfs_assert_spinning_writers_get(struct extent_buffer *eb)
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{
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WARN_ON(eb->spinning_writers);
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eb->spinning_writers++;
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}
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static inline void btrfs_assert_spinning_writers_put(struct extent_buffer *eb)
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{
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WARN_ON(eb->spinning_writers != 1);
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eb->spinning_writers--;
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}
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static inline void btrfs_assert_no_spinning_writers(struct extent_buffer *eb)
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{
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WARN_ON(eb->spinning_writers);
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}
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static inline void btrfs_assert_spinning_readers_get(struct extent_buffer *eb)
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{
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atomic_inc(&eb->spinning_readers);
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}
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static inline void btrfs_assert_spinning_readers_put(struct extent_buffer *eb)
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{
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WARN_ON(atomic_read(&eb->spinning_readers) == 0);
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atomic_dec(&eb->spinning_readers);
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}
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static inline void btrfs_assert_tree_read_locks_get(struct extent_buffer *eb)
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{
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atomic_inc(&eb->read_locks);
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}
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static inline void btrfs_assert_tree_read_locks_put(struct extent_buffer *eb)
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{
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atomic_dec(&eb->read_locks);
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}
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static inline void btrfs_assert_tree_read_locked(struct extent_buffer *eb)
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{
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BUG_ON(!atomic_read(&eb->read_locks));
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}
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static inline void btrfs_assert_tree_write_locks_get(struct extent_buffer *eb)
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{
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eb->write_locks++;
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}
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static inline void btrfs_assert_tree_write_locks_put(struct extent_buffer *eb)
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{
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eb->write_locks--;
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}
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#else
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static void btrfs_assert_spinning_writers_get(struct extent_buffer *eb) { }
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static void btrfs_assert_spinning_writers_put(struct extent_buffer *eb) { }
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static void btrfs_assert_no_spinning_writers(struct extent_buffer *eb) { }
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static void btrfs_assert_spinning_readers_put(struct extent_buffer *eb) { }
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static void btrfs_assert_spinning_readers_get(struct extent_buffer *eb) { }
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static void btrfs_assert_tree_read_locked(struct extent_buffer *eb) { }
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static void btrfs_assert_tree_read_locks_get(struct extent_buffer *eb) { }
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static void btrfs_assert_tree_read_locks_put(struct extent_buffer *eb) { }
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static void btrfs_assert_tree_write_locks_get(struct extent_buffer *eb) { }
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static void btrfs_assert_tree_write_locks_put(struct extent_buffer *eb) { }
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#endif
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/*
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* Mark already held read lock as blocking. Can be nested in write lock by the
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* same thread.
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*
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* Use when there are potentially long operations ahead so other thread waiting
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* on the lock will not actively spin but sleep instead.
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*
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* The rwlock is released and blocking reader counter is increased.
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*/
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void btrfs_set_lock_blocking_read(struct extent_buffer *eb)
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{
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trace_btrfs_set_lock_blocking_read(eb);
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/*
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* No lock is required. The lock owner may change if we have a read
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* lock, but it won't change to or away from us. If we have the write
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* lock, we are the owner and it'll never change.
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*/
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if (eb->lock_nested && current->pid == eb->lock_owner)
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return;
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btrfs_assert_tree_read_locked(eb);
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atomic_inc(&eb->blocking_readers);
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btrfs_assert_spinning_readers_put(eb);
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read_unlock(&eb->lock);
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}
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/*
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* Mark already held write lock as blocking.
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*
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* Use when there are potentially long operations ahead so other threads
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* waiting on the lock will not actively spin but sleep instead.
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*
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* The rwlock is released and blocking writers is set.
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*/
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void btrfs_set_lock_blocking_write(struct extent_buffer *eb)
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{
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trace_btrfs_set_lock_blocking_write(eb);
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/*
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* No lock is required. The lock owner may change if we have a read
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* lock, but it won't change to or away from us. If we have the write
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* lock, we are the owner and it'll never change.
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*/
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if (eb->lock_nested && current->pid == eb->lock_owner)
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return;
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if (eb->blocking_writers == 0) {
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btrfs_assert_spinning_writers_put(eb);
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btrfs_assert_tree_locked(eb);
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WRITE_ONCE(eb->blocking_writers, 1);
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write_unlock(&eb->lock);
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}
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}
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/*
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* Lock the extent buffer for read. Wait for any writers (spinning or blocking).
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* Can be nested in write lock by the same thread.
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*
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* Use when the locked section does only lightweight actions and busy waiting
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* would be cheaper than making other threads do the wait/wake loop.
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*
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* The rwlock is held upon exit.
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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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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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again:
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read_lock(&eb->lock);
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BUG_ON(eb->blocking_writers == 0 &&
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current->pid == eb->lock_owner);
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if (eb->blocking_writers) {
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if (current->pid == eb->lock_owner) {
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/*
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* This extent is already write-locked by our thread.
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* We allow an additional read lock to be added because
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* it's for the same thread. btrfs_find_all_roots()
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* depends on this as it may be called on a partly
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* (write-)locked tree.
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*/
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BUG_ON(eb->lock_nested);
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eb->lock_nested = true;
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read_unlock(&eb->lock);
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trace_btrfs_tree_read_lock(eb, start_ns);
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return;
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}
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read_unlock(&eb->lock);
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wait_event(eb->write_lock_wq,
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READ_ONCE(eb->blocking_writers) == 0);
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goto again;
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}
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btrfs_assert_tree_read_locks_get(eb);
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btrfs_assert_spinning_readers_get(eb);
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trace_btrfs_tree_read_lock(eb, start_ns);
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}
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/*
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* Lock extent buffer for read, optimistically expecting that there are no
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* contending blocking writers. If there are, don't wait.
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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_tree_read_lock_atomic(struct extent_buffer *eb)
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{
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if (READ_ONCE(eb->blocking_writers))
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return 0;
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read_lock(&eb->lock);
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/* Refetch value after lock */
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if (READ_ONCE(eb->blocking_writers)) {
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read_unlock(&eb->lock);
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return 0;
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}
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btrfs_assert_tree_read_locks_get(eb);
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btrfs_assert_spinning_readers_get(eb);
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trace_btrfs_tree_read_lock_atomic(eb);
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return 1;
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}
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/*
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* Try-lock for read. Don't block or wait for contending writers.
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*
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* Retrun 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 (READ_ONCE(eb->blocking_writers))
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return 0;
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if (!read_trylock(&eb->lock))
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return 0;
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/* Refetch value after lock */
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if (READ_ONCE(eb->blocking_writers)) {
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read_unlock(&eb->lock);
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return 0;
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}
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btrfs_assert_tree_read_locks_get(eb);
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btrfs_assert_spinning_readers_get(eb);
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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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/*
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* Try-lock for write. May block until the lock is uncontended, but does not
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* wait until it is free.
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*
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* Retrun 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 (READ_ONCE(eb->blocking_writers) || atomic_read(&eb->blocking_readers))
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return 0;
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write_lock(&eb->lock);
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/* Refetch value after lock */
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if (READ_ONCE(eb->blocking_writers) || atomic_read(&eb->blocking_readers)) {
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write_unlock(&eb->lock);
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return 0;
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}
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btrfs_assert_tree_write_locks_get(eb);
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btrfs_assert_spinning_writers_get(eb);
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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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/*
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* Release read lock. Must be used only if the lock is in spinning mode. If
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* the read lock is nested, must pair with read lock before the write unlock.
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*
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* The rwlock is not held upon exit.
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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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/*
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* if we're nested, we have the write lock. No new locking
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* is needed as long as we are the lock owner.
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* The write unlock will do a barrier for us, and the lock_nested
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* field only matters to the lock owner.
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*/
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if (eb->lock_nested && current->pid == eb->lock_owner) {
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eb->lock_nested = false;
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return;
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}
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btrfs_assert_tree_read_locked(eb);
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btrfs_assert_spinning_readers_put(eb);
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btrfs_assert_tree_read_locks_put(eb);
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read_unlock(&eb->lock);
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}
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/*
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* Release read lock, previously set to blocking by a pairing call to
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* btrfs_set_lock_blocking_read(). Can be nested in write lock by the same
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* thread.
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*
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* State of rwlock is unchanged, last reader wakes waiting threads.
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*/
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void btrfs_tree_read_unlock_blocking(struct extent_buffer *eb)
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{
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trace_btrfs_tree_read_unlock_blocking(eb);
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/*
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* if we're nested, we have the write lock. No new locking
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* is needed as long as we are the lock owner.
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* The write unlock will do a barrier for us, and the lock_nested
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* field only matters to the lock owner.
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*/
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if (eb->lock_nested && current->pid == eb->lock_owner) {
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eb->lock_nested = false;
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return;
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}
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btrfs_assert_tree_read_locked(eb);
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WARN_ON(atomic_read(&eb->blocking_readers) == 0);
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/* atomic_dec_and_test implies a barrier */
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if (atomic_dec_and_test(&eb->blocking_readers))
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cond_wake_up_nomb(&eb->read_lock_wq);
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btrfs_assert_tree_read_locks_put(eb);
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}
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/*
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* Lock for write. Wait for all blocking and spinning readers and writers. This
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* starts context where reader lock could be nested by the same thread.
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*
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* The rwlock is held for write upon exit.
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*/
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void btrfs_tree_lock(struct extent_buffer *eb)
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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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WARN_ON(eb->lock_owner == current->pid);
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again:
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wait_event(eb->read_lock_wq, atomic_read(&eb->blocking_readers) == 0);
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wait_event(eb->write_lock_wq, READ_ONCE(eb->blocking_writers) == 0);
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write_lock(&eb->lock);
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/* Refetch value after lock */
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if (atomic_read(&eb->blocking_readers) ||
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READ_ONCE(eb->blocking_writers)) {
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write_unlock(&eb->lock);
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goto again;
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}
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btrfs_assert_spinning_writers_get(eb);
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btrfs_assert_tree_write_locks_get(eb);
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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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/*
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* Release the write lock, either blocking or spinning (ie. there's no need
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* for an explicit blocking unlock, like btrfs_tree_read_unlock_blocking).
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* This also ends the context for nesting, the read lock must have been
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* released already.
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*
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* Tasks blocked and waiting are woken, rwlock is not held upon exit.
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*/
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void btrfs_tree_unlock(struct extent_buffer *eb)
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{
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/*
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* This is read both locked and unlocked but always by the same thread
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* that already owns the lock so we don't need to use READ_ONCE
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*/
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int blockers = eb->blocking_writers;
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BUG_ON(blockers > 1);
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btrfs_assert_tree_locked(eb);
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trace_btrfs_tree_unlock(eb);
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eb->lock_owner = 0;
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btrfs_assert_tree_write_locks_put(eb);
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if (blockers) {
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btrfs_assert_no_spinning_writers(eb);
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/* Unlocked write */
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WRITE_ONCE(eb->blocking_writers, 0);
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/*
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* We need to order modifying blocking_writers above with
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* actually waking up the sleepers to ensure they see the
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* updated value of blocking_writers
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*/
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cond_wake_up(&eb->write_lock_wq);
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} else {
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btrfs_assert_spinning_writers_put(eb);
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write_unlock(&eb->lock);
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}
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}
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/*
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* Set all locked nodes in the path to blocking locks. This should be done
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* before scheduling
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*/
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void btrfs_set_path_blocking(struct btrfs_path *p)
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{
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int i;
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for (i = 0; i < BTRFS_MAX_LEVEL; i++) {
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if (!p->nodes[i] || !p->locks[i])
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continue;
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/*
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* If we currently have a spinning reader or writer lock this
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* will bump the count of blocking holders and drop the
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* spinlock.
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*/
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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)
|
|
{
|
|
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);
|
|
}
|