Commit d4e253bb authored by David Sterba's avatar David Sterba

btrfs: document extent buffer locking

Signed-off-by: default avatarDavid Sterba <dsterba@suse.com>
parent a4477988
......@@ -13,6 +13,110 @@
#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)
{
......@@ -80,6 +184,15 @@ 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);
......@@ -96,6 +209,14 @@ void btrfs_set_lock_blocking_read(struct extent_buffer *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);
......@@ -115,8 +236,13 @@ void btrfs_set_lock_blocking_write(struct extent_buffer *eb)
}
/*
* take a spinning read lock. This will wait for any blocking
* writers
* 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)
{
......@@ -154,9 +280,10 @@ void btrfs_tree_read_lock(struct extent_buffer *eb)
}
/*
* take a spinning read lock.
* returns 1 if we get the read lock and 0 if we don't
* this won't wait for blocking writers
* 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)
{
......@@ -176,8 +303,9 @@ int btrfs_tree_read_lock_atomic(struct extent_buffer *eb)
}
/*
* returns 1 if we get the read lock and 0 if we don't
* this won't wait for blocking writers
* 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)
{
......@@ -199,8 +327,10 @@ int btrfs_try_tree_read_lock(struct extent_buffer *eb)
}
/*
* returns 1 if we get the read lock and 0 if we don't
* this won't wait for blocking writers or readers
* 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)
{
......@@ -221,7 +351,10 @@ int btrfs_try_tree_write_lock(struct extent_buffer *eb)
}
/*
* drop a spinning read lock
* 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)
{
......@@ -243,7 +376,11 @@ void btrfs_tree_read_unlock(struct extent_buffer *eb)
}
/*
* drop a blocking read 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)
{
......@@ -267,8 +404,10 @@ void btrfs_tree_read_unlock_blocking(struct extent_buffer *eb)
}
/*
* take a spinning write lock. This will wait for both
* blocking readers or writers
* 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)
{
......@@ -295,7 +434,12 @@ void btrfs_tree_lock(struct extent_buffer *eb)
}
/*
* drop a spinning or a blocking write lock.
* 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)
{
......
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