* 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:
+ * We use a rw_semaphore for tree locking, and the semantics are exactly the
+ * same:
*
* - 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.
+ * Additionally we need one level nesting recursion, see below. The rwsem
+ * implementation does opportunistic spinning which reduces number of times the
+ * locking task needs to sleep.
*
*
* Lock recursion
* 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.
*/
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_recursed && 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);
}
/*
*/
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_recursed && 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.
+ * __btrfs_tree_read_lock - lock extent buffer for read
+ * @eb: the eb to be locked
+ * @nest: the nesting level to be used for lockdep
+ * @recurse: if this lock is able to be recursed
*
- * Use when the locked section does only lightweight actions and busy waiting
- * would be cheaper than making other threads do the wait/wake loop.
+ * This takes the read lock on the extent buffer, using the specified nesting
+ * level for lockdep purposes.
*
- * The rwlock is held upon exit.
+ * If you specify recurse = true, then we will allow this to be taken if we
+ * currently own the lock already. This should only be used in specific
+ * usecases, and the subsequent unlock will not change the state of the lock.
*/
void __btrfs_tree_read_lock(struct extent_buffer *eb, enum btrfs_lock_nesting nest,
bool recurse)
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.
- */
- WARN_ON(!recurse);
- BUG_ON(eb->lock_recursed);
- eb->lock_recursed = true;
- read_unlock(&eb->lock);
- trace_btrfs_tree_read_lock(eb, start_ns);
- return;
+
+ if (unlikely(recurse)) {
+ /* First see if we can grab the lock outright */
+ if (down_read_trylock(&eb->lock))
+ goto out;
+
+ /*
+ * Ok still doesn't necessarily mean we are already holding the
+ * lock, check the owner.
+ */
+ if (eb->lock_owner != current->pid) {
+ down_read_nested(&eb->lock, nest);
+ goto out;
}
- read_unlock(&eb->lock);
- wait_event(eb->write_lock_wq,
- READ_ONCE(eb->blocking_writers) == 0);
- goto again;
+
+ /*
+ * Ok we have actually recursed, but we should only be recursing
+ * once, so blow up if we're already recursed, otherwise set
+ * ->lock_recursed and carry on.
+ */
+ BUG_ON(eb->lock_recursed);
+ eb->lock_recursed = true;
+ goto out;
}
- btrfs_assert_tree_read_locks_get(eb);
- btrfs_assert_spinning_readers_get(eb);
+ down_read_nested(&eb->lock, nest);
+out:
+ eb->lock_owner = current->pid;
trace_btrfs_tree_read_lock(eb, start_ns);
}
*/
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;
+ return btrfs_try_tree_read_lock(eb);
}
/*
- * Try-lock for read. Don't block or wait for contending writers.
+ * Try-lock for read.
*
* 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;
+ if (down_read_trylock(&eb->lock)) {
+ eb->lock_owner = current->pid;
+ trace_btrfs_try_tree_read_lock(eb);
+ return 1;
}
- btrfs_assert_tree_read_locks_get(eb);
- btrfs_assert_spinning_readers_get(eb);
- trace_btrfs_try_tree_read_lock(eb);
- return 1;
+ return 0;
}
/*
- * Try-lock for write. May block until the lock is uncontended, but does not
- * wait until it is free.
+ * Try-lock for write.
*
* 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;
+ if (down_write_trylock(&eb->lock)) {
+ eb->lock_owner = current->pid;
+ trace_btrfs_try_tree_write_lock(eb);
+ return 1;
}
- 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;
+ return 0;
}
/*
- * 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.
+ * Release read lock. If the read lock was recursed then the lock stays in the
+ * original state that it was before it was recursively locked.
*/
void btrfs_tree_read_unlock(struct extent_buffer *eb)
{
eb->lock_recursed = 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);
+ eb->lock_owner = 0;
+ up_read(&eb->lock);
}
/*
*/
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_recursed
- * field only matters to the lock owner.
- */
- if (eb->lock_recursed && current->pid == eb->lock_owner) {
- eb->lock_recursed = 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);
+ btrfs_tree_read_unlock(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.
+ * __btrfs_tree_lock - lock eb for write
+ * @eb: the eb to lock
+ * @nest: the nesting to use for the lock
*
- * The rwlock is held for write upon exit.
+ * Returns with the eb->lock write locked.
*/
void __btrfs_tree_lock(struct extent_buffer *eb, enum btrfs_lock_nesting nest)
__acquires(&eb->lock)
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);
+ down_write_nested(&eb->lock, nest);
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.
+ * Release the write lock.
*/
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);
- }
+ up_write(&eb->lock);
}
/*
projects / platform / kernel / linux-starfive.git / commitdiff