From 102a0743326a03cd1a1202ceda21e175b7d3575c Mon Sep 17 00:00:00 2001
From: hc <hc@nodka.com>
Date: Tue, 20 Feb 2024 01:20:52 +0000
Subject: [PATCH] add new system file
---
kernel/fs/btrfs/locking.c | 689 ++++++++++++++++++++++++++++++++++++++++++++------------
1 files changed, 538 insertions(+), 151 deletions(-)
diff --git a/kernel/fs/btrfs/locking.c b/kernel/fs/btrfs/locking.c
index 1da768e..66e02eb 100644
--- a/kernel/fs/btrfs/locking.c
+++ b/kernel/fs/btrfs/locking.c
@@ -8,209 +8,398 @@
#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"
-static void btrfs_assert_tree_read_locked(struct extent_buffer *eb);
-
/*
- * if we currently have a spinning reader or writer lock
- * (indicated by the rw flag) this will bump the count
- * of blocking holders and drop the spinlock.
+ * 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 recursion
+ * --------------
+ *
+ * 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
+ *
*/
-void btrfs_set_lock_blocking_rw(struct extent_buffer *eb, int rw)
+
+#ifdef CONFIG_BTRFS_DEBUG
+static inline void btrfs_assert_spinning_writers_get(struct extent_buffer *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_nested && current->pid == eb->lock_owner)
- return;
- if (rw == BTRFS_WRITE_LOCK) {
- if (atomic_read(&eb->blocking_writers) == 0) {
- WARN_ON(atomic_read(&eb->spinning_writers) != 1);
- atomic_dec(&eb->spinning_writers);
- btrfs_assert_tree_locked(eb);
- atomic_inc(&eb->blocking_writers);
- write_unlock(&eb->lock);
- }
- } else if (rw == BTRFS_READ_LOCK) {
- btrfs_assert_tree_read_locked(eb);
- atomic_inc(&eb->blocking_readers);
- WARN_ON(atomic_read(&eb->spinning_readers) == 0);
- atomic_dec(&eb->spinning_readers);
- read_unlock(&eb->lock);
- }
+ WARN_ON(eb->spinning_writers);
+ eb->spinning_writers++;
}
-/*
- * if we currently have a blocking lock, take the spinlock
- * and drop our blocking count
- */
-void btrfs_clear_lock_blocking_rw(struct extent_buffer *eb, int rw)
+static inline void btrfs_assert_spinning_writers_put(struct extent_buffer *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_nested && current->pid == eb->lock_owner)
- return;
-
- if (rw == BTRFS_WRITE_LOCK_BLOCKING) {
- BUG_ON(atomic_read(&eb->blocking_writers) != 1);
- write_lock(&eb->lock);
- WARN_ON(atomic_read(&eb->spinning_writers));
- atomic_inc(&eb->spinning_writers);
- /* atomic_dec_and_test implies a barrier */
- if (atomic_dec_and_test(&eb->blocking_writers))
- cond_wake_up_nomb(&eb->write_lock_wq);
- } else if (rw == BTRFS_READ_LOCK_BLOCKING) {
- BUG_ON(atomic_read(&eb->blocking_readers) == 0);
- read_lock(&eb->lock);
- atomic_inc(&eb->spinning_readers);
- /* atomic_dec_and_test implies a barrier */
- if (atomic_dec_and_test(&eb->blocking_readers))
- cond_wake_up_nomb(&eb->read_lock_wq);
- }
+ WARN_ON(eb->spinning_writers != 1);
+ eb->spinning_writers--;
}
-/*
- * take a spinning read lock. This will wait for any blocking
- * writers
- */
-void btrfs_tree_read_lock(struct extent_buffer *eb)
+static inline void btrfs_assert_no_spinning_writers(struct extent_buffer *eb)
{
-again:
- BUG_ON(!atomic_read(&eb->blocking_writers) &&
- current->pid == eb->lock_owner);
+ WARN_ON(eb->spinning_writers);
+}
- read_lock(&eb->lock);
- if (atomic_read(&eb->blocking_writers) &&
- 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.
- */
- BUG_ON(eb->lock_nested);
- eb->lock_nested = 1;
- read_unlock(&eb->lock);
- return;
- }
- if (atomic_read(&eb->blocking_writers)) {
- read_unlock(&eb->lock);
- wait_event(eb->write_lock_wq,
- atomic_read(&eb->blocking_writers) == 0);
- goto again;
- }
- atomic_inc(&eb->read_locks);
+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
+
/*
- * 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
+ * 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);
+ /*
+ * 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);
+}
+
+/*
+ * 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);
+ /*
+ * 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.
+ *
+ * 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, enum btrfs_lock_nesting nest,
+ bool recurse)
+{
+ u64 start_ns = 0;
+
+ 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;
+ }
+ read_unlock(&eb->lock);
+ wait_event(eb->write_lock_wq,
+ READ_ONCE(eb->blocking_writers) == 0);
+ goto again;
+ }
+ btrfs_assert_tree_read_locks_get(eb);
+ btrfs_assert_spinning_readers_get(eb);
+ 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, false);
+}
+
+/*
+ * 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)
{
- if (atomic_read(&eb->blocking_writers))
+ if (READ_ONCE(eb->blocking_writers))
return 0;
read_lock(&eb->lock);
- if (atomic_read(&eb->blocking_writers)) {
+ /* Refetch value after lock */
+ if (READ_ONCE(eb->blocking_writers)) {
read_unlock(&eb->lock);
return 0;
}
- atomic_inc(&eb->read_locks);
- atomic_inc(&eb->spinning_readers);
+ btrfs_assert_tree_read_locks_get(eb);
+ btrfs_assert_spinning_readers_get(eb);
+ trace_btrfs_tree_read_lock_atomic(eb);
return 1;
}
/*
- * 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)
{
- if (atomic_read(&eb->blocking_writers))
+ if (READ_ONCE(eb->blocking_writers))
return 0;
if (!read_trylock(&eb->lock))
return 0;
- if (atomic_read(&eb->blocking_writers)) {
+ /* Refetch value after lock */
+ if (READ_ONCE(eb->blocking_writers)) {
read_unlock(&eb->lock);
return 0;
}
- atomic_inc(&eb->read_locks);
- atomic_inc(&eb->spinning_readers);
+ btrfs_assert_tree_read_locks_get(eb);
+ btrfs_assert_spinning_readers_get(eb);
+ trace_btrfs_try_tree_read_lock(eb);
return 1;
}
/*
- * 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)
{
- if (atomic_read(&eb->blocking_writers) ||
- atomic_read(&eb->blocking_readers))
+ if (READ_ONCE(eb->blocking_writers) || atomic_read(&eb->blocking_readers))
return 0;
write_lock(&eb->lock);
- if (atomic_read(&eb->blocking_writers) ||
- atomic_read(&eb->blocking_readers)) {
+ /* Refetch value after lock */
+ if (READ_ONCE(eb->blocking_writers) || atomic_read(&eb->blocking_readers)) {
write_unlock(&eb->lock);
return 0;
}
- atomic_inc(&eb->write_locks);
- atomic_inc(&eb->spinning_writers);
+ 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;
}
/*
- * 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)
{
+ trace_btrfs_tree_read_unlock(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_nested
+ * The write unlock will do a barrier for us, and the lock_recursed
* field only matters to the lock owner.
*/
- if (eb->lock_nested && current->pid == eb->lock_owner) {
- eb->lock_nested = 0;
+ 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->spinning_readers) == 0);
- atomic_dec(&eb->spinning_readers);
- atomic_dec(&eb->read_locks);
+ btrfs_assert_spinning_readers_put(eb);
+ btrfs_assert_tree_read_locks_put(eb);
read_unlock(&eb->lock);
}
/*
- * 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)
{
+ 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_nested
+ * The write unlock will do a barrier for us, and the lock_recursed
* field only matters to the lock owner.
*/
- if (eb->lock_nested && current->pid == eb->lock_owner) {
- eb->lock_nested = 0;
+ if (eb->lock_recursed && current->pid == eb->lock_owner) {
+ eb->lock_recursed = false;
return;
}
btrfs_assert_tree_read_locked(eb);
@@ -218,70 +407,268 @@
/* atomic_dec_and_test implies a barrier */
if (atomic_dec_and_test(&eb->blocking_readers))
cond_wake_up_nomb(&eb->read_lock_wq);
- atomic_dec(&eb->read_locks);
+ btrfs_assert_tree_read_locks_put(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)
+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();
+
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, atomic_read(&eb->blocking_writers) == 0);
+ wait_event(eb->write_lock_wq, READ_ONCE(eb->blocking_writers) == 0);
write_lock(&eb->lock);
- if (atomic_read(&eb->blocking_readers)) {
+ /* Refetch value after lock */
+ if (atomic_read(&eb->blocking_readers) ||
+ READ_ONCE(eb->blocking_writers)) {
write_unlock(&eb->lock);
- wait_event(eb->read_lock_wq,
- atomic_read(&eb->blocking_readers) == 0);
goto again;
}
- if (atomic_read(&eb->blocking_writers)) {
- write_unlock(&eb->lock);
- wait_event(eb->write_lock_wq,
- atomic_read(&eb->blocking_writers) == 0);
- goto again;
- }
- WARN_ON(atomic_read(&eb->spinning_writers));
- atomic_inc(&eb->spinning_writers);
- atomic_inc(&eb->write_locks);
+ btrfs_assert_spinning_writers_get(eb);
+ btrfs_assert_tree_write_locks_get(eb);
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);
}
/*
- * 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)
{
- int blockers = atomic_read(&eb->blocking_writers);
+ /*
+ * 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;
- atomic_dec(&eb->write_locks);
+ btrfs_assert_tree_write_locks_put(eb);
if (blockers) {
- WARN_ON(atomic_read(&eb->spinning_writers));
- atomic_dec(&eb->blocking_writers);
- /* Use the lighter barrier after atomic */
- smp_mb__after_atomic();
- cond_wake_up_nomb(&eb->write_lock_wq);
+ 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 {
- WARN_ON(atomic_read(&eb->spinning_writers) != 1);
- atomic_dec(&eb->spinning_writers);
+ btrfs_assert_spinning_writers_put(eb);
write_unlock(&eb->lock);
}
}
-void btrfs_assert_tree_locked(struct extent_buffer *eb)
+/*
+ * Set all locked nodes in the path to blocking locks. This should be done
+ * before scheduling
+ */
+void btrfs_set_path_blocking(struct btrfs_path *p)
{
- BUG_ON(!atomic_read(&eb->write_locks));
+ int i;
+
+ for (i = 0; i < BTRFS_MAX_LEVEL; i++) {
+ if (!p->nodes[i] || !p->locks[i])
+ continue;
+ /*
+ * If we currently have a spinning reader or writer lock this
+ * will bump the count of blocking holders and drop the
+ * spinlock.
+ */
+ 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;
+ }
+ }
}
-static void btrfs_assert_tree_read_locked(struct extent_buffer *eb)
+/*
+ * 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)
{
- BUG_ON(!atomic_read(&eb->read_locks));
+ 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,
+ bool recurse)
+{
+ struct extent_buffer *eb;
+
+ while (1) {
+ eb = btrfs_root_node(root);
+ __btrfs_tree_read_lock(eb, BTRFS_NESTING_NORMAL, recurse);
+ 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);
}
--
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