1 files changed, 173 insertions, 34 deletions

diff --git a/fs/btrfs/locking.c b/fs/btrfs/locking.c
index 39bae7761db6..85506c4a3af7 100644
--- a/fs/btrfs/locking.c
+++ b/fs/btrfs/locking.c
@@ -25,64 +25,203 @@
 #include "extent_io.h"
 #include "locking.h"
 
+static inline void spin_nested(struct extent_buffer *eb)
+{
+        spin_lock(&eb->lock);
+}
+
 /*
- * locks the per buffer mutex in an extent buffer.  This uses adaptive locks
- * and the spin is not tuned very extensively.  The spinning does make a big
- * difference in almost every workload, but spinning for the right amount of
- * time needs some help.
- *
- * In general, we want to spin as long as the lock holder is doing btree
- * searches, and we should give up if they are in more expensive code.
+ * Setting a lock to blocking will drop the spinlock and set the
+ * flag that forces other procs who want the lock to wait.  After
+ * this you can safely schedule with the lock held.
  */
+void btrfs_set_lock_blocking(struct extent_buffer *eb)
+{
+        if (!test_bit(EXTENT_BUFFER_BLOCKING, &eb->bflags)) {
+                set_bit(EXTENT_BUFFER_BLOCKING, &eb->bflags);
+                spin_unlock(&eb->lock);
+        }
+        /* exit with the spin lock released and the bit set */
+}
 
-int btrfs_tree_lock(struct extent_buffer *eb)
+/*
+ * clearing the blocking flag will take the spinlock again.
+ * After this you can't safely schedule
+ */
+void btrfs_clear_lock_blocking(struct extent_buffer *eb)
 {
-        int i;
+        if (test_bit(EXTENT_BUFFER_BLOCKING, &eb->bflags)) {
+                spin_nested(eb);
+                clear_bit(EXTENT_BUFFER_BLOCKING, &eb->bflags);
+                smp_mb__after_clear_bit();
+        }
+        /* exit with the spin lock held */
+}
 
-        if (mutex_trylock(&eb->mutex))
-                return 0;
+/*
+ * unfortunately, many of the places that currently set a lock to blocking
+ * don't end up blocking for every long, and often they don't block
+ * at all.  For a dbench 50 run, if we don't spin one the blocking bit
+ * at all, the context switch rate can jump up to 400,000/sec or more.
+ *
+ * So, we're still stuck with this crummy spin on the blocking bit,
+ * at least until the most common causes of the short blocks
+ * can be dealt with.
+ */
+static int btrfs_spin_on_block(struct extent_buffer *eb)
+{
+        int i;
         for (i = 0; i < 512; i++) {
                 cpu_relax();
-                if (mutex_trylock(&eb->mutex))
-                        return 0;
+                if (!test_bit(EXTENT_BUFFER_BLOCKING, &eb->bflags))
+                        return 1;
+                if (need_resched())
+                        break;
         }
-        cpu_relax();
-        mutex_lock_nested(&eb->mutex, BTRFS_MAX_LEVEL - btrfs_header_level(eb));
         return 0;
 }
 
-int btrfs_try_tree_lock(struct extent_buffer *eb)
+/*
+ * This is somewhat different from trylock.  It will take the
+ * spinlock but if it finds the lock is set to blocking, it will
+ * return without the lock held.
+ *
+ * returns 1 if it was able to take the lock and zero otherwise
+ *
+ * After this call, scheduling is not safe without first calling
+ * btrfs_set_lock_blocking()
+ */
+int btrfs_try_spin_lock(struct extent_buffer *eb)
 {
-        return mutex_trylock(&eb->mutex);
+        int i;
+
+        spin_nested(eb);
+        if (!test_bit(EXTENT_BUFFER_BLOCKING, &eb->bflags))
+                return 1;
+        spin_unlock(&eb->lock);
+
+        /* spin for a bit on the BLOCKING flag */
+        for (i = 0; i < 2; i++) {
+                if (!btrfs_spin_on_block(eb))
+                        break;
+
+                spin_nested(eb);
+                if (!test_bit(EXTENT_BUFFER_BLOCKING, &eb->bflags))
+                        return 1;
+                spin_unlock(&eb->lock);
+        }
+        return 0;
 }
 
-int btrfs_tree_unlock(struct extent_buffer *eb)
+/*
+ * the autoremove wake function will return 0 if it tried to wake up
+ * a process that was already awake, which means that process won't
+ * count as an exclusive wakeup.  The waitq code will continue waking
+ * procs until it finds one that was actually sleeping.
+ *
+ * For btrfs, this isn't quite what we want.  We want a single proc
+ * to be notified that the lock is ready for taking.  If that proc
+ * already happen to be awake, great, it will loop around and try for
+ * the lock.
+ *
+ * So, btrfs_wake_function always returns 1, even when the proc that we
+ * tried to wake up was already awake.
+ */
+static int btrfs_wake_function(wait_queue_t *wait, unsigned mode,
+                               int sync, void *key)
 {
-        mutex_unlock(&eb->mutex);
-        return 0;
+        autoremove_wake_function(wait, mode, sync, key);
+        return 1;
 }
 
-int btrfs_tree_locked(struct extent_buffer *eb)
+/*
+ * returns with the extent buffer spinlocked.
+ *
+ * This will spin and/or wait as required to take the lock, and then
+ * return with the spinlock held.
+ *
+ * After this call, scheduling is not safe without first calling
+ * btrfs_set_lock_blocking()
+ */
+int btrfs_tree_lock(struct extent_buffer *eb)
 {
-        return mutex_is_locked(&eb->mutex);
+        DEFINE_WAIT(wait);
+        wait.func = btrfs_wake_function;
+
+        while(1) {
+                spin_nested(eb);
+
+                /* nobody is blocking, exit with the spinlock held */
+                if (!test_bit(EXTENT_BUFFER_BLOCKING, &eb->bflags))
+                        return 0;
+
+                /*
+                 * we have the spinlock, but the real owner is blocking.
+                 * wait for them
+                 */
+                spin_unlock(&eb->lock);
+
+                /*
+                 * spin for a bit, and if the blocking flag goes away,
+                 * loop around
+                 */
+                if (btrfs_spin_on_block(eb))
+                        continue;
+
+                prepare_to_wait_exclusive(&eb->lock_wq, &wait,
+                                          TASK_UNINTERRUPTIBLE);
+
+                if (test_bit(EXTENT_BUFFER_BLOCKING, &eb->bflags))
+                        schedule();
+
+                finish_wait(&eb->lock_wq, &wait);
+        }
+        return 0;
 }
 
 /*
- * btrfs_search_slot uses this to decide if it should drop its locks
- * before doing something expensive like allocating free blocks for cow.
+ * Very quick trylock, this does not spin or schedule.  It returns
+ * 1 with the spinlock held if it was able to take the lock, or it
+ * returns zero if it was unable to take the lock.
+ *
+ * After this call, scheduling is not safe without first calling
+ * btrfs_set_lock_blocking()
  */
-int btrfs_path_lock_waiting(struct btrfs_path *path, int level)
+int btrfs_try_tree_lock(struct extent_buffer *eb)
 {
-        int i;
-        struct extent_buffer *eb;
-        for (i = level; i <= level + 1 && i < BTRFS_MAX_LEVEL; i++) {
-                eb = path->nodes[i];
-                if (!eb)
-                        break;
-                smp_mb();
-                if (!list_empty(&eb->mutex.wait_list))
-                        return 1;
+        if (spin_trylock(&eb->lock)) {
+                if (test_bit(EXTENT_BUFFER_BLOCKING, &eb->bflags)) {
+                        /*
+                         * we've got the spinlock, but the real owner is
+                         * blocking.  Drop the spinlock and return failure
+                         */
+                        spin_unlock(&eb->lock);
+                        return 0;
+                }
+                return 1;
         }
+        /* someone else has the spinlock giveup */
         return 0;
 }
 
+int btrfs_tree_unlock(struct extent_buffer *eb)
+{
+        /*
+         * if we were a blocking owner, we don't have the spinlock held
+         * just clear the bit and look for waiters
+         */
+        if (test_and_clear_bit(EXTENT_BUFFER_BLOCKING, &eb->bflags))
+                smp_mb__after_clear_bit();
+        else
+                spin_unlock(&eb->lock);
+
+        if (waitqueue_active(&eb->lock_wq))
+                wake_up(&eb->lock_wq);
+        return 0;
+}
+
+int btrfs_tree_locked(struct extent_buffer *eb)
+{
+        return test_bit(EXTENT_BUFFER_BLOCKING, &eb->bflags) ||
+                        spin_is_locked(&eb->lock);
+}