Btrfs: add nested locking mode for paths

This patch adds the possibilty to read-lock an extent even if it is already
write-locked from the same thread. btrfs_find_all_roots() needs this
capability.

Signed-off-by: Arne Jansen <sensille@gmx.net>
Signed-off-by: Jan Schmidt <list.btrfs@jan-o-sch.net>

diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
index be1bf627a14b292655ad10a4478c015d2bff9a44..dd8d140eb27bfffa5d8b9dc0397f705174a6dbe3 100644 (file)
--- a/fs/btrfs/extent_io.c
+++ b/fs/btrfs/extent_io.c
@@ -3571,6 +3571,7 @@ static struct extent_buffer *__alloc_extent_buffer(struct extent_io_tree *tree,
        atomic_set(&eb->blocking_writers, 0);
        atomic_set(&eb->spinning_readers, 0);
        atomic_set(&eb->spinning_writers, 0);
+       eb->lock_nested = 0;
        init_waitqueue_head(&eb->write_lock_wq);
        init_waitqueue_head(&eb->read_lock_wq);
 

diff --git a/fs/btrfs/extent_io.h b/fs/btrfs/extent_io.h
index 7604c30013227fd823b1523503f8faaeccf283c4..bc6a042cb6fc496e6910d21fb3cdaec000f43b8e 100644 (file)
--- a/fs/btrfs/extent_io.h
+++ b/fs/btrfs/extent_io.h
@@ -129,6 +129,7 @@ struct extent_buffer {
        struct list_head leak_list;
        struct rcu_head rcu_head;
        atomic_t refs;
+       pid_t lock_owner;
 
        /* count of read lock holders on the extent buffer */
        atomic_t write_locks;
@@ -137,6 +138,7 @@ struct extent_buffer {
        atomic_t blocking_readers;
        atomic_t spinning_readers;
        atomic_t spinning_writers;
+       int lock_nested;
 
        /* protects write locks */
        rwlock_t lock;

diff --git a/fs/btrfs/locking.c b/fs/btrfs/locking.c
index d77b67c4b275731417c11e04ad38b3dc9c4d456b..5e178d8f7167f496e928613b6c1f0000c2ea242e 100644 (file)
--- a/fs/btrfs/locking.c
+++ b/fs/btrfs/locking.c
@@ -33,6 +33,14 @@ void btrfs_assert_tree_read_locked(struct extent_buffer *eb);
  */
 void btrfs_set_lock_blocking_rw(struct extent_buffer *eb, int rw)
 {
+       if (eb->lock_nested) {
+               read_lock(&eb->lock);
+               if (eb->lock_nested && current->pid == eb->lock_owner) {
+                       read_unlock(&eb->lock);
+                       return;
+               }
+               read_unlock(&eb->lock);
+       }
        if (rw == BTRFS_WRITE_LOCK) {
                if (atomic_read(&eb->blocking_writers) == 0) {
                        WARN_ON(atomic_read(&eb->spinning_writers) != 1);
@@ -57,6 +65,14 @@ void btrfs_set_lock_blocking_rw(struct extent_buffer *eb, int rw)
  */
 void btrfs_clear_lock_blocking_rw(struct extent_buffer *eb, int rw)
 {
+       if (eb->lock_nested) {
+               read_lock(&eb->lock);
+               if (&eb->lock_nested && current->pid == eb->lock_owner) {
+                       read_unlock(&eb->lock);
+                       return;
+               }
+               read_unlock(&eb->lock);
+       }
        if (rw == BTRFS_WRITE_LOCK_BLOCKING) {
                BUG_ON(atomic_read(&eb->blocking_writers) != 1);
                write_lock(&eb->lock);
@@ -81,12 +97,25 @@ void btrfs_clear_lock_blocking_rw(struct extent_buffer *eb, int rw)
 void btrfs_tree_read_lock(struct extent_buffer *eb)
 {
 again:
+       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;
+       }
+       read_unlock(&eb->lock);
        wait_event(eb->write_lock_wq, atomic_read(&eb->blocking_writers) == 0);
        read_lock(&eb->lock);
        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);
@@ -129,6 +158,7 @@ int btrfs_try_tree_write_lock(struct extent_buffer *eb)
        }
        atomic_inc(&eb->write_locks);
        atomic_inc(&eb->spinning_writers);
+       eb->lock_owner = current->pid;
        return 1;
 }
 
@@ -137,6 +167,15 @@ int btrfs_try_tree_write_lock(struct extent_buffer *eb)
  */
 void btrfs_tree_read_unlock(struct extent_buffer *eb)
 {
+       if (eb->lock_nested) {
+               read_lock(&eb->lock);
+               if (eb->lock_nested && current->pid == eb->lock_owner) {
+                       eb->lock_nested = 0;
+                       read_unlock(&eb->lock);
+                       return;
+               }
+               read_unlock(&eb->lock);
+       }
        btrfs_assert_tree_read_locked(eb);
        WARN_ON(atomic_read(&eb->spinning_readers) == 0);
        atomic_dec(&eb->spinning_readers);
@@ -149,6 +188,15 @@ void btrfs_tree_read_unlock(struct extent_buffer *eb)
  */
 void btrfs_tree_read_unlock_blocking(struct extent_buffer *eb)
 {
+       if (eb->lock_nested) {
+               read_lock(&eb->lock);
+               if (eb->lock_nested && current->pid == eb->lock_owner) {
+                       eb->lock_nested = 0;
+                       read_unlock(&eb->lock);
+                       return;
+               }
+               read_unlock(&eb->lock);
+       }
        btrfs_assert_tree_read_locked(eb);
        WARN_ON(atomic_read(&eb->blocking_readers) == 0);
        if (atomic_dec_and_test(&eb->blocking_readers))
@@ -181,6 +229,7 @@ again:
        WARN_ON(atomic_read(&eb->spinning_writers));
        atomic_inc(&eb->spinning_writers);
        atomic_inc(&eb->write_locks);
+       eb->lock_owner = current->pid;
        return 0;
 }