Btrfs: fix potential race in extent buffer freeing

This sounds sort of impossible but it is the only thing I can think of and
at the very least it is theoretically possible so here it goes.

If we are in try_release_extent_buffer we will check that the ref count on
the extent buffer is 1 and not under IO, and then go down and clear the tree
ref.  If between this check and clearing the tree ref somebody else comes in
and grabs a ref on the eb and the marks it dirty before
try_release_extent_buffer() does it's tree ref clear we can end up with a
dirty eb that will be freed while it is still dirty which will result in a
panic.  Thanks,

Signed-off-by: Josef Bacik <jbacik@fusionio.com>

1 files changed, 3 insertions, 6 deletions

diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
index e6243f787434..e1939a6c7478 100644
--- a/fs/btrfs/extent_io.c
+++ b/fs/btrfs/extent_io.c
@@ -4123,11 +4123,10 @@ static void check_buffer_tree_ref(struct extent_buffer *eb)
          * So bump the ref count first, then set the bit.  If someone
          * beat us to it, drop the ref we added.
          */
-        if (!test_bit(EXTENT_BUFFER_TREE_REF, &eb->bflags)) {
+        spin_lock(&eb->refs_lock);
+        if (!test_and_set_bit(EXTENT_BUFFER_TREE_REF, &eb->bflags))
                 atomic_inc(&eb->refs);
-                if (test_and_set_bit(EXTENT_BUFFER_TREE_REF, &eb->bflags))
-                        atomic_dec(&eb->refs);
-        }
+        spin_unlock(&eb->refs_lock);
 }
 
 static void mark_extent_buffer_accessed(struct extent_buffer *eb)
@@ -4239,9 +4238,7 @@ again:
                 goto free_eb;
         }
         /* add one reference for the tree */
-        spin_lock(&eb->refs_lock);
         check_buffer_tree_ref(eb);
-        spin_unlock(&eb->refs_lock);
         spin_unlock(&tree->buffer_lock);
         radix_tree_preload_end();