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>

diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
index e6243f787434627ccd7b18c993b9681fdb7814bb..e1939a6c747867dd353830f915b85d605e7c8918 100644 (file)
--- 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();