1 files changed, 19 insertions, 0 deletions

diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
index 43af5a61ad25..c32d226bfecc 100644
--- a/fs/btrfs/extent_io.c
+++ b/fs/btrfs/extent_io.c
@@ -4772,6 +4772,25 @@ struct extent_buffer *find_extent_buffer(struct btrfs_fs_info *fs_info,
                                start >> PAGE_CACHE_SHIFT);
         if (eb && atomic_inc_not_zero(&eb->refs)) {
                 rcu_read_unlock();
+                /*
+                 * Lock our eb's refs_lock to avoid races with
+                 * free_extent_buffer. When we get our eb it might be flagged
+                 * with EXTENT_BUFFER_STALE and another task running
+                 * free_extent_buffer might have seen that flag set,
+                 * eb->refs == 2, that the buffer isn't under IO (dirty and
+                 * writeback flags not set) and it's still in the tree (flag
+                 * EXTENT_BUFFER_TREE_REF set), therefore being in the process
+                 * of decrementing the extent buffer's reference count twice.
+                 * So here we could race and increment the eb's reference count,
+                 * clear its stale flag, mark it as dirty and drop our reference
+                 * before the other task finishes executing free_extent_buffer,
+                 * which would later result in an attempt to free an extent
+                 * buffer that is dirty.
+                 */
+                if (test_bit(EXTENT_BUFFER_STALE, &eb->bflags)) {
+                        spin_lock(&eb->refs_lock);
+                        spin_unlock(&eb->refs_lock);
+                }
                 mark_extent_buffer_accessed(eb, NULL);
                 return eb;
         }