ext4: Fix WARN_ON_ONCE in ext4_commit_super()

If there are racing calls to ext4_commit_super() it's possible for
another writeback of the superblock to result in the buffer being
marked with an error after we check if the buffer is marked as having
a write error and the buffer up-to-date flag is set again.  If that
happens mark_buffer_dirty() can end up throwing a WARN_ON_ONCE.

Fix this by moving this check to write before we call
write_buffer_dirty(), and keeping the buffer locked during this whole
sequence.

Signed-off-by: Pranay Kr. Srivastava <pranjas@gmail.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

1 files changed, 16 insertions, 14 deletions

diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index 6e2f9d628c48..5664ee66b301 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -4327,20 +4327,6 @@ static int ext4_commit_super(struct super_block *sb, int sync)
 
         if (!sbh || block_device_ejected(sb))
                 return error;
-        if (buffer_write_io_error(sbh)) {
-                /*
-                 * Oh, dear.  A previous attempt to write the
-                 * superblock failed.  This could happen because the
-                 * USB device was yanked out.  Or it could happen to
-                 * be a transient write error and maybe the block will
-                 * be remapped.  Nothing we can do but to retry the
-                 * write and hope for the best.
-                 */
-                ext4_msg(sb, KERN_ERR, "previous I/O error to "
-                       "superblock detected");
-                clear_buffer_write_io_error(sbh);
-                set_buffer_uptodate(sbh);
-        }
         /*
          * If the file system is mounted read-only, don't update the
          * superblock write time.  This avoids updating the superblock
@@ -4371,7 +4357,23 @@ static int ext4_commit_super(struct super_block *sb, int sync)
                                 &EXT4_SB(sb)->s_freeinodes_counter));
         BUFFER_TRACE(sbh, "marking dirty");
         ext4_superblock_csum_set(sb);
+        lock_buffer(sbh);
+        if (buffer_write_io_error(sbh)) {
+                /*
+                 * Oh, dear.  A previous attempt to write the
+                 * superblock failed.  This could happen because the
+                 * USB device was yanked out.  Or it could happen to
+                 * be a transient write error and maybe the block will
+                 * be remapped.  Nothing we can do but to retry the
+                 * write and hope for the best.
+                 */
+                ext4_msg(sb, KERN_ERR, "previous I/O error to "
+                       "superblock detected");
+                clear_buffer_write_io_error(sbh);
+                set_buffer_uptodate(sbh);
+        }
         mark_buffer_dirty(sbh);
+        unlock_buffer(sbh);
         if (sync) {
                 error = __sync_dirty_buffer(sbh,
                         test_opt(sb, BARRIER) ? WRITE_FUA : WRITE_SYNC);