ext4: fix deadlock during page writeback

Commit 06bd3c36a733 (ext4: fix data exposure after a crash) uncovered a
deadlock in ext4_writepages() which was previously much harder to hit.
After this commit xfstest generic/130 reproduces the deadlock on small
filesystems.

The problem happens when ext4_do_update_inode() sets LARGE_FILE feature
and marks current inode handle as synchronous. That subsequently results
in ext4_journal_stop() called from ext4_writepages() to block waiting for
transaction commit while still holding page locks, reference to io_end,
and some prepared bio in mpd structure each of which can possibly block
transaction commit from completing and thus results in deadlock.

Fix the problem by releasing page locks, io_end reference, and
submitting prepared bio before calling ext4_journal_stop().

[ Changed to defer the call to ext4_journal_stop() only if the handle
  is synchronous.  --tytso ]

Reported-and-tested-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
CC: stable@vger.kernel.org
Signed-off-by: Jan Kara <jack@suse.cz>

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 44ee5d9..321a31c 100644 (file)
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -2754,13 +2754,36 @@ retry:
                                done = true;
                        }
                }
-               ext4_journal_stop(handle);
+               /*
+                * Caution: If the handle is synchronous,
+                * ext4_journal_stop() can wait for transaction commit
+                * to finish which may depend on writeback of pages to
+                * complete or on page lock to be released.  In that
+                * case, we have to wait until after after we have
+                * submitted all the IO, released page locks we hold,
+                * and dropped io_end reference (for extent conversion
+                * to be able to complete) before stopping the handle.
+                */
+               if (!ext4_handle_valid(handle) || handle->h_sync == 0) {
+                       ext4_journal_stop(handle);
+                       handle = NULL;
+               }
                /* Submit prepared bio */
                ext4_io_submit(&mpd.io_submit);
                /* Unlock pages we didn't use */
                mpage_release_unused_pages(&mpd, give_up_on_write);
-               /* Drop our io_end reference we got from init */
-               ext4_put_io_end(mpd.io_submit.io_end);
+               /*
+                * Drop our io_end reference we got from init. We have
+                * to be careful and use deferred io_end finishing if
+                * we are still holding the transaction as we can
+                * release the last reference to io_end which may end
+                * up doing unwritten extent conversion.
+                */
+               if (handle) {
+                       ext4_put_io_end_defer(mpd.io_submit.io_end);
+                       ext4_journal_stop(handle);
+               } else
+                       ext4_put_io_end(mpd.io_submit.io_end);
 
                if (ret == -ENOSPC && sbi->s_journal) {
                        /*