reiserfs: Protect dquot_writeback_dquots() by s_umount semaphore

dquot_writeback_dquots() expects s_umount semaphore to be held to
protect it from other concurrent quota operations. reiserfs_sync_fs()
can call dquot_writeback_dquots() without holding s_umount semaphore
when called from flush_old_commits().

Fix the problem by grabbing s_umount in flush_old_commits(). However we
have to be careful and use only trylock since reiserfs_cancel_old_sync()
can be waiting for flush_old_commits() to complete while holding
s_umount semaphore. Possible postponing of sync work is not a big deal
though as that is only an opportunistic flush.

Fixes: 9d1ccbe70e0b14545caad12dc73adb3605447df0
Reported-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Jan Kara <jack@suse.cz>

1 files changed, 14 insertions, 0 deletions

diff --git a/fs/reiserfs/super.c b/fs/reiserfs/super.c
index 91cf5cbd6332..f536e12c4b1d 100644
--- a/fs/reiserfs/super.c
+++ b/fs/reiserfs/super.c
@@ -89,6 +89,19 @@ static void flush_old_commits(struct work_struct *work)
         sbi = container_of(work, struct reiserfs_sb_info, old_work.work);
         s = sbi->s_journal->j_work_sb;
 
+        /*
+         * We need s_umount for protecting quota writeback. We have to use
+         * trylock as reiserfs_cancel_old_flush() may be waiting for this work
+         * to complete with s_umount held.
+         */
+        if (!down_read_trylock(&s->s_umount)) {
+                /* Requeue work if we are not cancelling it */
+                spin_lock(&sbi->old_work_lock);
+                if (sbi->work_queued == 1)
+                        queue_delayed_work(system_long_wq, &sbi->old_work, HZ);
+                spin_unlock(&sbi->old_work_lock);
+                return;
+        }
         spin_lock(&sbi->old_work_lock);
         /* Avoid clobbering the cancel state... */
         if (sbi->work_queued == 1)
@@ -96,6 +109,7 @@ static void flush_old_commits(struct work_struct *work)
         spin_unlock(&sbi->old_work_lock);
 
         reiserfs_sync_fs(s, 1);
+        up_read(&s->s_umount);
 }
 
 void reiserfs_schedule_old_flush(struct super_block *s)