document rwsem_release() in sb_wait_write()

Not only we need to avoid the warning from lockdep_sys_exit(), the
caller of freeze_super() can never release this lock. Another thread
can do this, so there is another reason for rwsem_release().

Plus the comment should explain why we have to fool lockdep.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Reviewed-by: Jan Kara <jack@suse.com>

diff --git a/fs/super.c b/fs/super.c
index 24a76bcd62a59ad4cb44a65428ec4c8529effe4d..8aa3cbc571d18aabc4cfb8c8cd153d58a96fa995 100644 (file)
--- a/fs/super.c
+++ b/fs/super.c
@@ -1236,11 +1236,17 @@ static void sb_wait_write(struct super_block *sb, int level)
 {
        s64 writers;
 
+       rwsem_acquire(&sb->s_writers.lock_map[level-1], 0, 0, _THIS_IP_);
        /*
-        * We just cycle-through lockdep here so that it does not complain
-        * about returning with lock to userspace
+        * We are going to return to userspace and forget about this lock, the
+        * ownership goes to the caller of thaw_super() which does unlock.
+        *
+        * FIXME: we should do this before return from freeze_super() after we
+        * called sync_filesystem(sb) and s_op->freeze_fs(sb), and thaw_super()
+        * should re-acquire these locks before s_op->unfreeze_fs(sb). However
+        * this leads to lockdep false-positives, so currently we do the early
+        * release right after acquire.
         */
-       rwsem_acquire(&sb->s_writers.lock_map[level-1], 0, 0, _THIS_IP_);
        rwsem_release(&sb->s_writers.lock_map[level-1], 1, _THIS_IP_);
 
        do {