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document rwsem_release() in sb_wait_write()

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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>
This commit is contained in:
Oleg Nesterov 2015-08-11 16:28:29 +02:00
parent f4b554af99
commit 0e28e01f1e
1 changed files with 10 additions and 4 deletions
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14
fs/super.c
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@ -1236,11 +1236,17 @@ static void sb_wait_write(struct super_block *sb, int level)
{
s64 writers;
/*
* We just cycle-through lockdep here so that it does not complain
* about returning with lock to userspace
*/
rwsem_acquire(&sb->s_writers.lock_map[level-1], 0, 0, _THIS_IP_);
/*
* 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_release(&sb->s_writers.lock_map[level-1], 1, _THIS_IP_);
do {