[PATCH v3 3/8] document rwsem_release() in sb_wait_write()

Oleg Nesterov
2015-08-14T17:22:16Z

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>
---
fs/super.c | 12 +++++++++---
1 files changed, 9 insertions(+), 3 deletions(-)

diff --git a/fs/super.c b/fs/super.c
index d0fdd49..89b58fb 100644
--- 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 {
--
1.5.5.1