[PATCH 4/4] change thaw_super() to re-acquire s_writers.lock_map

[Oleg Nesterov <oleg@redhat.com>]

Change thaw_super() to re-acquire the "write" locks we are going to
release. This way s_op->unfreeze_fs(sb) is called with these locks
held as it seen by lockdep, and this matches the reality. This adds
another trivial helper, sb_freeze_acquire().

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
 fs/super.c |   12 ++++++++++++
 1 files changed, 12 insertions(+), 0 deletions(-)

diff --git a/fs/super.c b/fs/super.c
index e7ea9f1..b4db3ee 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -1263,6 +1263,14 @@ static void sb_freeze_release(struct super_block *sb)
 		rwsem_release(sb->s_writers.lock_map + level, 1, _THIS_IP_);
 }
 
+static void sb_freeze_acquire(struct super_block *sb)
+{
+	int level;
+
+	for (level = 0; level < SB_FREEZE_LEVELS; ++level)
+		rwsem_acquire(sb->s_writers.lock_map + level, 0, 1, _THIS_IP_);
+}
+
 /**
  * freeze_super - lock the filesystem and force it into a consistent state
  * @sb: the super to lock
@@ -1386,16 +1394,20 @@ int thaw_super(struct super_block *sb)
 	if (sb->s_flags & MS_RDONLY)
 		goto out;
 
+	sb_freeze_acquire(sb);
+
 	if (sb->s_op->unfreeze_fs) {
 		error = sb->s_op->unfreeze_fs(sb);
 		if (error) {
 			printk(KERN_ERR
 				"VFS:Filesystem thaw failed\n");
+			sb_freeze_release(sb);
 			up_write(&sb->s_umount);
 			return error;
 		}
 	}
 
+	sb_freeze_release(sb);
 out:
 	sb->s_writers.frozen = SB_UNFROZEN;
 	smp_wmb();
-- 
1.5.5.1