[PATCH 2/2] fs/super.c: don't fool lockdep in freeze_super() and thaw_super() paths

Oleg Nesterov
2016-09-26T16:08:10Z

sb_wait_write()->percpu_rwsem_release() fools lockdep to avoid the
false-positives. Now that xfs was fixed by Dave's commit dbad7c993053
("xfs: stop holding ILOCK over filldir callbacks") we can remove it and
change freeze_super() and thaw_super() to run with s_writers.rw_sem
locks held; we add two trivial helpers for that, sb_freeze_release()
and sb_freeze_acquire().

xfstests-dev/check `grep -il freeze tests/*/???` does not trigger any
warning from lockdep.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
fs/super.c | 37 +++++++++++++++++++++++++------------
1 file changed, 25 insertions(+), 12 deletions(-)

diff --git a/fs/super.c b/fs/super.c
index 2549896c..a9757e1 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -1214,25 +1214,34 @@ EXPORT_SYMBOL(__sb_start_write);
static void sb_wait_write(struct super_block *sb, int level)
{
percpu_down_write(sb->s_writers.rw_sem + level-1);
- /*
- * 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.
- */
- percpu_rwsem_release(sb->s_writers.rw_sem + level-1, 0, _THIS_IP_);
}

-static void sb_freeze_unlock(struct super_block *sb)
+/*
+ * We are going to return to userspace and forget about these locks, the
+ * ownership goes to the caller of thaw_super() which does unlock().
+ */
+static void sb_freeze_release(struct super_block *sb)
+{
+ int level;
+
+ for (level = SB_FREEZE_LEVELS - 1; level >= 0; level--)
+ percpu_rwsem_release(sb->s_writers.rw_sem + level, 0, _THIS_IP_);
+}
+
+/*
+ * Tell lockdep we are holding these locks before we call ->unfreeze_fs(sb).
+ */
+static void sb_freeze_acquire(struct super_block *sb)
{
int level;

for (level = 0; level < SB_FREEZE_LEVELS; ++level)
percpu_rwsem_acquire(sb->s_writers.rw_sem + level, 0, _THIS_IP_);
+}
+
+static void sb_freeze_unlock(struct super_block *sb)
+{
+ int level;

for (level = SB_FREEZE_LEVELS - 1; level >= 0; level--)
percpu_up_write(sb->s_writers.rw_sem + level);
@@ -1328,6 +1337,7 @@ int freeze_super(struct super_block *sb)
* when frozen is set to SB_FREEZE_COMPLETE, and for thaw_super().
*/
sb->s_writers.frozen = SB_FREEZE_COMPLETE;
+ sb_freeze_release(sb);
up_write(&sb->s_umount);
return 0;
}
@@ -1354,11 +1364,14 @@ int thaw_super(struct super_block *sb)
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;
}
--
2.5.0