[PATCH v2 9/8] don't fool lockdep in freeze_super() and thaw_super() paths

Oleg Nesterov
2015-08-12T13:13:55Z

On 08/11, Oleg Nesterov wrote:
>
> The only essential change is that I dropped the lockdep improvements
> as we discussed. This means that 8/8 was changed a bit, and I decided
> to add the new documentation patch, see 3/8.

Update: The recent

[PATCH 0/2] xfs: kill lockdep false positives from readdir

changes from Dave fixed the problems ILOCK false-positives. So we can
add the additional patch which (modulo comments) just turns v2 back into
v1.

Dave, Jan, you seem to agree with these patches. How should we route
this all?

-------------------------------------------------------------------------------
Subject: [PATCH v2 9/8] don't fool lockdep in freeze_super() and thaw_super() paths

sb_wait_write()->percpu_rwsem_release() fools lockdep to avoid the
false-positives. Now that xfs was fixed by Dave 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().

While at it, kill the outdated part of the comment above sb_wait_write.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
fs/super.c | 41 ++++++++++++++++++++++++++---------------
1 files changed, 26 insertions(+), 15 deletions(-)

diff --git a/fs/super.c b/fs/super.c
index 8762997..91c9756 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -1208,32 +1208,39 @@ EXPORT_SYMBOL(__sb_start_write);
* @level: type of writers we wait for (normal vs page fault)
*
* This function waits until there are no writers of given type to given file
- * system. Caller of this function should make sure there can be no new writers
- * of type @level before calling this function. Otherwise this function can
- * livelock.
+ * system.
*/
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; --level >= 0; )
+ 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; --level >= 0; )
percpu_up_write(sb->s_writers.rw_sem + level);
@@ -1329,6 +1336,7 @@ int freeze_super(struct super_block *sb)
* sees write activity when frozen is set to SB_FREEZE_COMPLETE.
*/
sb->s_writers.frozen = SB_FREEZE_COMPLETE;
+ sb_freeze_release(sb);
up_write(&sb->s_umount);
return 0;
}
@@ -1355,11 +1363,14 @@ 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;
}
--
1.5.5.1