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

Jan Kara
2015-08-13T11:02:02Z

On Wed 12-08-15 15:11:38, Oleg Nesterov wrote:
> 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?

Regarding the routing, ideally Al Viro should take these as a VFS
maintainer.

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 While at it, kill the outdated part of the comment above sb_wait_write.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

The patch looks good. Just one nit:

> + for (level = SB_FREEZE_LEVELS; --level >= 0; )
> + percpu_rwsem_release(sb->s_writers.rw_sem + level, 0, _THIS_IP_);

It is more common (and to me more readable) to have the loop written as:

for (level = SB_FREEZE_LEVELS - 1; level >= 0; level--)

I agree what you do is shorter but IMHO it's just an unnecessary
obfuscation :)

Otherwise feel free to add:

Reviewed-by: Jan Kara <jack@suse.com>

Honza

> +}
> +
> +/*
> + * 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
>
>
--
Jan Kara <jack@suse.com>
SUSE Labs, CR