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

Jan Kara
2016-09-26T16:18:58Z

On Mon 26-09-16 18:08:06, Oleg Nesterov wrote:
> 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.
...
> +/*
> + * 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)

Can we call this lockdep_sb_freeze_acquire() or something like that so that
it is clear this is only about lockdep annotations? Similarly with
sb_freeze_unlock()...

Other than that the change looks good to me (and I hope you really tested
there are no more lockdep false positives ;).

Honza

> {
> 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
>
>
--
Jan Kara <jack@suse.com>
SUSE Labs, CR