linux-kernel - Re: [PATCH 2/2] fs/super.c: don't fool lockdep in freeze_super() and thaw

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Date:   Mon, 26 Sep 2016 18:18:56 +0200
From:   Jan Kara <jack@...e.cz>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     Al Viro <viro@...iv.linux.org.uk>,
        Dave Chinner <david@...morbit.com>, Jan Kara <jack@...e.cz>,
        Nikolay Borisov <kernel@...p.com>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] fs/super.c: don't fool lockdep in freeze_super() and
 thaw_super() paths

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@...e.com>
SUSE Labs, CR