Re: [RFC][PATCH 2/2] fs: conditionally do memory barrier in __sb_end_write()

[Jan Kara <jack@suse.cz>]

On Fri 19-06-15 15:32:23, Dave Hansen wrote:
> If I sit in a loop and do write()s to small tmpfs files,
> __sb_end_write() is third-hottest kernel function due to its
> smp_mb().
> 
> The stated purpose for the smp_mb() in __sb_end_write() is to
> ensure "s_writers are updated before we wake up waiters".  We
> only wake up waiters if waitqueue_active(), but we do the
> smp_mb() unconditionally.
> 
> It seems like we should be able to avoid it unless we are
> actually doing the wake_up().
...
> diff -puN fs/super.c~selectively-do-barriers-in-__sb_end_write fs/super.c
> --- a/fs/super.c~selectively-do-barriers-in-__sb_end_write	2015-06-19 15:20:37.953726659 -0700
> +++ b/fs/super.c	2015-06-19 15:20:37.956726794 -0700
> @@ -1147,13 +1147,14 @@ out:
>  void __sb_end_write(struct super_block *sb, int level)
>  {
>  	percpu_counter_dec(&sb->s_writers.counter[level-1]);
> -	/*
> -	 * Make sure s_writers are updated before we wake up waiters in
> -	 * freeze_super().
> -	 */
> -	smp_mb();
> -	if (waitqueue_active(&sb->s_writers.wait))
> +	if (waitqueue_active(&sb->s_writers.wait)) {
> +		/*
> +		 * Make sure other CPUs can see our s_writers update
> +		 * before we wake up waiters in freeze_super().
> +		 */
> +		smp_mb();

I think this is actually wrong. The barrier has to be before the
waitqueue_active() check. Otherwise that read can be reordered before the
percpu counter increment and a race window opens...

But we could make things faster by something like:

__sb_end_write()
	rcu_read_lock();
	percpu_counter_dec(&sb->s_writers.counter[level-1]);
	if (unlikely(sb->s_writers.frozen >= level))
		wake_up(&sb->s_writers.wait);
	rcu_read_unlock();

So the synchronize_rcu() calls you've added in the first patch will make
sure that all __sb_end_write() calls after we've started the freeze
procedure will end up calling wake_up() and so the process waiting in
sb_wait_write() will be woken as necessary. But please add a detailed
comment about the synchronization because its tricky and uncommon...

								Honza


>  		wake_up(&sb->s_writers.wait);
> +	}
>  	rwsem_release(&sb->s_writers.lock_map[level-1], 1, _RET_IP_);
>  }
>  EXPORT_SYMBOL(__sb_end_write);
> _
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR