Re: dm: fix inflight IO check

[Mike Snitzer <snitzer@redhat.com>]

On Mon, Dec 10 2018 at  5:45pm -0500,
Jens Axboe <axboe@kernel.dk> wrote:

> After switching to percpu inflight counters, the inflight check
> is totally buggy. It's perfectly valid for some counters to be
> non-zero while having a total inflight IO count of 0, that's how
> these kinds of counters work (inc on one CPU, dec on another).
> Fix the md_in_flight() check to sum all counters before returning
> a false positive, potentially.
> 
> While at it, remove the inflight read for IO completion. We don't
> need it, just wake anyone that's waiting for the IO count to drop
> to zero. The caller needs to re-check that value anyway when woken,
> which it does.
> 
> Fixes: 6f75723190d8 ("dm: remove the pending IO accounting")
> Reported-by: Christoph Hellwig <hch@lst.de>
> Signed-off-by: Jens Axboe <axboe@kernel.dk>

I'm seeing that device-mapper-test-suite's "resize_io" test doesn't
pass.  Glad this resolves the xfstest issue but I think more work is
needed, so I'll build any additional changes on this fix.

Thanks.

Acked-by: Mike Snitzer <snitzer@redhat.com>


> 
> ---
> 
> diff --git a/drivers/md/dm.c b/drivers/md/dm.c
> index 70568f8b6c53..79ad4b3d215c 100644
> --- a/drivers/md/dm.c
> +++ b/drivers/md/dm.c
> @@ -650,14 +650,14 @@ static bool md_in_flight(struct mapped_device *md)
>  {
>  	int cpu;
>  	struct hd_struct *part = &dm_disk(md)->part0;
> +	long sum = 0;
>  
>  	for_each_possible_cpu(cpu) {
> -		if (part_stat_local_read_cpu(part, in_flight[0], cpu) ||
> -		    part_stat_local_read_cpu(part, in_flight[1], cpu))
> -			return true;
> +		sum += part_stat_local_read_cpu(part, in_flight[0], cpu);
> +		sum += part_stat_local_read_cpu(part, in_flight[1], cpu);
>  	}
>  
> -	return false;
> +	return sum != 0;
>  }

Heh, amazing any tests passed.. sorry for this