Re: [Xen-devel] [PATCH 3/3] xen-blkback: fix shutdown race

["Roger Pau Monné" <roger.pau@citrix.com>]

On 29/01/14 08:52, Jan Beulich wrote:
>>>> On 28.01.14 at 18:43, Roger Pau Monne <roger.pau@citrix.com> wrote:
>> --- a/drivers/block/xen-blkback/blkback.c
>> +++ b/drivers/block/xen-blkback/blkback.c
>> @@ -985,17 +985,31 @@ static void __end_block_io_op(struct pending_req 
>> *pending_req, int error)
>>  	 * the proper response on the ring.
>>  	 */
>>  	if (atomic_dec_and_test(&pending_req->pendcnt)) {
>> -		xen_blkbk_unmap(pending_req->blkif,
>> +		struct xen_blkif *blkif = pending_req->blkif;
>> +
>> +		xen_blkbk_unmap(blkif,
>>  		                pending_req->segments,
>>  		                pending_req->nr_pages);
>> -		make_response(pending_req->blkif, pending_req->id,
>> +		make_response(blkif, pending_req->id,
>>  			      pending_req->operation, pending_req->status);
>> -		xen_blkif_put(pending_req->blkif);
>> -		if (atomic_read(&pending_req->blkif->refcnt) <= 2) {
>> -			if (atomic_read(&pending_req->blkif->drain))
>> -				complete(&pending_req->blkif->drain_complete);
>> +		free_req(blkif, pending_req);
>> +		/*
>> +		 * Make sure the request is freed before releasing blkif,
>> +		 * or there could be a race between free_req and the
>> +		 * cleanup done in xen_blkif_free during shutdown.
>> +		 *
>> +		 * NB: The fact that we might try to wake up pending_free_wq
>> +		 * before drain_complete (in case there's a drain going on)
>> +		 * it's not a problem with our current implementation
>> +		 * because we can assure there's no thread waiting on
>> +		 * pending_free_wq if there's a drain going on, but it has
>> +		 * to be taken into account if the current model is changed.
>> +		 */
>> +		xen_blkif_put(blkif);
>> +		if (atomic_read(&blkif->refcnt) <= 2) {
>> +			if (atomic_read(&blkif->drain))
>> +				complete(&blkif->drain_complete);
>>  		}
>> -		free_req(pending_req->blkif, pending_req);
>>  	}
>>  }
> 
> The put is still too early imo - you're explicitly accessing field in the
> structure immediately afterwards. This may not be an issue at
> present, but I think it's at least a latent one.

Yes, thanks for catching that one, it's an issue that we should solve
now on this patch or else I would just be solving a race by introducing
a new one.

> Apart from that, the two if()s would - at least to me - be more
> clear if combined into one.

Ack, will see how the patch ends up looking after getting rid of the new
race.

Roger.