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

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On 29/01/14 08:52, Jan Beulich wrote:
>>>> On 28.01.14 at 18:43, Roger Pau Monne <roger.pau@xxxxxxxxxx> 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.


_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
http://lists.xen.org/xen-devel