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

From: Roger Pau Monné
Date: Wed Jan 29 2014 - 06:31:03 EST


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.

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