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

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Date:	Tue, 4 Feb 2014 09:39:05 +0100
From:	Roger Pau Monné <roger.pau@...rix.com>
To:	Jan Beulich <JBeulich@...e.com>
CC:	Matt Rushton <mrushton@...zon.com>, Matt Wilson <msw@...zon.com>,
	DavidVrabel <david.vrabel@...rix.com>,
	Ian Campbell <Ian.Campbell@...rix.com>,
	<xen-devel@...ts.xenproject.org>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [Xen-devel] [PATCH 3/3] xen-blkback: fix shutdown race

On 04/02/14 09:31, Jan Beulich wrote:
>>>> On 04.02.14 at 09:16, Roger Pau Monné<roger.pau@...rix.com> wrote:
>> On 04/02/14 09:02, Jan Beulich wrote:
>>>>>> On 03.02.14 at 17:58, Roger Pau Monné<roger.pau@...rix.com> wrote:
>>>> On 29/01/14 09:52, Jan Beulich wrote:
>>>>>>>> On 28.01.14 at 18:43, Roger Pau Monne <roger.pau@...rix.com> wrote:
>>>>>> +		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.
>>>>>
>>>>> Apart from that, the two if()s would - at least to me - be more
>>>>> clear if combined into one.
>>>>
>>>> In order to get rid of the race I had to introduce yet another atomic_t 
>>>> in xen_blkif struct, which is something I don't really like, but I 
>>>> could not see any other way to solve this. If that's fine I will resend 
>>>> the series, here is the reworked patch:
>>>
>>> Mind explaining why you can't simply move the xen_blkif_put()
>>> down between the if() and the free_ref().
>>
>> You mean doing something like:
>>
>> if (atomic_read(&blkif->refcnt) <= 3) {
>> 	if (atomic_read(&blkif->drain))
>> 		complete(&blkif->drain_complete);
>> }
>> xen_blkif_put(blkif);
>> free_req(blkif, pending_req);
> 
> Actually, I got the description wrong. I really meant
> 
> free_req();
> if (atomic_read ...)
> 	complete();
> xen_blkif_put();

IMHO this is still a race, since we evaluate refcnt before decrementing
it. If we have for example 2 in flight requests, both could read refcnt,
both could see it's greater than 3 (so no one would call complete), and
then both will decrement it, without anyone actually calling complete.

Roger.

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