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

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Date:	Tue, 04 Feb 2014 08:31:54 +0000
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"Roger Pau Monné" <roger.pau@...rix.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 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();

Jan
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