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

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Date:	Tue, 04 Feb 2014 08:02:17 +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 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().

Jan
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