linux-kernel - Re: [PATCH] Correctly release and allocate a new request on TUR retries

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Message-ID: <493D2042.6030800@hp.com>
Date:	Mon, 08 Dec 2008 08:25:22 -0500
From:	"Alan D. Brunelle" <Alan.Brunelle@...com>
To:	Jens Axboe <jens.axboe@...cle.com>
CC:	Mike Anderson <andmike@...ux.vnet.ibm.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	LKML-scsi <linux-scsi@...r.kernel.org>,
	James.Bottomley@...senPartnership.com
Subject: Re: [PATCH] Correctly release and allocate a new request on TUR	retries

Jens Axboe wrote:
> On Mon, Dec 08 2008, Alan D. Brunelle wrote:
>> Mike Anderson wrote:
>>> Jens Axboe <jens.axboe@...cle.com> wrote:
>>>> On Fri, Dec 05 2008, Alan D. Brunelle wrote:
>>>>> Commands needing to be retried (TUR in this case) would result in a block
>>>>> I/O request being re-used, without being re-initialized properly. This
>>>>> patch ensures that the requests are correctly re-initialized via
>>>>> standard allocation means.
>>>>>
>>>>> Prior to this patch, boots were failing consistently as in:
>>>>> http://lkml.org/lkml/2008/12/5/161
>>>>>
>>>>> With this patch in place, the system is booting reliably.
>>>>>
>>>>> Signed-off-by: Alan D. Brunelle <alan.brunelle@...com>
>>>>> Cc: Jens Axboe <jens.axboe@...cle.com>
>>>> Looks good.
>>>>
>>>> Acked-by: Jens Axboe <jens.axboe@...cle.com>
>>>>
>>>> Perhaps James can push it in, I'm about to shutdown for the day...
>>>>
>>> I know a failure was not detected in the hp_sw_start_stop function, but it
>>> uses the same retry method as hp_sw_tur we should update this function
>>> also.
>>>
>>> I made a quick scope of callers of blk_get_request and I did not see a
>>> repeated of this retry usage model. I will make another pass to see if I
>>> missed something.
>> drivers/cdrom/cdrom.c:cdrom_read_cdda_bpc() is even worse: it gets one
>> request, then sits in a while loop re-using the same request over and
>> over again.
> 
> Sigh, it does indeed look messy...
> 
>> Since blk_rq_init() is an exported symbol, perhaps instead of having the
>> three callers realloc, it _may_ be sufficient to just have them call
>> that before re-use? (See attached un-tested patch for an example.)
> 
> I think that's a really bad idea, since it basically just clears the
> 'rq'. If you have that rq on some list (timeout, for instance), the
> kernel will not be happy. I think we have to, for now at least, put and
> get a request before looping. Then for 2.6.29 we can hopefully improve
> this situation!

OK, I'll work that one up for 2.6.28 and test it out this morning.

Alan
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