linux-kernel - Re: [PATCH] block: fix blk_queue_end

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Date:	Wed, 21 Dec 2011 16:16:46 +0800
From:	Tao Ma <tm@....ma>
To:	"Williams, Dan J" <dan.j.williams@...el.com>
CC:	Meelis Roos <mroos@...ux.ee>, axboe@...nel.dk,
	Tao Ma <boyu.mt@...bao.com>,
	Linux Kernel list <linux-kernel@...r.kernel.org>,
	linux-scsi@...r.kernel.org, Ed Nadolski <edmund.nadolski@...el.com>
Subject: Re: [PATCH] block: fix blk_queue_end_tag()

On 12/21/2011 03:30 PM, Williams, Dan J wrote:
> On Tue, Dec 20, 2011 at 10:48 PM, Tao Ma <tm@....ma> wrote:
>> On 12/21/2011 02:36 PM, Meelis Roos wrote:
>>>>> -   if (unlikely(tag >= bqt->max_depth)) {
>>>>> +   if (WARN_ONCE(tag >= bqt->real_max_depth,
>>>>> +                 "%s: tag %d greater than tag map size: %d\n",
>>>>> +                 __func__, tag, bqt->real_max_depth)) {
>>>>>             /*
>>>>>              * This can happen after tag depth has been reduced.
>>>> Please also change the comments here since it should never happen in the
>>>> right workload.
>>>
>>> What do you mean by right workload? Normal workload?
>> yeah, so real_max_depth is the maximum depth we ever have. So in normal
>> case(shrinking queue depth is also a normal user case), we should never
>> arrive here. In another word, if tag >= real_max_depth, we should have a
>> bug in the kernel.
> 
> So this is what Ed Nadolski suggested, just cut to the chase and do,
> the following.  Seems like the comment is what got us into trouble in
> the first place.
> 
> diff --git a/block/blk-tag.c b/block/blk-tag.c
> index e74d6d1..e297d9d7 100644
> --- a/block/blk-tag.c
> +++ b/block/blk-tag.c
> @@ -284,16 +284,7 @@ void blk_queue_end_tag(struct request_queue *q,
> struct request *rq)
>         struct blk_queue_tag *bqt = q->queue_tags;
>         int tag = rq->tag;
> 
> -       BUG_ON(tag == -1);
> -
> -       if (unlikely(tag >= bqt->max_depth)) {
> -               /*
> -                * This can happen after tag depth has been reduced.
> -                * But tag shouldn't be larger than real_max_depth.
> -                */
> -               WARN_ON(tag >= bqt->real_max_depth);
> -               return;
> -       }
> +       BUG_ON(tag == -1 || tag > bqt->real_max_depth);
I guess tag = bqt->real_max_depth should also be a problem.

Thanks
Tao
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