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

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Message-ID: <CABE8wwsRDjNs=UjBG071=AF7-h7RB+NNrQzPMREypNwSpYAVPA@mail.gmail.com>
Date:	Wed, 21 Dec 2011 00:22:49 -0800
From:	"Williams, Dan J" <dan.j.williams@...el.com>
To:	Tao Ma <tm@....ma>
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 Wed, Dec 21, 2011 at 12:16 AM, Tao Ma <tm@....ma> wrote:
> 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.

Yes, sorry, should have been >=
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