linux-kernel - Re: [PATCH RESEND] block: warn if tag is greater than real_max

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Date:	Mon, 19 Dec 2011 17:45:14 -0800
From:	Dan Williams <dan.j.williams@...el.com>
To:	Tao Ma <tm@....ma>
Cc:	Jens Axboe <axboe@...nel.dk>, linux-kernel@...r.kernel.org,
	linux-scsi <linux-scsi@...r.kernel.org>,
	"edmund.nadolski" <edmund.nadolski@...el.com>, mroos@...ee
Subject: Re: [PATCH RESEND] block: warn if tag is greater than real_max_depth.

On Mon, Dec 19, 2011 at 5:11 PM, Tao Ma <tm@....ma> wrote:
>>> Looks good, better than what we had. Applied.
>>
>> This appears to interact badly with scsi_adjust_queue_depth() when the
>> tag space shrinks.  I can reproduce a similar crash as reported in
>> "3.2-rc2+git: kernel BUG at block/blk-core.c:1000!
>> (__scsi_queue_insert)" [1].
>>
>> I can hit "kernel BUG at block/blk-core.c:2268!" which is the same
>> BUG_ON(blk_queued_rq(rq)) check reliably with:
>> # for i in $(seq 0 10); do dd if=/dev/zero of=/dev/sdX & done
>> # echo 4 > /sys/class/block/sdX/device/queue_depth
>>
>> The following fixes it for me, if this looks ok (versus reverting
>> commit 5e081591) I'll roll it into a formal patch with Ed and Meelis'
>> Reported-by.
> Interesting. If I read the code correctly, real_max_depth is the maximum
> queue depth we ever have and max_depth is the current depth.
>
> In your fix, we never resize the tag size to be smaller than max_depth.
> So I think this patch does expose some problem, but not lead to the BUG.

Yes, if we keep the "if (unlikely(tag >= bqt->max_depth))" check in
blk_queue_end_tag() then the side effect is that we can never shrink
the tag depth, which I don't think was intended.

> And in your new comment, you mentioned that "request between new_depth
> and max_depth can be in-flight", but max_depth <= real_max_depth, so
> what's wrong with the comment? Sorry, but am I missing something here?

Prior to the change blk_queue_end_tag() would continue to complete
requests with a tag > max_depth, now it silently drops them on the
floor leaving BUG_ON(blk_queued_rq(rq)) to trigger when we try to end
the request

> Having said that, I have tried the test in my machine here, but have no
> luck by now. My kernel version is 3.2.0-rc5+.

What controller are you using?  I am using the isci driver with a sas
disk.  Meelis saw it with aic7xxx on a parallel scsi disk.

--
Dan
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