linux-kernel - Re: Kernel crash in icq_free_icq

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Message-ID: <CANejiEUE0ZNNf=t5dUTD8pTB5G3gAGAxkSRkvFomrh3Bm+dY=Q@mail.gmail.com>
Date:	Wed, 18 Jan 2012 10:26:38 +0800
From:	Shaohua Li <shaohua.li@...el.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	Vivek Goyal <vgoyal@...hat.com>,
	linux kernel mailing list <linux-kernel@...r.kernel.org>,
	Jens Axboe <axboe@...nel.dk>
Subject: Re: Kernel crash in icq_free_icq_rcu

2012/1/18 Shaohua Li <shaohua.li@...el.com>:
> On Tue, 2012-01-17 at 17:11 -0800, Tejun Heo wrote:
>> On Wed, Jan 18, 2012 at 09:05:26AM +0800, Shaohua Li wrote:
>> > > Vivek is seeing the problem while switching elevators.  Are you too?
>> > > Or is it during normal operation?
>> > same here. I had some problems when I debug my ioscheduler, but
>> > eventually found even switching cfq and noop can trigger oops.
>>
>> Hmmm... maybe quiescing isn't working as expected and kmem cache is
>> being destroyed with live icq's.  I'll try to reproduce it.
> this debug patch seems to fix for me.
>
> diff --git a/block/blk-core.c b/block/blk-core.c
> index e6c05a9..c6a8ef5 100644
> --- a/block/blk-core.c
> +++ b/block/blk-core.c
> @@ -872,11 +872,11 @@ retry:
>        spin_unlock_irq(q->queue_lock);
>
>        /* create icq if missing */
> -       if (unlikely(et->icq_cache && !icq))
> +       if (unlikely(et->icq_cache && !icq && (rw_flags & REQ_ELVPRIV)))
>                icq = ioc_create_icq(q, gfp_mask);
>
>        /* rqs are guaranteed to have icq on elv_set_request() if requested */
> -       if (likely(!et->icq_cache || icq))
> +       if (likely(!et->icq_cache || icq || !(rw_flags & REQ_ELVPRIV)))
>                rq = blk_alloc_request(q, icq, rw_flags, gfp_mask);
>
>        if (unlikely(!rq)) {
this passed my test, but I didn't get reason why it can help ...
blk_alloc_request doesn't use icq if REQ_ELVPRIV isn't set, so the code
has problem for sure.
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