From: Jeff Moyer <jmoyer@redhat.com>
To: Vivek Goyal <vgoyal@redhat.com>
Cc: Jens Axboe <jens.axboe@oracle.com>,
       linux kernel mailing list <linux-kernel@vger.kernel.org>,
       Gui Jianfeng <guijianfeng@cn.fujitsu.com>
Subject: Re: [PATCH] Fix a CFQ crash in "for-2.6.33" branch of block tree
References: <20091210170845.GA8327@redhat.com>
Date: Thu, 10 Dec 2009 13:13:07 -0500
In-Reply-To: <20091210170845.GA8327@redhat.com> (Vivek Goyal's message of
	"Thu, 10 Dec 2009 12:08:45 -0500")
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Vivek Goyal <vgoyal@redhat.com> writes:

> I think I have hit following BUG_ON() in cfq_dispatch_request().
>
> BUG_ON(RB_EMPTY_ROOT(&cfqq->sort_list));
>
> Please find attached the patch to fix it. I have done some stress testing
> with it and have not seen it happening again.
>
>
> o We should wait on a queue even after slice expiry only if it is empty. If
>   queue is not empty then continue to expire it.
>
> o If we decide to keep the queue then make cfqq=NULL. Otherwise select_queue()
>   will return a valid cfqq and cfq_dispatch_request() can hit following
>   BUG_ON().
>
>   BUG_ON(RB_EMPTY_ROOT(&cfqq->sort_list))
>
> Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Reviewed-by: Jeff Moyer <jmoyer@redhat.com>
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