linux-kernel - Re: [PATCH] scsi_debug: deadlock between completions and surprise module removal

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Date:	Sat, 06 Sep 2014 10:40:06 -0400
From:	Douglas Gilbert <dgilbert@...erlog.com>
To:	Bart Van Assche <bvanassche@....org>,
	Christoph Hellwig <hch@...radead.org>
CC:	SCSI development list <linux-scsi@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	James Bottomley <james.bottomley@...senpartnership.com>,
	Milan Broz <gmazyland@...il.com>
Subject: Re: [PATCH] scsi_debug: deadlock between completions and surprise
 module removal

On 14-09-05 11:25 AM, Bart Van Assche wrote:
> On 09/05/14 15:56, Douglas Gilbert wrote:
>> With scsi-mq I think many LLDs probably have a new
>> race possibility between a surprise rmmod of the LLD
>> and another thread presenting a new command at about
>> the same time (or another thread's command completing
>> around that time). Does anything above the LLD stop
>> this happening?
>>
>> Looking at mpt3sas and hpsa module exit calls, they don't
>> seem to guard against this possibility.
>>
>> The test is pretty easy: build the LLD as a module, load
>> it and fire up a multi-thread, libaio fio test on one or
>> more devices (SSDs would probably be good) on that LLD.
>> While the test is running, do 'rmmod LLD'.
>
> An LLD must call scsi_remove_host() directly or indirectly from the module
> cleanup path. scsi_remove_host() triggers a call to blk_cleanup_queue(). That
> last function sets the flag QUEUE_FLAG_DYING which prevents that new I/O is
> queued and waits until previously queued requests have finished before returning.

And they do call scsi_remove_host(). But they do that toward
the end of their clean-up. The problem that I observed has
already happened before that.

IOW I think the QUEUE_FLAG_DYING state needs to be set and
acknowledged as the first order of business by the code
that implements 'rmmod LLD'.

Doug Gilbert
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