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Date: Fri, 05 Sep 2014 17:25:04 +0200
From: Bart Van Assche <bvanassche@acm.org>
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To: dgilbert@interlog.com, Christoph Hellwig <hch@infradead.org>
CC: SCSI development list <linux-scsi@vger.kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        James Bottomley <james.bottomley@hansenpartnership.com>,
        Milan Broz <gmazyland@gmail.com>
Subject: Re: [PATCH] scsi_debug: deadlock between completions and surprise
 module removal
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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.

Bart.