linux-kernel - Re: [PATCH RESEND] remove the queue unlock in scsi_requset

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Date:	Thu, 16 Aug 2012 09:10:01 +0100
From:	James Bottomley <James.Bottomley@...senPartnership.com>
To:	Bart Van Assche <bvanassche@....org>
Cc:	Chanho Min <chanho0207@...il.com>,
	Mike Christie <michaelc@...wisc.edu>,
	linux-scsi@...r.kernel.org, linux-kernel@...r.kernel.org,
	Jens Axboe <axboe@...nel.dk>, Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH RESEND] remove the queue unlock in scsi_requset_fn

On Thu, 2012-08-16 at 07:52 +0000, Bart Van Assche wrote:
> On 08/16/12 01:35, Chanho Min wrote:
> >> functions will occur in line.  I also don't see why the sdev reference
> >> couldn't drop to zero here.
> > scsi_request_fn is called under the lock of request_queue->queue_lock.
> > If we drop the sdev reference to zero here,
> > scsi_device_dev_release_usercontext is
> > invoked and make request_queue to NULL. When caller of scsi_request_fn try to
> > unlock request_queue->queue_lock, the oops is occurred.
> 
> Whether or not your patch is applied, if the put_device() call in
> scsi_request_fn() decreases the sdev reference count to zero, the
> scsi_request_fn() caller will still try to unlock the queue lock after
> scsi_request_fn() finished and hence will trigger a use-after-free. I'm
> afraid the only real solution is to modify the SCSI and/or block layers
> such that scsi_remove_device() can't finish while scsi_request_fn() is
> in progress. And once that is guaranteed the get_device() / put_device()
> pair can be left out from scsi_request_fn().

Well, no.  The only way to destroy a queue is with blk_cleanup_queue()
which does the final put.  blk_cleanup_queue has a rather clunky drain
check that looks at both queued and in-flight requests.  Even if we have
a scsi_remove_device() racing with the scsi_request_fn() and it gets to
blk_cleanup_queue(), that call gets held at the drain wait until the
scsi_request_fn() has exited.  The same is true for all other request
functions, so we're safe.

James


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