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

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Date:	Thu, 16 Aug 2012 07:52:35 +0000
From:	Bart Van Assche <bvanassche@....org>
To:	Chanho Min <chanho0207@...il.com>
CC:	James Bottomley <James.Bottomley@...senpartnership.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 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().

Bart.

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