From mboxrd@z Thu Jan  1 00:00:00 1970
From: Bart Van Assche <bvanassche@acm.org>
Subject: Re: [PATCH v11 2/9] Remove get_device() / put_device() pair from
 scsi_request_fn()
Date: Mon, 24 Jun 2013 09:13:16 +0200
Message-ID: <51C7F18C.7010904@acm.org>
References: <51B86E26.6030108@acm.org> <51B86EB5.8030602@acm.org> <1372041391.2386.20.camel@dabdike>
Mime-Version: 1.0
Content-Type: text/plain; charset=ISO-8859-15; format=flowed
Content-Transfer-Encoding: 7bit
Return-path: <linux-scsi-owner@vger.kernel.org>
Received: from gerard.telenet-ops.be ([195.130.132.48]:47871 "EHLO
	gerard.telenet-ops.be" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1752310Ab3FXHNU (ORCPT
	<rfc822;linux-scsi@vger.kernel.org>); Mon, 24 Jun 2013 03:13:20 -0400
In-Reply-To: <1372041391.2386.20.camel@dabdike>
Sender: linux-scsi-owner@vger.kernel.org
List-Id: linux-scsi@vger.kernel.org
To: James Bottomley <jbottomley@parallels.com>
Cc: linux-scsi <linux-scsi@vger.kernel.org>, Joe Lawrence <jdl1291@gmail.com>, Tejun Heo <tj@kernel.org>, Chanho Min <chanho.min@lge.com>, David Milburn <dmilburn@redhat.com>, Hannes Reinecke <hare@suse.de>, Mike Christie <michaelc@cs.wisc.edu>

On 06/24/13 04:36, James Bottomley wrote:
> On Wed, 2013-06-12 at 14:51 +0200, Bart Van Assche wrote:
>> Now that all scsi_request_fn() callers hold a reference on the
>> SCSI device that function is invoked for
>
> What makes you think that this is a true statement?  The usual caller is
> the block layer, which doesn't really know anything about the
> sdev->sdev_gendev.

The reasoning behind that comment is as follows:
* The block layer guarantees that the reference count of a request
   queue is >= 1 as long as a request_fn() call is in progress (see also
   blk_cleanup_queue(), the __blk_drain_queue() call in that function
   and the loop in __blk_drain_queue() that waits until
   request_fn_active == 0).
* The SCSI core guarantees that blk_cleanup_queue() is invoked before
   the final put on sdev->sdev_gendev.

Bart.