Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754648Ab0HPPow (ORCPT ); Mon, 16 Aug 2010 11:44:52 -0400 Received: from mx1.redhat.com ([209.132.183.28]:21049 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754615Ab0HPPov (ORCPT ); Mon, 16 Aug 2010 11:44:51 -0400 Date: Mon, 16 Aug 2010 11:44:43 -0400 From: Mike Snitzer To: Peter Oberparleiter Cc: Linux Kernel Mailing List , Jens Axboe , unsik Kim , Martin Schwidefsky , Heiko Carstens Subject: Re: Switching block device elevators Message-ID: <20100816154443.GA9860@redhat.com> References: <4C6950B3.8020702@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <4C6950B3.8020702@linux.vnet.ibm.com> User-Agent: Mutt/1.5.20 (2009-12-10) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2033 Lines: 46 On Mon, Aug 16 2010 at 10:52am -0400, Peter Oberparleiter wrote: > After commit 1abec4fdbb142e3ccb6ce99832fae42129134a96, "block: make > blk_init_free_list and elevator_init idempotent", we're seeing > kernel panics in our s390 tape block device driver. The panic is > triggered because our driver tries to replace the default elevator > with a noop elevator by calling elevator_exit() directly followed by > elevator_init(). Maybe we should look to export elevator_switch() -- rather than confining its use to the sysfs interface. > Since the commit, elevator_init() returns 0 if > request_queue->elevator is non-null, even though it does not install > a new elevator. As a result, the next access to the elevator finds a > pointer to the old one which was already freed and a panic is > triggered. Our current fix consists of setting the elevator pointer > to NULL after elevator_exit(). elevator_exit() triggers a call, via kobj, to elevator_release() which doesn't have access to the request_queue to reset it. Unfortunately, commit 1abec4fdbb imposes that the elevator_exit() caller must take care to reset q->elevator to NULL -- like dasd_alloc_queue() does. Though I suppose we _could_ pass request_queue to elevator_exit. > There is at least one other driver where the problem currently > exists (drivers/block/mg_disk.c, author on cc) and another s390 > driver where the problem was only accidentally fixed before 2.6.35. > I'm wondering if there's a better solution (apart from not forcing > an elevator) and would like to hear everyone's opinion on this > matter. How about declaring elevator_switch() non-static, for > example? Right, updating drivers/block/mg_disk.c and drivers/s390/block/dasd.c to use elevator_switch would work. Mike -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/