Return-path: Received: from hera.kernel.org ([140.211.167.34]:32949 "EHLO hera.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751852Ab0HBQd3 (ORCPT ); Mon, 2 Aug 2010 12:33:29 -0400 Message-ID: <4C56F30D.3020304@kernel.org> Date: Mon, 02 Aug 2010 18:32:13 +0200 From: Tejun Heo MIME-Version: 1.0 To: Linus Torvalds CC: "Rafael J. Wysocki" , Jens Axboe , Linux Kernel Mailing List , Maciej Rutecki , Andrew Morton , Kernel Testers List , Network Development , Linux ACPI , Linux PM List , Linux SCSI List , Linux Wireless List , DRI Subject: Re: 2.6.35-rc6-git6: Reported regressions from 2.6.34 References: <2dWFdr9kTFM.A.P0D.spXVMB@chimera> In-Reply-To: Content-Type: text/plain; charset=ISO-8859-1 Sender: linux-wireless-owner@vger.kernel.org List-ID: Hello, Linus. On 08/01/2010 08:01 PM, Linus Torvalds wrote: > This has a proposed patch. I don't know what the status of it is, though. Jens? > > http://marc.info/?l=linux-kernel&m=127950018204029&w=2 > >> Bug-Entry : http://bugzilla.kernel.org/show_bug.cgi?id=16393 >> Subject : kernel BUG at fs/block_dev.c:765! >> Submitter : Markus Trippelsdorf >> Date : 2010-07-14 13:52 (19 days old) >> Message-ID : <20100714135217.GA1797@arch.tripp.de> >> References : http://marc.info/?l=linux-kernel&m=127911564213748&w=2 > > This one is interesting. And I think I perhaps see where it's coming from. > > bd_start_claiming() (through bd_prepare_to_claim()) has two separate > success cases: either there was no holder (bd_claiming is NULL) or the > new holder was already claiming it (bd_claiming == holder). > > Note in particular the case of the holder _already_ holding it. What happens is: > > - bd_start_claiming() succeeds because we had _already_ claimed it > with the same holder > > - then some error happens, and we call bd_abort_claiming(), which > does whole->bd_claiming = NULL; > > - the original holder thinks it still holds the bd, but it has been released! > > - a new claimer comes in, and succeeds because bd_claiming is now NULL. > > - we now have two "owners" of the bd, but bd_claiming only points to > the second one. > > I think bd_start_claiming() needs to do some kind of refcount for the > nested holder case, and bd_abort_claiming() needs to decrement the > refcount and only clear the bd_claiming field when it goes down to > zero. > > I dunno. Maybe there's something else going on, but it does look > suspicious, and the above would explain the BUG_ON(). Yeah, that definitely sounds plausible. I think the condition check in bd_prepare_to_claim() should have been "if (whole->bd_claiming)" instead of "if (whole->bd_claiming && whole->bd_claiming != holder)". It doesn't make much sense to allow multiple parallel claiming operations anyway and the comment above already says - "This function fails if @bdev is already claimed by another holder and waits if another claiming is in progress." I'll try to build a test case and verify it. Thank you. -- tejun