From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751574Ab1AEQT2 (ORCPT ); Wed, 5 Jan 2011 11:19:28 -0500 Received: from mx1.redhat.com ([209.132.183.28]:34521 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751026Ab1AEQT1 (ORCPT ); Wed, 5 Jan 2011 11:19:27 -0500 Message-ID: <4D249A07.5020507@redhat.com> Date: Wed, 05 Jan 2011 17:19:19 +0100 From: Jerome Marchand User-Agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.9.1.4pre) Gecko/20091014 Fedora/3.0-2.8.b4.fc11 Thunderbird/3.0b4 MIME-Version: 1.0 To: Greg KH CC: Vivek Goyal , Jens Axboe , Satoru Takeuchi , Linus Torvalds , Yasuaki Ishimatsu , "linux-kernel@vger.kernel.org" Subject: Re: [PATCH 2/2] block: fix accounting bug on cross partition merges References: <4D0B68AF.80804@redhat.com> <4D0BB4A1.8080305@fusionio.com> <4D13664C.3020500@redhat.com> <20101223153915.GE9502@redhat.com> <4D13810B.8000304@redhat.com> <20101224192916.GB2082@redhat.com> <4D23423A.60707@redhat.com> <4D2342E1.8010405@redhat.com> <20110104210011.GB4180@kroah.com> <4D247760.9050307@redhat.com> <20110105160034.GE2072@kroah.com> In-Reply-To: <20110105160034.GE2072@kroah.com> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 01/05/2011 05:00 PM, Greg KH wrote: > On Wed, Jan 05, 2011 at 02:51:28PM +0100, Jerome Marchand wrote: >> On 01/04/2011 10:00 PM, Greg KH wrote: >>> On Tue, Jan 04, 2011 at 04:55:13PM +0100, Jerome Marchand wrote: >>>> Also add a refcount to struct hd_struct to keep the partition in >>>> memory as long as users exist. We use kref_test_and_get() to ensure >>>> we don't add a reference to a partition which is going away. >>> >>> No, don't do this, use a kref correctly and no such function should be >>> needed. >>> >>>> + } else { >>>> + part = disk_map_sector_rcu(rq->rq_disk, blk_rq_pos(rq)); >>> >>> That is the function that should properly increment the reference count >>> on the object. >> >> Agreed. >> >>> If the object is "being removed", then it will return >>> NULL and you need to check that. Do that and you do not need to add: >> >> The object is actually removed in a rcu callback function. We could >> certainly add a flag to hd_struct, set by the release function, to >> indicate disk_map_sector_rcu() that the partition is being removed, but >> why not use the refcount instead? > > Because you have to properly serialize the grabbing of a kref if you > don't have a valid pointer in the first place, otherwise it will not > work properly at all. Your new function still does not properly handle > the race condition of dropping the last reference and then having the > kref be cleaned up. You are giving false hope to the user of the api > that what they are doing is correct. > For clarification, is your objection only about not adding that misleading function to kref api (I understand that), or is my code actually racy? thanks, Jerome