From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752884Ab1AEUrf (ORCPT ); Wed, 5 Jan 2011 15:47:35 -0500 Received: from kroah.org ([198.145.64.141]:38752 "EHLO coco.kroah.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751322Ab1AEUre (ORCPT ); Wed, 5 Jan 2011 15:47:34 -0500 Date: Wed, 5 Jan 2011 12:08:25 -0800 From: Greg KH To: Jens Axboe Cc: Jerome Marchand , Vivek Goyal , Satoru Takeuchi , Linus Torvalds , Yasuaki Ishimatsu , "linux-kernel@vger.kernel.org" Subject: Re: [PATCH 2/2] block: fix accounting bug on cross partition merges Message-ID: <20110105200825.GA11372@kroah.com> References: <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> <4D247867.9040502@fusionio.com> <20110105155816.GD2072@kroah.com> <4D24BC88.4090804@fusionio.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <4D24BC88.4090804@fusionio.com> User-Agent: Mutt/1.5.20 (2009-06-14) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jan 05, 2011 at 07:46:32PM +0100, Jens Axboe wrote: > On 2011-01-05 16:58, Greg KH wrote: > > On Wed, Jan 05, 2011 at 02:55:51PM +0100, Jens Axboe wrote: > >> On 2011-01-04 22:00, 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. 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: > >> > >> It doesn't matter if you do it in there of after the fact, since the > >> "lock" (RCU) is being held across the call. See my original suggestion > >> here: > >> > >> https://lkml.org/lkml/2010/12/17/275 > > > > Ok, that's fine, just do it without adding that kref function and I have > > no objection :) > > Why? The code is perfectly fine. I originally objected to making an API > like this for simple reference counting - seems I was right. Please > actually look at the code and use. Alexey asked whether this was a toy > API or a real one, I'd like to know that as well. If this is meant just > for very basic get/put references, fine, then document that. But then > what's the point of having this API in the first place? The point is that you shouldn't have to roll your own reference count code all over the place, 99% of the time, you should just use the debugged, and documented, interface that the kernel provides with the kref interface. As for it being a "toy", it properly handles a very large majority of the kernel reference counting logic today, in a race-free manner, so I would not call that a "toy" at all. Just use it properly. As this patch series points out, adding this type of function to the api is not a good idea, as it will be incorrect when used. thanks, greg k-h