From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754370Ab1ATUVP (ORCPT ); Thu, 20 Jan 2011 15:21:15 -0500 Received: from cantor.suse.de ([195.135.220.2]:47351 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753411Ab1ATUVO (ORCPT ); Thu, 20 Jan 2011 15:21:14 -0500 Date: Thu, 20 Jan 2011 21:21:12 +0100 From: Jan Kara To: Nick Piggin Cc: Jan Kara , Jeff Moyer , Andrew Morton , linux-fsdevel , linux-kernel@vger.kernel.org Subject: Re: [patch] fs: aio fix rcu lookup Message-ID: <20110120202111.GB19797@quack.suse.cz> References: <20110118190114.GA5070@quack.suse.cz> <20110118235236.GA14087@quack.suse.cz> <20110119132123.GC4246@quack.suse.cz> <20110119165011.GA16682@quack.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: 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 Thu 20-01-11 04:37:55, Nick Piggin wrote: > On Thu, Jan 20, 2011 at 3:50 AM, Jan Kara wrote: > > On Thu 20-01-11 03:03:23, Nick Piggin wrote: > >> On Thu, Jan 20, 2011 at 12:21 AM, Jan Kara wrote: > >> >  Well, we are not required to cancel all the outstanding AIO because of the > >> > API requirement, that's granted. But we must do it because of the way how > >> > the code is written. Outstanding IO requests reference ioctx but they are > >> > not counted in ctx->users but in ctx->reqs_active. So the code relies on > >> > the fact that the reference held by the hash table protects ctx from being > >> > freed and io_destroy() waits for requests before dropping the last > >> > reference to ctx. But there's the second race I describe making it possible > >> > for new IO to be created after io_destroy() has waited for all IO to > >> > finish... > >> > >> Yes there is that race too I agree. I just didn't follow through the code far > >> enough to see it was a problem -- I thought it was by design. > >> > >> I'd like to solve it without synchronize_rcu() though. > >  Ah, OK. I don't find io_destroy() performance critical but I can > > Probably not performance critical, but it could be a very > large slowdown so somebody might complain. > > > understand that you need not like synchronize_rcu() there. ;) Then it > > should be possible to make IO requests count in ctx->users which would > > solve the race as well. We'd just have to be prepared that request > > completion might put the last reference to ioctx and free it but that > > shouldn't be an issue. Do you like that solution better? > > I think so, if it can be done without slowing things down > and adding locks or atomics if possible. Actually, I found that freeing ioctx upon IO completion isn't straightforward because freeing ioctx may need to sleep (it is destroying work queue) and aio_complete() can be called from an interrupt context. We could offload the sleeping work to the RCU callback (basically we'd have to offload the whole __put_ioctx() to RCU callback) but I'm not convinced it's worth it so I rather chose a bit more subtle approach for fixing the race (see my patch). Honza -- Jan Kara SUSE Labs, CR