From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754855AbbHCRcO (ORCPT ); Mon, 3 Aug 2015 13:32:14 -0400 Received: from mx1.redhat.com ([209.132.183.28]:52251 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754617AbbHCRcM (ORCPT ); Mon, 3 Aug 2015 13:32:12 -0400 Date: Mon, 3 Aug 2015 19:30:07 +0200 From: Oleg Nesterov To: Jan Kara Cc: Al Viro , Dave Chinner , Dave Hansen , "Paul E. McKenney" , Peter Zijlstra , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 4/4] change sb_writers to use percpu_rw_semaphore Message-ID: <20150803173007.GA19627@redhat.com> References: <20150722211513.GA19986@redhat.com> <20150722211541.GA20017@redhat.com> <20150728083401.GA24030@quack.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150728083401.GA24030@quack.suse.cz> User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Jan, Thanks for your review and sorry for delay, I was on vacation. On 07/28, Jan Kara wrote: > > On Wed 22-07-15 23:15:41, Oleg Nesterov wrote: > > > > Perhaps we should also cleanup the usage of ->frozen. It would be > > better to set/clear (say) SB_FREEZE_WRITE with the corresponding > > write-lock held. Currently freeze_super() has to set SB_FREEZE_WRITE > > before sb_wait_write(SB_FREEZE_WRITE) to avoid the race with itself, > > we can add another state. The "From now on, no new normal writers > > can start" removed by this patch was not really correct. > > The patch looks good, just one question: Why wasn't the above comment > really correct? It is not that I think it was wrong, just not 100% accurate even before this change. "w_writers.frozen = SB_FREEZE_WRITE" itself can't guarantee that "no new normal writers can start". We do not know when other CPU's will see the result of this STORE. > Do you mean it wouldn't be correct after your changes? I > agree with that. Yes, yes, this was the actual reason to remove this comment. Sorry for confusion. > Also when you'd like to "cleanup the usage of ->frozen", you have to be > careful no only about races with freeze_super() itself but also about races > with remount (that's one of the reasons why we use s_umount for protecting > modifications of ->frozen). So I'm not sure how much we can actually > improve on code readability... Yes, me too. Probably I should simply remove this (confusing) part of the changelog. > Anyway, you can add: > > Reviewed-by: Jan Kara Thanks! OK. Now I'll try to actually test this all. Hopefully this week. Oleg.