From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Srivatsa S. Bhat" Subject: Re: [PATCH] VFS: br_write_lock locks on possible CPUs other than online CPUs Date: Tue, 20 Dec 2011 20:05:32 +0530 Message-ID: <4EF09D34.1060607@linux.vnet.ibm.com> References: <20111219121100.GI2203@ZenIV.linux.org.uk> <4EEF9D4E.1000008@linux.vnet.ibm.com> <20111219205251.GK2203@ZenIV.linux.org.uk> <4EF01565.2000700@linux.vnet.ibm.com> <20111220062710.GC23916@ZenIV.linux.org.uk> <4EF03915.60902@linux.vnet.ibm.com> <1324373854.21588.16.camel@mengcong> <4EF0654B.4060904@linux.vnet.ibm.com> <4EF06C9B.4010703@linux.vnet.ibm.com> <4EF084A4.3000106@linux.vnet.ibm.com> <20111220140628.GD23916@ZenIV.linux.org.uk> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Cc: mc@linux.vnet.ibm.com, Stephen Boyd , linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, Nick Piggin , david@fromorbit.com, "akpm@linux-foundation.org" , Maciej Rutecki To: Al Viro Return-path: Received: from e28smtp09.in.ibm.com ([122.248.162.9]:56000 "EHLO e28smtp09.in.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752121Ab1LTOjV (ORCPT ); Tue, 20 Dec 2011 09:39:21 -0500 Received: from /spool/local by e28smtp09.in.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Tue, 20 Dec 2011 20:09:18 +0530 In-Reply-To: <20111220140628.GD23916@ZenIV.linux.org.uk> Sender: linux-fsdevel-owner@vger.kernel.org List-ID: On 12/20/2011 07:36 PM, Al Viro wrote: > On Tue, Dec 20, 2011 at 06:20:44PM +0530, Srivatsa S. Bhat wrote: > >>> Hey, actually there is a simple solution: just nip it (or rather delay it) >>> in the bud ;) That is, we watch out for CPU_UP_PREPARE event and lock it >>> up there itself using our spinlock.. that way, that cpu will not come up >>> until we are done executing br_write_unlock(). In fact, we can even fail >>> the onlining of that cpu by returning appropriate value from our callback, >>> but that would be too harsh.. so we can settle for delaying the cpu online > > Eeeek... Are you serious about leaving a spinlock grabbed by notifier > callback and not released until another callback call? That would be one > hell of a constraint on what these notifiers can do - _nothing_ between > these calls (including other notifier callbacks, etc.) would be allowed > to block. > > That sounds extremely brittle... > Sorry but I didn't quite get your point... No two cpu hotplug operations can race because of the cpu_hotplug lock they use. Hence, if a cpu online operation begins, it has to succeed or fail eventually. No other cpu hotplug operation can intervene. Ditto for cpu offline operations. Hence a CPU_UP_PREPARE event *will* be followed by a corresponding CPU_UP_CANCELED or CPU_ONLINE event for the same cpu. (And we ignore the CPU_STARTING event that comes in between, on purpose, so as to avoid the race with cpu_online_mask). Similar is the story for offline operation. And if the notifier grabs the spinlock and keeps it locked between these 2 points of a cpu hotplug operation, it ensures that our br locks will spin, instead of block till the cpu hotplug operation is complete. Isn't this what we desired all along? "A non-blocking way to sync br locks with cpu hotplug"? Or am I missing something? Regards, Srivatsa S. Bhat