Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1760111AbZFBIAa (ORCPT ); Tue, 2 Jun 2009 04:00:30 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1759600AbZFBIAX (ORCPT ); Tue, 2 Jun 2009 04:00:23 -0400 Received: from bombadil.infradead.org ([18.85.46.34]:53751 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757901AbZFBIAW (ORCPT ); Tue, 2 Jun 2009 04:00:22 -0400 Subject: Re: [PATCH] sched: sched_clock() clocksource handling. From: Peter Zijlstra To: Paul Mundt Cc: Ingo Molnar , Thomas Gleixner , Daniel Walker , Linus Walleij , Andrew Victor , Haavard Skinnemoen , Andrew Morton , John Stultz , linux-arm-kernel@lists.arm.linux.org.uk, linux-sh@vger.kernel.org, linux-kernel@vger.kernel.org In-Reply-To: <20090602075409.GA19294@linux-sh.org> References: <20090602071718.GA17710@linux-sh.org> <1243927502.23657.5619.camel@twins> <20090602073515.GB17710@linux-sh.org> <1243928495.23657.5642.camel@twins> <20090602075409.GA19294@linux-sh.org> Content-Type: text/plain Content-Transfer-Encoding: 7bit Date: Tue, 02 Jun 2009 10:00:03 +0200 Message-Id: <1243929603.23657.5662.camel@twins> Mime-Version: 1.0 X-Mailer: Evolution 2.26.1 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1984 Lines: 45 On Tue, 2009-06-02 at 16:54 +0900, Paul Mundt wrote: > On Tue, Jun 02, 2009 at 09:41:35AM +0200, Peter Zijlstra wrote: > > On Tue, 2009-06-02 at 16:35 +0900, Paul Mundt wrote: > > > > > > We already do via select_clocksource(), if we are unregistering the > > > current one then a new one with the flag set is selected. Before that, > > > the override is likewise given preference, and we fall back on jiffies if > > > there is nothing else. I suppose we could try and find the "best" one, > > > but I think the override and manual clocksource selection should be fine > > > for this. > > > > Ah, ok. So unregister calls select_clocksource again? That does leave us > > a small window with jiffies, but I guess that's ok. > > > A synchronize_rcu() would fix that up, but I think a small window with > jiffies is less painful than sorting out RCU ordering and synchronization > for a corner case of a corner case ;-) > > > > Now that you mention it though, the sched_clocksource() assignment within > > > select_clocksource() happens underneath the clocksource_lock, but is not > > > using rcu_assign_pointer(). > > > > Right, that would want fixing indeed. > > > > > If the assignment there needs to use > > > rcu_assign_pointer() then presumably all of the unlock paths that do > > > select_clocksource() will have to synchronize_rcu()? > > > > No, you only have to do sync_rcu() when stuff that could have referenced > > is going away and you cannot use call_rcu(). > > > > So when selecting a new clocksource, you don't need synchonization > > because stuff doesn't go away (I think :-) > > Ok, that keeps things more simplified then. How does this look? Looks fine to me, Acked-by: Peter Zijlstra -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/