From mboxrd@z Thu Jan 1 00:00:00 1970 From: Peter Zijlstra Date: Tue, 02 Jun 2009 07:41:35 +0000 Subject: Re: [PATCH] sched: sched_clock() clocksource handling. Message-Id: <1243928495.23657.5642.camel@twins> List-Id: References: <20090602071718.GA17710@linux-sh.org> <1243927502.23657.5619.camel@twins> <20090602073515.GB17710@linux-sh.org> In-Reply-To: <20090602073515.GB17710@linux-sh.org> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit 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 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. > 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 :-)