From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753847Ab0AJRpO (ORCPT ); Sun, 10 Jan 2010 12:45:14 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1753663Ab0AJRpM (ORCPT ); Sun, 10 Jan 2010 12:45:12 -0500 Received: from e6.ny.us.ibm.com ([32.97.182.146]:39197 "EHLO e6.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753460Ab0AJRpL (ORCPT ); Sun, 10 Jan 2010 12:45:11 -0500 Date: Sun, 10 Jan 2010 09:45:12 -0800 From: "Paul E. McKenney" To: Steven Rostedt Cc: Mathieu Desnoyers , Oleg Nesterov , Peter Zijlstra , linux-kernel@vger.kernel.org, Ingo Molnar , akpm@linux-foundation.org, josh@joshtriplett.org, tglx@linutronix.de, Valdis.Kletnieks@vt.edu, dhowells@redhat.com, laijs@cn.fujitsu.com, dipankar@in.ibm.com Subject: Re: [RFC PATCH] introduce sys_membarrier(): process-wide memory barrier Message-ID: <20100110174512.GH9044@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20100109054215.GB9044@linux.vnet.ibm.com> <20100109192006.GA23672@Krystal> <1263078327.28171.3792.camel@gandalf.stny.rr.com> <1263079000.28171.3795.camel@gandalf.stny.rr.com> <20100110000318.GD9044@linux.vnet.ibm.com> <1263084099.2231.5.camel@frodo> <20100110014456.GG25790@Krystal> <1263089578.2231.22.camel@frodo> <20100110052508.GG9044@linux.vnet.ibm.com> <1263124209.28171.3798.camel@gandalf.stny.rr.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1263124209.28171.3798.camel@gandalf.stny.rr.com> User-Agent: Mutt/1.5.15+20070412 (2007-04-11) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun, Jan 10, 2010 at 06:50:09AM -0500, Steven Rostedt wrote: > On Sat, 2010-01-09 at 21:25 -0800, Paul E. McKenney wrote: > > On Sat, Jan 09, 2010 at 09:12:58PM -0500, Steven Rostedt wrote: > > > > > < user space > > > > > > > > > < misses that CPU 2 is in rcu section > > > > > > > > > > If the TLB flush misses that CPU 2 has a threaded task, and does not > > > flush CPU 2s TLB, it can also risk the same type of crash. > > > > But isn't the VM's locking helping us out in that case? > > > > > > [CPU 2's ->curr update now visible] > > > > > > > > [CPU 2's rcu_read_lock() store now visible] > > > > > > > > free(obj); > > > > > > > > use_object(obj); <=== crash! > > > > > > > > > > Think about it. If you change a process mmap, say you updated a mmap of > > > a file by flushing out one page and replacing it with another. If the > > > above missed sending to CPU 2, then CPU 2 may still be accessing the old > > > page of the file, and not the new one. > > > > > > I think this may be the safe bet. > > > > You might well be correct that we can access that bitmap locklessly, > > but there are additional things (like the loading of the arch-specific > > page-table register) that are likely to be helping in the VM case, but > > not necessarily helping in this case. > > Then perhaps the sys_membarrier() should just do a flush_tlb()? That > should guarantee the synchronization, right? Isn't just grabbing the runqueue locks a bit more straightforward and more clearly correct? Again, this is the URCU slowpath, so it is hard to get too excited about a 15% performance penalty. Thanx, Paul