From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Paul E. McKenney" Subject: Re: Alternative to signals/sys_membarrier() in liburcu Date: Thu, 12 Mar 2015 14:12:58 -0700 Message-ID: <20150312211258.GX5412@linux.vnet.ibm.com> References: <666590480.287502.1426193588471.JavaMail.zimbra@efficios.com> <1243872207.287578.1426193760572.JavaMail.zimbra@efficios.com> Reply-To: paulmck@linux.vnet.ibm.com Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Received: from e31.co.us.ibm.com ([32.97.110.149]:43055 "EHLO e31.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754855AbbCLVND (ORCPT ); Thu, 12 Mar 2015 17:13:03 -0400 Received: from /spool/local by e31.co.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Thu, 12 Mar 2015 15:13:03 -0600 Content-Disposition: inline In-Reply-To: <1243872207.287578.1426193760572.JavaMail.zimbra@efficios.com> Sender: linux-arch-owner@vger.kernel.org List-ID: To: Mathieu Desnoyers Cc: Michael Sullivan , Peter Zijlstra , LKML , Steven Rostedt , lttng-dev@lists.lttng.org, Thomas Gleixner , Linus Torvalds , Ingo Molnar , linux-arch@vger.kernel.org On Thu, Mar 12, 2015 at 08:56:00PM +0000, Mathieu Desnoyers wrote: > (sorry for re-send, my mail client tricked me into posting HTML > to lkml) > > Hi, > > Michael Sullivan proposed a clever hack abusing mprotect() to > perform the same effect as sys_membarrier() I submitted a few > years ago ( https://lkml.org/lkml/2010/4/18/15 ). > > At that time, the sys_membarrier implementation was deemed > technically sound, but there were not enough users of the system call > to justify its inclusion. > > So far, the number of users of liburcu has increased, but liburcu > still appears to be the only direct user of sys_membarrier. On this > front, we could argue that many other system calls have only > one user: glibc. In that respect, liburcu is quite similar to glibc. > > So the question as it stands appears to be: would you be comfortable > having users abuse mprotect(), relying on its side-effect of issuing > a smp_mb() on each targeted CPU for the TLB shootdown, as > an effective implementation of process-wide memory barrier ? > > Thoughts ? Are there any architectures left that use hardware-assisted global TLB invalidation? On such an architecture, you might not get a memory barrier except on the CPU executing the mprotect() or munmap(). (Here is hoping that no one does -- it is a cute abuse^Whack otherwise!) Thanx, Paul > Thanks! > > Mathieu > > > > > > From: "Michael Sullivan" > To: "Mathieu Desnoyers" > Cc: lttng-dev@lists.lttng.org > Sent: Thursday, March 12, 2015 12:04:07 PM > Subject: Re: [lttng-dev] Alternative to signals/sys_membarrier() in liburcu > > On Thu, Mar 12, 2015 at 10:57 AM, Mathieu Desnoyers < mathieu.desnoyers@efficios.com > wrote: > > > > > Even though it depends on internal behavior not currently specified by mprotect, > I'd very much like to see the prototype you have, > > > I ended up posting my code at https://github.com/msullivan/userspace-rcu/tree/msync-barrier . > The interesting patch is https://github.com/msullivan/userspace-rcu/commit/04656b468d418efbc5d934ab07954eb8395a7ab0 . > > Quick blog post I wrote about it at http://www.msully.net/blog/2015/02/24/forcing-memory-barriers-on-other-cpus-with-mprotect2/ . > (I talked briefly about sys_membarrier in the post as best as I could piece together from LKML; if my comment on it is inaccurate I can edit the post.) > > -Michael Sullivan > > > > -- > Mathieu Desnoyers > EfficiOS Inc. > http://www.efficios.com > > _______________________________________________ > lttng-dev mailing list > lttng-dev@lists.lttng.org > http://lists.lttng.org/cgi-bin/mailman/listinfo/lttng-dev >