From mboxrd@z Thu Jan 1 00:00:00 1970 From: Mathieu Desnoyers Subject: Re: Alternative to signals/sys_membarrier() in liburcu Date: Fri, 13 Mar 2015 00:43:58 +0000 (UTC) Message-ID: <1510717200.289089.1426207438871.JavaMail.zimbra@efficios.com> References: <666590480.287502.1426193588471.JavaMail.zimbra@efficios.com> <1243872207.287578.1426193760572.JavaMail.zimbra@efficios.com> <20150312235938.3f7b3245@lxorguk.ukuu.org.uk> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20150312235938.3f7b3245@lxorguk.ukuu.org.uk> Sender: linux-kernel-owner@vger.kernel.org To: One Thousand Gnomes Cc: Michael Sullivan , Peter Zijlstra , LKML , Steven Rostedt , lttng-dev@lists.lttng.org, Thomas Gleixner , "Paul E. McKenney" , Linus Torvalds , Ingo Molnar List-Id: lttng-dev@lists.lttng.org ----- Original Message ----- > From: "One Thousand Gnomes" > To: "Mathieu Desnoyers" > Cc: "Michael Sullivan" , "Peter Zijlstra" , "LKML" > , "Steven Rostedt" , lttng-dev@lists.lttng.org, "Thomas Gleixner" > , "Paul E. McKenney" , "Linus Torvalds" > , "Ingo Molnar" > Sent: Thursday, March 12, 2015 7:59:38 PM > Subject: Re: Alternative to signals/sys_membarrier() in liburcu > > On Thu, 12 Mar 2015 20:56:00 +0000 (UTC) > 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 ? > > What are you going to do if some future ARM or x86 CPU update with > hardware TLB shootdown appears ? All your code will start to fail on new > kernels using that property, and in nasty insidious ways. I'd claim that removing the IPIs breaks userspace, of course. :-P If we start relying on mprotect() implying memory barriers issued on all CPUs associated with the memory mapping in core user-space libraries, then whenever those shiny new CPUs show up, we might be stuck with the IPIs, otherwise we could claim that removing them breaks userspace. I would really hate to tie in an assumption like that on mprotect, because that would really be painting ourselves in a corner. > > Also doesn't sun4d have hardware shootdown for 16 processors or less ? That's possible. I'm no sun expert though. > > I would have thought a membarrier was a lot safer and it can be made to > do whatever horrible things are needed on different processors (indeed it > could even be a pure libc hotpath if some future cpu grows this ability) I'd really prefer a well-documented system call for that purpose too. Thanks, Mathieu > > Alan > -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com