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Tue, 4 Aug 2020 10:48:41 -0400 (EDT) Received: from mail03.efficios.com (mail03.efficios.com [167.114.26.124]) by mail.efficios.com (Postfix) with ESMTP id D2D9A2D7A50; Tue, 4 Aug 2020 10:48:41 -0400 (EDT) Date: Tue, 4 Aug 2020 10:48:41 -0400 (EDT) From: Mathieu Desnoyers To: Peter Zijlstra Cc: linux-kernel , Will Deacon , paulmck , Nicholas Piggin , Andy Lutomirski , Thomas Gleixner , Linus Torvalds , Alan Stern , linux-mm Message-ID: <709073430.39864.1596552521779.JavaMail.zimbra@efficios.com> In-Reply-To: <20200804143419.GL2657@hirez.programming.kicks-ass.net> References: <20200728160010.3314-1-mathieu.desnoyers@efficios.com> <20200804143419.GL2657@hirez.programming.kicks-ass.net> Subject: Re: [RFC PATCH 1/2] sched: Fix exit_mm vs membarrier MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit X-Originating-IP: [167.114.26.124] X-Mailer: Zimbra 8.8.15_GA_3955 (ZimbraWebClient - FF79 (Linux)/8.8.15_GA_3953) Thread-Topic: sched: Fix exit_mm vs membarrier Thread-Index: 4wlPtxcjq37vbD29WsZHDlFzoriwyA== X-Rspamd-Queue-Id: CB2FD180442C0 X-Spamd-Result: default: False [0.00 / 100.00] X-Rspamd-Server: rspam05 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: ----- On Aug 4, 2020, at 10:34 AM, Peter Zijlstra peterz@infradead.org wrote: > On Tue, Jul 28, 2020 at 12:00:09PM -0400, Mathieu Desnoyers wrote: >> exit_mm should issue memory barriers after user-space memory accesses, >> before clearing current->mm, to order user-space memory accesses >> performed prior to exit_mm before clearing tsk->mm, which has the >> effect of skipping the membarrier private expedited IPIs. >> >> The membarrier system call can be issued concurrently with do_exit >> if we have thread groups created with CLONE_VM but not CLONE_THREAD. > > I'm still wonder what the exact failure case is though; exit_mm() is on > the exit path (as the name very much implies) and the thread is about to > die. The context switch that follows guarantees a full barrier before we > run anything else again. Here is the scenario I have in mind: Two thread groups are created, A and B. Thread group B is created by issuing clone from group A with flag CLONE_VM set, but not CLONE_THREAD. Let's assume we have a single thread within each thread group (Thread A and Thread B). The AFAIU we can have: Userspace variables: int x = 0, y = 0; CPU 0 CPU 1 Thread A Thread B (in thread group A) (in thread group B) x = 1 barrier() y = 1 exit() exit_mm() current->mm = NULL; r1 = load y membarrier() skips CPU 0 (no IPI) because its current mm is NULL r2 = load x BUG_ON(r1 == 1 && r2 == 0) Thanks, Mathieu -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com