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Tue, 04 Aug 2020 16:51:21 +0000 Received: from hirez.programming.kicks-ass.net (hirez.programming.kicks-ass.net [192.168.1.225]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (Client did not present a certificate) by noisy.programming.kicks-ass.net (Postfix) with ESMTPS id 6ED96301E02; Tue, 4 Aug 2020 18:51:18 +0200 (CEST) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 533302B90EB46; Tue, 4 Aug 2020 18:51:18 +0200 (CEST) Date: Tue, 4 Aug 2020 18:51:18 +0200 From: peterz@infradead.org To: Mathieu Desnoyers Cc: linux-kernel , Will Deacon , paulmck , Nicholas Piggin , Andy Lutomirski , Thomas Gleixner , Linus Torvalds , Alan Stern , linux-mm Subject: Re: [RFC PATCH 1/2] sched: Fix exit_mm vs membarrier Message-ID: <20200804165118.GN2657@hirez.programming.kicks-ass.net> References: <20200728160010.3314-1-mathieu.desnoyers@efficios.com> <20200804143419.GL2657@hirez.programming.kicks-ass.net> <709073430.39864.1596552521779.JavaMail.zimbra@efficios.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <709073430.39864.1596552521779.JavaMail.zimbra@efficios.com> X-Rspamd-Queue-Id: 9D6321819E769 X-Spamd-Result: default: False [0.00 / 100.00] X-Rspamd-Server: rspam04 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 Tue, Aug 04, 2020 at 10:48:41AM -0400, Mathieu Desnoyers wrote: > Here is the scenario I have in mind: > 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) > Ah, yes of course. We really should have a bunch of these scenarios in membarrier.c. Now, the above cannot happen because we have an unconditional atomic_dec_and_test() in do_exit() before exit_mm(), but I'm sure relying on that is a wee bit dodgy.