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Wed, 07 Oct 2020 15:09:10 +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 D84B23019CE; Wed, 7 Oct 2020 17:09:08 +0200 (CEST) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id C57F929C75E31; Wed, 7 Oct 2020 17:09:08 +0200 (CEST) Date: Wed, 7 Oct 2020 17:09:08 +0200 From: Peter Zijlstra To: Mathieu Desnoyers Cc: Boqun Feng , linux-kernel , Will Deacon , paulmck , Nicholas Piggin , Andy Lutomirski , Thomas Gleixner , Linus Torvalds , Alan Stern , linux-mm Subject: Re: [RFC PATCH 1/3] sched: fix exit_mm vs membarrier (v3) Message-ID: <20201007150908.GI2628@hirez.programming.kicks-ass.net> References: <20200924172508.8724-1-mathieu.desnoyers@efficios.com> <20200924172508.8724-2-mathieu.desnoyers@efficios.com> <20201007142947.GG2628@hirez.programming.kicks-ass.net> <1929468067.11124.1602082672412.JavaMail.zimbra@efficios.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1929468067.11124.1602082672412.JavaMail.zimbra@efficios.com> 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 Wed, Oct 07, 2020 at 10:57:52AM -0400, Mathieu Desnoyers wrote: > ----- On Oct 7, 2020, at 10:29 AM, Peter Zijlstra peterz@infradead.org wrote: > > > On Thu, Sep 24, 2020 at 01:25:06PM -0400, Mathieu Desnoyers wrote: > >> diff --git a/kernel/exit.c b/kernel/exit.c > >> index 733e80f334e7..0767a2dbf245 100644 > >> --- a/kernel/exit.c > >> +++ b/kernel/exit.c > >> @@ -475,7 +475,19 @@ static void exit_mm(void) > >> BUG_ON(mm != current->active_mm); > >> /* more a memory barrier than a real lock */ > >> task_lock(current); > >> + /* > >> + * When a thread stops operating on an address space, the loop > >> + * in membarrier_private_expedited() may not observe that > >> + * tsk->mm, and the loop in membarrier_global_expedited() may > >> + * not observe a MEMBARRIER_STATE_GLOBAL_EXPEDITED > >> + * rq->membarrier_state, so those would not issue an IPI. > >> + * Membarrier requires a memory barrier after accessing > >> + * user-space memory, before clearing tsk->mm or the > >> + * rq->membarrier_state. > >> + */ > >> + smp_mb__after_spinlock(); > >> current->mm = NULL; > >> + membarrier_update_current_mm(NULL); > >> mmap_read_unlock(mm); > >> enter_lazy_tlb(mm, current); > >> task_unlock(current); > > > > This site seems to be lacking in IRQ disabling. As proposed it will > > explode on RT. > > Right, so irq off is needed for accessing this_rq()'s fields safely, > correct ? Yes, but also we're having IRQs disabled on ever other site that mucks with ->mm these days.