From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751640AbdIUMZM (ORCPT ); Thu, 21 Sep 2017 08:25:12 -0400 Received: from merlin.infradead.org ([205.233.59.134]:35522 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751387AbdIUMZL (ORCPT ); Thu, 21 Sep 2017 08:25:11 -0400 Date: Thu, 21 Sep 2017 14:25:00 +0200 From: Peter Zijlstra To: Mathieu Desnoyers Cc: "Paul E . McKenney" , linux-kernel@vger.kernel.org, Boqun Feng , Andrew Hunter , Maged Michael , gromer@google.com, Avi Kivity , Benjamin Herrenschmidt , Paul Mackerras , Michael Ellerman , Dave Watson Subject: Re: [PATCH v3] membarrier: Document scheduler barrier requirements Message-ID: <20170921122500.mai63v5lqe4hnsff@hirez.programming.kicks-ass.net> References: <20170919220205.19681-1-mathieu.desnoyers@efficios.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170919220205.19681-1-mathieu.desnoyers@efficios.com> User-Agent: NeoMutt/20170609 (1.8.3) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Sep 19, 2017 at 06:02:05PM -0400, Mathieu Desnoyers wrote: > diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c > index 1ab3821f9e26..74f94fe4aded 100644 > --- a/arch/x86/mm/tlb.c > +++ b/arch/x86/mm/tlb.c > @@ -144,6 +144,11 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct mm_struct *next, > } > #endif > > + /* > + * The membarrier system call requires a full memory barrier > + * before returning to user-space, after storing to rq->curr. > + * Writing to CR3 provides that full memory barrier. > + */ > if (real_prev == next) { > VM_BUG_ON(this_cpu_read(cpu_tlbstate.ctxs[prev_asid].ctx_id) != > next->context.ctx_id); > diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h > index 3a19c253bdb1..766cc47c4d7c 100644 > --- a/include/linux/sched/mm.h > +++ b/include/linux/sched/mm.h > @@ -38,6 +38,11 @@ static inline void mmgrab(struct mm_struct *mm) > extern void __mmdrop(struct mm_struct *); > static inline void mmdrop(struct mm_struct *mm) > { > + /* > + * The implicit full barrier implied by atomic_dec_and_test is > + * required by the membarrier system call before returning to > + * user-space, after storing to rq->curr. > + */ > if (unlikely(atomic_dec_and_test(&mm->mm_count))) > __mmdrop(mm); > } > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > index 18a6966567da..7977b25acf54 100644 > --- a/kernel/sched/core.c > +++ b/kernel/sched/core.c > @@ -2658,6 +2658,12 @@ static struct rq *finish_task_switch(struct task_struct *prev) > finish_arch_post_lock_switch(); > > fire_sched_in_preempt_notifiers(current); > + /* > + * When transitioning from a kernel thread to a userspace > + * thread, mmdrop()'s implicit full barrier is required by the > + * membarrier system call, because the current active_mm can > + * become the current mm without going through switch_mm(). > + */ > if (mm) > mmdrop(mm); > if (unlikely(prev_state == TASK_DEAD)) { I would also put a comment in context_switch() that explains we either pass through switch_mm() or do mmdrop(). And I think that for the weak archs that don't have native RELEASE we actually rely on rq_unlock() for the smp_mb(). So there's 4 schemes: - switch_mm()/mmdrop() (x86,s390, sparc?) - finish_lock_switch() (weak, !release) - switch_to (arm64) - member arch hook (ppc) And I don't think that's spelled out clearly enough. > @@ -3299,6 +3305,9 @@ static void __sched notrace __schedule(bool preempt) > * Make sure that signal_pending_state()->signal_pending() below > * can't be reordered with __set_current_state(TASK_INTERRUPTIBLE) > * done by the caller to avoid the race with signal_wake_up(). > + * > + * The membarrier system call requires a full memory barrier > + * after coming from user-space, before storing to rq->curr. > */ > rq_lock(rq, &rf); > smp_mb__after_spinlock(); Right, this is the only part that's actually trivial :-)