From mboxrd@z Thu Jan 1 00:00:00 1970 From: Peter Zijlstra Subject: Re: [PATCH 08/11] membarrier: Provide core serializing command (v2) Date: Mon, 29 Jan 2018 19:04:14 +0100 Message-ID: <20180129180414.GO2249@hirez.programming.kicks-ass.net> References: <20180123155733.3404-1-mathieu.desnoyers@efficios.com> <20180123155733.3404-9-mathieu.desnoyers@efficios.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <20180123155733.3404-9-mathieu.desnoyers@efficios.com> Sender: linux-kernel-owner@vger.kernel.org To: Mathieu Desnoyers Cc: Ingo Molnar , Thomas Gleixner , linux-kernel@vger.kernel.org, linux-api@vger.kernel.org, Andy Lutomirski , "Paul E . McKenney" , Boqun Feng , Andrew Hunter , Maged Michael , Avi Kivity , Benjamin Herrenschmidt , Paul Mackerras , Michael Ellerman , Dave Watson , "H . Peter Anvin" , Andrea Parri , Russell King , Greg Hackmann , Will Deacon , David Sehr Linus Torvalds List-Id: linux-arch.vger.kernel.org On Tue, Jan 23, 2018 at 10:57:30AM -0500, Mathieu Desnoyers wrote: > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > index f38c4c7e256a..041893128f51 100644 > --- a/kernel/sched/core.c > +++ b/kernel/sched/core.c > @@ -2662,9 +2662,13 @@ static struct rq *finish_task_switch(struct task_struct *prev) > * 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(). > + * membarrier also requires a core serializing instruction > + * before going back to user-space after storing to rq->curr. > */ > - if (mm) > + if (mm) { > + membarrier_mm_sync_core_before_usermode(mm); > mmdrop(mm); > + } *confused*, when we switch from process A to process B, context_switch() will not set rq->prev_mm and the above mm will be NULL and we'll not pass through your_function_names_are_waaay_too_long and we'll not get cookies. And if there's anything more complicated going on, the comment/changelog are not adequate. > if (unlikely(prev_state == TASK_DEAD)) { > if (prev->sched_class->task_dead) > prev->sched_class->task_dead(prev); From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from merlin.infradead.org ([205.233.59.134]:58708 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751072AbeA2SE6 (ORCPT ); Mon, 29 Jan 2018 13:04:58 -0500 Date: Mon, 29 Jan 2018 19:04:14 +0100 From: Peter Zijlstra Subject: Re: [PATCH 08/11] membarrier: Provide core serializing command (v2) Message-ID: <20180129180414.GO2249@hirez.programming.kicks-ass.net> References: <20180123155733.3404-1-mathieu.desnoyers@efficios.com> <20180123155733.3404-9-mathieu.desnoyers@efficios.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180123155733.3404-9-mathieu.desnoyers@efficios.com> Sender: linux-arch-owner@vger.kernel.org List-ID: To: Mathieu Desnoyers Cc: Ingo Molnar , Thomas Gleixner , linux-kernel@vger.kernel.org, linux-api@vger.kernel.org, Andy Lutomirski , "Paul E . McKenney" , Boqun Feng , Andrew Hunter , Maged Michael , Avi Kivity , Benjamin Herrenschmidt , Paul Mackerras , Michael Ellerman , Dave Watson , "H . Peter Anvin" , Andrea Parri , Russell King , Greg Hackmann , Will Deacon , David Sehr , Linus Torvalds , x86@kernel.org, linux-arch@vger.kernel.org Message-ID: <20180129180414.pbQWHKodNStvJCiQVx5UZqRqNBZTdl1n27hquWW4kSk@z> On Tue, Jan 23, 2018 at 10:57:30AM -0500, Mathieu Desnoyers wrote: > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > index f38c4c7e256a..041893128f51 100644 > --- a/kernel/sched/core.c > +++ b/kernel/sched/core.c > @@ -2662,9 +2662,13 @@ static struct rq *finish_task_switch(struct task_struct *prev) > * 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(). > + * membarrier also requires a core serializing instruction > + * before going back to user-space after storing to rq->curr. > */ > - if (mm) > + if (mm) { > + membarrier_mm_sync_core_before_usermode(mm); > mmdrop(mm); > + } *confused*, when we switch from process A to process B, context_switch() will not set rq->prev_mm and the above mm will be NULL and we'll not pass through your_function_names_are_waaay_too_long and we'll not get cookies. And if there's anything more complicated going on, the comment/changelog are not adequate. > if (unlikely(prev_state == TASK_DEAD)) { > if (prev->sched_class->task_dead) > prev->sched_class->task_dead(prev);