From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751745AbdISWyI (ORCPT ); Tue, 19 Sep 2017 18:54:08 -0400 Received: from mx0b-001b2d01.pphosted.com ([148.163.158.5]:45445 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1751361AbdISWyG (ORCPT ); Tue, 19 Sep 2017 18:54:06 -0400 Date: Tue, 19 Sep 2017 15:53:59 -0700 From: "Paul E. McKenney" To: Mathieu Desnoyers Cc: linux-kernel@vger.kernel.org, Peter Zijlstra , 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 Reply-To: paulmck@linux.vnet.ibm.com 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: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 17091922-0024-0000-0000-000002D4D8B2 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00007764; HX=3.00000241; KW=3.00000007; PH=3.00000004; SC=3.00000230; SDB=6.00919472; UDB=6.00461940; IPR=6.00699698; BA=6.00005598; NDR=6.00000001; ZLA=6.00000005; ZF=6.00000009; ZB=6.00000000; ZP=6.00000000; ZH=6.00000000; ZU=6.00000002; MB=3.00017215; XFM=3.00000015; UTC=2017-09-19 22:54:03 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 17091922-0025-0000-0000-0000457AB9D4 Message-Id: <20170919225359.GM3521@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10432:,, definitions=2017-09-19_09:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 spamscore=0 suspectscore=0 malwarescore=0 phishscore=0 adultscore=0 bulkscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1707230000 definitions=main-1709190313 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: > Document the membarrier requirement on having a full memory barrier in > __schedule() after coming from user-space, before storing to rq->curr. > It is provided by smp_mb__after_spinlock() in __schedule(). > > Document that membarrier requires a full barrier on transition from > kernel thread to userspace thread. We currently have an implicit barrier > from atomic_dec_and_test() in mmdrop() that ensures this. > > The x86 switch_mm_irqs_off() full barrier is currently provided by many > cpumask update operations as well as write_cr3(). Document that > write_cr3() provides this barrier. > > Changes since v1: > - Update comments to match reality for code paths which are after > storing to rq->curr, before returning to user-space. > Changes since v2: > - Update changelog (smp_mb__before_spinlock -> smp_mb__after_spinlock). > > Signed-off-by: Mathieu Desnoyers I replaced the version that I had queued with this one, thank you! Thanx, Paul > CC: Peter Zijlstra > CC: Paul E. McKenney > CC: Boqun Feng > CC: Andrew Hunter > CC: Maged Michael > CC: gromer@google.com > CC: Avi Kivity > CC: Benjamin Herrenschmidt > CC: Paul Mackerras > CC: Michael Ellerman > CC: Dave Watson > --- > arch/x86/mm/tlb.c | 5 +++++ > include/linux/sched/mm.h | 5 +++++ > kernel/sched/core.c | 9 +++++++++ > 3 files changed, 19 insertions(+) > > 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)) { > @@ -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(); > -- > 2.11.0 >