From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933020AbdIRRIA (ORCPT ); Mon, 18 Sep 2017 13:08:00 -0400 Received: from mx0a-001b2d01.pphosted.com ([148.163.156.1]:47002 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932904AbdIRRH6 (ORCPT ); Mon, 18 Sep 2017 13:07:58 -0400 Date: Mon, 18 Sep 2017 10:07:52 -0700 From: "Paul E. McKenney" To: mathieu.desnoyers@efficios.com Cc: linux-kernel@vger.kernel.org, peterz@infradead.org, boqun.feng@gmail.com, ahh@google.com, maged.michael@gmail.com, gromer@google.com, avi@scylladb.com, benh@kernel.crashing.org, paulus@samba.org, mpe@ellerman.id.au, davejwatson@fb.com Subject: sys_membarrier() scheduler barrier requirements: dropped Reply-To: paulmck@linux.vnet.ibm.com MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 17091817-0052-0000-0000-0000026351DC X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00007757; HX=3.00000241; KW=3.00000007; PH=3.00000004; SC=3.00000230; SDB=6.00918877; UDB=6.00461617; IPR=6.00699104; BA=6.00005595; NDR=6.00000001; ZLA=6.00000005; ZF=6.00000009; ZB=6.00000000; ZP=6.00000000; ZH=6.00000000; ZU=6.00000002; MB=3.00017196; XFM=3.00000015; UTC=2017-09-18 17:07:55 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 17091817-0053-0000-0000-0000520C2AF1 Message-Id: <20170918170752.GA12654@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10432:,, definitions=2017-09-18_06:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 spamscore=0 suspectscore=1 malwarescore=0 phishscore=0 adultscore=0 bulkscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1707230000 definitions=main-1709180243 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hello! Commit 3ed668659e95 ("membarrier: Document scheduler barrier requirements") did not make it into the v4.14 merge window, and rebasing to v4.14-rc1 results in conflicts. I have therefore dropped it. If someone would be willing to forward-port it, I would be quite happy to pull it back in for the v4.15 merge window. Or potentially as part of the fix to sys_membarrier() for 4.14, for that matter. Thanx, Paul ------------------------------------------------------------------------ commit 3ed668659e95ecfb6f6be0a3e7ff0fa6d27b2f5c Author: Mathieu Desnoyers Date: Fri Aug 18 21:39:16 2017 -0700 membarrier: Document scheduler barrier requirements 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__before_spinlock() in __schedule(). Document that membarrier requires a full barrier on transition from kernel thread to userspace thread, which skips the call to switch_mm(). 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 load_cr3(). Document that load_cr3() is providing this barrier. [ Rebased on top of linux-rcu for-mingo branch. Applies on top of "membarrier: Provide expedited private command". ] Signed-off-by: Mathieu Desnoyers 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 Signed-off-by: Paul E. McKenney diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c index 014d07a80053..cd815b63420a 100644 --- a/arch/x86/mm/tlb.c +++ b/arch/x86/mm/tlb.c @@ -133,6 +133,9 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct mm_struct *next, * and neither LOCK nor MFENCE orders them. * Fortunately, load_cr3() is serializing and gives the * ordering guarantee we need. + * + * This full barrier is also required by the membarrier + * system call. */ load_cr3(next->pgd); diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h index 2b24a6974847..fe29d06e2800 100644 --- a/include/linux/sched/mm.h +++ b/include/linux/sched/mm.h @@ -38,6 +38,10 @@ 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. + */ if (unlikely(atomic_dec_and_test(&mm->mm_count))) __mmdrop(mm); } diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 3f29c6a89d80..b0f199f9ec62 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -2654,6 +2654,12 @@ 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)) { @@ -3295,6 +3301,9 @@ 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. */ smp_mb__before_spinlock(); rq_lock(rq, &rf);