Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S969456AbdIZRwv (ORCPT ); Tue, 26 Sep 2017 13:52:51 -0400 Received: from mail.efficios.com ([167.114.142.141]:46487 "EHLO mail.efficios.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S969396AbdIZRwV (ORCPT ); Tue, 26 Sep 2017 13:52:21 -0400 From: Mathieu Desnoyers To: "Paul E . McKenney" , Peter Zijlstra Cc: linux-kernel@vger.kernel.org, Mathieu Desnoyers , Boqun Feng , Andrew Hunter , Maged Michael , gromer@google.com, Avi Kivity , Benjamin Herrenschmidt , Paul Mackerras , Michael Ellerman , Dave Watson Subject: [PATCH v4 for 4.14 3/3] membarrier: Document scheduler barrier requirements Date: Tue, 26 Sep 2017 13:51:51 -0400 Message-Id: <20170926175151.14264-3-mathieu.desnoyers@efficios.com> X-Mailer: git-send-email 2.11.0 In-Reply-To: <20170926175151.14264-1-mathieu.desnoyers@efficios.com> References: <20170926175151.14264-1-mathieu.desnoyers@efficios.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 5589 Lines: 142 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). Changes since v3: - Clarify comments following feeback from Peter Zijlstra. 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 --- arch/x86/mm/tlb.c | 5 +++++ include/linux/sched/mm.h | 5 +++++ kernel/sched/core.c | 38 +++++++++++++++++++++++++++----------- 3 files changed, 37 insertions(+), 11 deletions(-) diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c index 93fe97cce581..5ba86b85953b 100644 --- a/arch/x86/mm/tlb.c +++ b/arch/x86/mm/tlb.c @@ -143,6 +143,11 @@ 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 d3b81e48784d..1bd10c2c0893 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 b9d731283946..6254f87645de 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -2649,6 +2649,12 @@ 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)) { @@ -2754,6 +2760,13 @@ context_switch(struct rq *rq, struct task_struct *prev, */ arch_start_context_switch(prev); + /* + * If mm is non-NULL, we pass through switch_mm(). If mm is + * NULL, we will pass through mmdrop() in finish_task_switch(). + * Both of these contain the full memory barrier required by + * membarrier after storing to rq->curr, before returning to + * user-space. + */ if (!mm) { next->active_mm = oldmm; mmgrab(oldmm); @@ -3290,6 +3303,9 @@ 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(); @@ -3337,17 +3353,17 @@ static void __sched notrace __schedule(bool preempt) /* * The membarrier system call requires each architecture * to have a full memory barrier after updating - * rq->curr, before returning to user-space. For TSO - * (e.g. x86), the architecture must provide its own - * barrier in switch_mm(). For weakly ordered machines - * for which spin_unlock() acts as a full memory - * barrier, finish_lock_switch() in common code takes - * care of this barrier. For weakly ordered machines for - * which spin_unlock() acts as a RELEASE barrier (only - * arm64 and PowerPC), arm64 has a full barrier in - * switch_to(), and PowerPC has - * smp_mb__after_unlock_lock() before - * finish_lock_switch(). + * rq->curr, before returning to user-space. + * + * Here are the schemes providing that barrier on the + * various architectures: + * - mm ? switch_mm() : mmdrop() for x86, s390, sparc, + * - finish_lock_switch() for weakly-ordered + * architectures where spin_unlock is a full barrier, + * - switch_to() for arm64 (weakly-ordered, spin_unlock + * is a RELEASE barrier), + * - membarrier_arch_sched_in() for PowerPC, + * (weakly-ordered, spin_unlock is a RELEASE barrier). */ ++*switch_count; -- 2.11.0