From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Cyrus-Session-Id: sloti22d1t05-4192269-1516208315-2-9213057623594469680 X-Sieve: CMU Sieve 3.0 X-Spam-known-sender: no X-Spam-score: 0.0 X-Spam-hits: BAYES_00 -1.9, HEADER_FROM_DIFFERENT_DOMAINS 0.25, ME_NOAUTH 0.01, RCVD_IN_DNSWL_HI -5, T_RP_MATCHES_RCVD -0.01, LANGUAGES en, BAYES_USED global, SA_VERSION 3.4.0 X-Spam-source: IP='209.132.180.67', Host='vger.kernel.org', Country='US', FromHeader='com', MailFrom='org' X-Spam-charsets: X-Resolved-to: greg@kroah.com X-Delivered-to: greg@kroah.com X-Mail-from: linux-api-owner@vger.kernel.org ARC-Seal: i=1; a=rsa-sha256; cv=none; d=messagingengine.com; s=arctest; t=1516208314; b=g4KCMYm/QKTrSzNtnDYgLKD5VL9Ab6WXHpXZ1d3kqH1urQV V6Pqsfmd/tzimog52tfny/W3GEh20m5O3m5NTOT4UDCsFSxOLM1rN0BiQsBcLFRK 8RSsiJWIGzyUaX9cnH3TyQRS6t0LpQbrnJzCwqUhIEtyzk4Zytb8mkrR8c5DxZgZ SqpPAi5G5W31n/5JtLv7HOKtgjnjtX3P2wxAlZorngobTNgrri4bjznrB4q8VBsG doJWk40zMAGTg7748AcaHh3nDmG0YdkYH63cwySbvbrLN5rxHr5wpFBjkTi4FKKN 9mqxIULUHxK3NyDLvkkLsWtYZv5kxux0VTdBQ2g== ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d= messagingengine.com; h=from:to:cc:subject:date:message-id :in-reply-to:references:sender:list-id; s=arctest; t=1516208314; bh=iduw/KiVubIW5MszfcKgjxiHgOGoNPprUUPB5p/Ra6E=; b=CAlHJEtuoiuX SBekyTKLbItSxBc16InfD0mNA1nS0mP3wZPdbhfBxBWai9Jjw84F3caljTETf1m0 nmLw2H2iklfBi/Cl/ud0mrjdl5Ij3HF5Qhf4c/T7IqR6+uhC8NfF1WPY+W2IBhjB MNHOEXQsNwuinQYFkQ2FVT2J7soF+wP0zs6gOLhOTyDtFQ/m5DxdYAR2SImFE8AF VFtFYz9wkEINrFgLmT4aCpv0ORl34J4Rr9R8fxZQBIUlwQBCgCHIZS+gzqIELRQT 1dkvEgwm26Z1bIzMD+FV0aJ/UXTIezqDRLb0ZDharThC6LvtA3kTIKDK6bqZo2mE xDYC2UA/ug== ARC-Authentication-Results: i=1; mx1.messagingengine.com; arc=none (no signatures found); dkim=none (no signatures found); dmarc=none (p=none,has-list-id=yes,d=none) header.from=efficios.com; iprev=pass policy.iprev=209.132.180.67 (vger.kernel.org); spf=none smtp.mailfrom=linux-api-owner@vger.kernel.org smtp.helo=vger.kernel.org; x-aligned-from=fail; x-ptr=pass x-ptr-helo=vger.kernel.org x-ptr-lookup=vger.kernel.org; x-return-mx=pass smtp.domain=vger.kernel.org smtp.result=pass smtp_org.domain=kernel.org smtp_org.result=pass smtp_is_org_domain=no header.domain=efficios.com header.result=pass header_is_org_domain=yes Authentication-Results: mx1.messagingengine.com; arc=none (no signatures found); dkim=none (no signatures found); dmarc=none (p=none,has-list-id=yes,d=none) header.from=efficios.com; iprev=pass policy.iprev=209.132.180.67 (vger.kernel.org); spf=none smtp.mailfrom=linux-api-owner@vger.kernel.org smtp.helo=vger.kernel.org; x-aligned-from=fail; x-ptr=pass x-ptr-helo=vger.kernel.org x-ptr-lookup=vger.kernel.org; x-return-mx=pass smtp.domain=vger.kernel.org smtp.result=pass smtp_org.domain=kernel.org smtp_org.result=pass smtp_is_org_domain=no header.domain=efficios.com header.result=pass header_is_org_domain=yes Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932238AbeAQQ4R (ORCPT ); Wed, 17 Jan 2018 11:56:17 -0500 Received: from mail.efficios.com ([167.114.142.141]:46240 "EHLO mail.efficios.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932220AbeAQQ4M (ORCPT ); Wed, 17 Jan 2018 11:56:12 -0500 From: Mathieu Desnoyers To: Ingo Molnar , Peter Zijlstra , Thomas Gleixner Cc: 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, Mathieu Desnoyers Subject: [PATCH for 4.16 03/11] membarrier: Document scheduler barrier requirements (v5) Date: Wed, 17 Jan 2018 11:54:50 -0500 Message-Id: <20180117165458.13330-4-mathieu.desnoyers@efficios.com> X-Mailer: git-send-email 2.11.0 In-Reply-To: <20180117165458.13330-1-mathieu.desnoyers@efficios.com> References: <20180117165458.13330-1-mathieu.desnoyers@efficios.com> Sender: linux-api-owner@vger.kernel.org X-Mailing-List: linux-api@vger.kernel.org X-getmail-retrieved-from-mailbox: INBOX X-Mailing-List: linux-kernel@vger.kernel.org List-ID: 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. Signed-off-by: Mathieu Desnoyers CC: Peter Zijlstra CC: Paul E. McKenney CC: Boqun Feng CC: Andrew Hunter CC: Maged Michael CC: Avi Kivity CC: Benjamin Herrenschmidt CC: Paul Mackerras CC: Michael Ellerman CC: Dave Watson CC: Thomas Gleixner CC: Ingo Molnar CC: "H. Peter Anvin" CC: Andrea Parri CC: x86@kernel.org --- Changes since v1: - Update comments to match reality for code paths which are after storing to rq->curr, before returning to user-space, based on feedback from Andrea Parri. Changes since v2: - Update changelog (smp_mb__before_spinlock -> smp_mb__after_spinlock). Based on feedback from Andrea Parri. Changes since v3: - Clarify comments following feeback from Peter Zijlstra. Changes since v4: - Update comment regarding powerpc barrier. --- arch/x86/mm/tlb.c | 5 +++++ include/linux/sched/mm.h | 5 +++++ kernel/sched/core.c | 37 ++++++++++++++++++++++++++----------- 3 files changed, 36 insertions(+), 11 deletions(-) diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c index a1561957dccb..c28cd5592b0d 100644 --- a/arch/x86/mm/tlb.c +++ b/arch/x86/mm/tlb.c @@ -200,6 +200,11 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct mm_struct *next, #endif this_cpu_write(cpu_tlbstate.is_lazy, false); + /* + * 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_WARN_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 1754396795f6..28aef7051d73 100644 --- a/include/linux/sched/mm.h +++ b/include/linux/sched/mm.h @@ -39,6 +39,11 @@ 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 524b705892db..62f269980e29 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -2657,6 +2657,12 @@ 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)) { @@ -2762,6 +2768,13 @@ 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); @@ -3298,6 +3311,9 @@ 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(); @@ -3345,17 +3361,16 @@ 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, PowerPC. + * switch_mm() rely on membarrier_arch_switch_mm() on PowerPC. + * - 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), */ ++*switch_count; -- 2.11.0