From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-17.0 required=3.0 tests=BAYES_00,INCLUDES_CR_TRAILER, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_GIT autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id DBEEAC4361A for ; Fri, 4 Dec 2020 05:08:06 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 8B3EF22571 for ; Fri, 4 Dec 2020 05:08:06 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727522AbgLDFHv (ORCPT ); Fri, 4 Dec 2020 00:07:51 -0500 Received: from mail.kernel.org ([198.145.29.99]:43780 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725554AbgLDFHu (ORCPT ); Fri, 4 Dec 2020 00:07:50 -0500 From: Andy Lutomirski Authentication-Results: mail.kernel.org; dkim=permerror (bad message/signature format) To: x86@kernel.org, Mathieu Desnoyers Cc: LKML , Nicholas Piggin , Arnd Bergmann , Anton Blanchard , Andy Lutomirski , stable@vger.kernel.org Subject: [PATCH v3 1/4] x86/membarrier: Get rid of a dubious optimization Date: Thu, 3 Dec 2020 21:07:03 -0800 Message-Id: <5afc7632be1422f91eaf7611aaaa1b5b8580a086.1607058304.git.luto@kernel.org> X-Mailer: git-send-email 2.28.0 In-Reply-To: References: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org sync_core_before_usermode() had an incorrect optimization. If we're in an IRQ, we can get to usermode without IRET -- we just have to schedule to a different task in the same mm and do SYSRET. Fortunately, there were no callers of sync_core_before_usermode() that could have had in_irq() or in_nmi() equal to true, because it's only ever called from the scheduler. While we're at it, clarify a related comment. Cc: stable@vger.kernel.org Reviewed-by: Mathieu Desnoyers Signed-off-by: Andy Lutomirski --- arch/x86/include/asm/sync_core.h | 9 +++++---- arch/x86/mm/tlb.c | 10 ++++++++-- 2 files changed, 13 insertions(+), 6 deletions(-) diff --git a/arch/x86/include/asm/sync_core.h b/arch/x86/include/asm/sync_core.h index 0fd4a9dfb29c..ab7382f92aff 100644 --- a/arch/x86/include/asm/sync_core.h +++ b/arch/x86/include/asm/sync_core.h @@ -98,12 +98,13 @@ static inline void sync_core_before_usermode(void) /* With PTI, we unconditionally serialize before running user code. */ if (static_cpu_has(X86_FEATURE_PTI)) return; + /* - * Return from interrupt and NMI is done through iret, which is core - * serializing. + * Even if we're in an interrupt, we might reschedule before returning, + * in which case we could switch to a different thread in the same mm + * and return using SYSRET or SYSEXIT. Instead of trying to keep + * track of our need to sync the core, just sync right away. */ - if (in_irq() || in_nmi()) - return; sync_core(); } diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c index 11666ba19b62..569ac1d57f55 100644 --- a/arch/x86/mm/tlb.c +++ b/arch/x86/mm/tlb.c @@ -474,8 +474,14 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct mm_struct *next, /* * The membarrier system call requires a full memory barrier and * core serialization before returning to user-space, after - * storing to rq->curr. Writing to CR3 provides that full - * memory barrier and core serializing instruction. + * storing to rq->curr, when changing mm. This is because + * membarrier() sends IPIs to all CPUs that are in the target mm + * to make them issue memory barriers. However, if another CPU + * switches to/from the target mm concurrently with + * membarrier(), it can cause that CPU not to receive an IPI + * when it really should issue a memory barrier. Writing to CR3 + * provides that full memory barrier and core serializing + * instruction. */ if (real_prev == next) { VM_WARN_ON(this_cpu_read(cpu_tlbstate.ctxs[prev_asid].ctx_id) != -- 2.28.0