From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Cyrus-Session-Id: sloti22d1t05-3815709-1517250937-2-1796223172431114063 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 0.001, 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', XOriginatingCountry='CA' X-Spam-charsets: plain='utf-8' 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=1517250936; b=YBICwV1RbVeTMjD9S0jLcevN9xi3z8siZVusiWAhM6mtU5d nkB8/MuWWSfjBj+a56E1Db4thBFg9UB59l7h/HLHv9VAo0iWasyjSOV6gncOwRT3 kfQRoOyjGg79xJPaDLDLU10jBuYeZH4Dln+o/t/FLS5PIJCJ3JhvcNM49vtJB/Bg yUYbytzxv8wtjAxRr7mGfnVxgJg4IE9UAi6frsAUH5EOwSCAkDsksMPUK/+qXJwq VSDjupHwvoGkQDgvb6MoZ3F9jv33EWyPoYVXVYKnvtEPD8GKy29U3QV5kfZ8L4FL eWRt6H640M+dQx2uPSbrY28VTM2PQuQ4Rd3YgTg== ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d= messagingengine.com; h=date:from:to:cc:message-id:in-reply-to :references:subject:mime-version:content-type :content-transfer-encoding:sender:list-id; s=arctest; t= 1517250936; bh=26NMpTYJf5n11Cj53esoWpp6f/SM5/t6tdYYADdNqZI=; b=f kIZ9K8eLgZbAdPy3KdwO4tcQJIKlqk5vhA/CdB08Y6tTQclKXUd6bJLVZEUp/tRp MC7nkDLZzwt47kuEs2bs481pJs+uBs7sqVITIBi6cYszCcLWoKoS5ZsVL0OrUmIk Lwrcm9f+QphTuv+NqGmpqdbCI7vKaZkimA2NwIBPr2Gq+Z8C/ynj311X+S1cmeT4 mON3v6+7HQyKmwEKs8xlJO+OAZzJmp5OxNpYf3BDRb+NF2Zh1QGD7RfHqgvpBLDs 4KVj9YmuFVSgSOys9o9xuEYbobEwO0q9lKxocviFnUK0dhdJQOw/1MlivBWKi8wQ OUKXlWw3zqLN/PTjPvBdg== ARC-Authentication-Results: i=1; mx2.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: mx2.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 S1751436AbeA2Sff (ORCPT ); Mon, 29 Jan 2018 13:35:35 -0500 Received: from mail.efficios.com ([167.114.142.141]:51734 "EHLO mail.efficios.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751357AbeA2Sfe (ORCPT ); Mon, 29 Jan 2018 13:35:34 -0500 Date: Mon, 29 Jan 2018 18:36:05 +0000 (UTC) From: Mathieu Desnoyers To: Peter Zijlstra Cc: Ingo Molnar , Thomas Gleixner , linux-kernel , linux-api , 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, ARM Linux" , Greg Hackmann , Will Deacon , David Sehr , Linus Torvalds , x86 , linux-arch Message-ID: <485936677.11601.1517250965043.JavaMail.zimbra@efficios.com> In-Reply-To: <20180129181529.GG2295@hirez.programming.kicks-ass.net> References: <20180123155733.3404-1-mathieu.desnoyers@efficios.com> <20180123155733.3404-9-mathieu.desnoyers@efficios.com> <20180129180414.GO2249@hirez.programming.kicks-ass.net> <20180129181529.GG2295@hirez.programming.kicks-ass.net> Subject: Re: [PATCH 08/11] membarrier: Provide core serializing command (v2) MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit X-Originating-IP: [167.114.142.141] X-Mailer: Zimbra 8.7.11_GA_1854 (ZimbraWebClient - FF52 (Linux)/8.7.11_GA_1854) Thread-Topic: membarrier: Provide core serializing command (v2) Thread-Index: wYgJZQvxQ/i8zr4IqfaW2yWcLsylfw== 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: ----- On Jan 29, 2018, at 1:15 PM, Peter Zijlstra peterz@infradead.org wrote: > On Mon, Jan 29, 2018 at 07:04:14PM +0100, Peter Zijlstra wrote: >> On Tue, Jan 23, 2018 at 10:57:30AM -0500, Mathieu Desnoyers wrote: >> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c >> > index f38c4c7e256a..041893128f51 100644 >> > --- a/kernel/sched/core.c >> > +++ b/kernel/sched/core.c >> > @@ -2662,9 +2662,13 @@ static struct rq *finish_task_switch(struct task_struct >> > *prev) >> > * 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(). >> > + * membarrier also requires a core serializing instruction >> > + * before going back to user-space after storing to rq->curr. >> > */ >> > - if (mm) >> > + if (mm) { >> > + membarrier_mm_sync_core_before_usermode(mm); >> > mmdrop(mm); >> > + } >> >> *confused*, when we switch from process A to process B, context_switch() >> will not set rq->prev_mm and the above mm will be NULL and we'll not >> pass through your_function_names_are_waaay_too_long and we'll not get >> cookies. >> >> And if there's anything more complicated going on, the comment/changelog >> are not adequate. > > Aaah, its the case where we do not pass through switch_mm(), the partial > comment got to me. I only realized after reading the next patch. Indeed, if we read the entire comment, it's made clear that this case is for when switch_mm is not invoked, where the current mm is changed without going through switch_mm(), when scheduling between uthread->kthread->uthread for instance. /* * 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(). * membarrier also requires a core serializing instruction * before going back to user-space after storing to rq->curr. */ Is there something I should improve in the wording of this added sentence to make it clearer ? Thanks, Mathieu > >> > if (unlikely(prev_state == TASK_DEAD)) { >> > if (prev->sched_class->task_dead) >> > prev->sched_class->task_dead(prev); >> -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com