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=-0.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS 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 7BC20C67863 for ; Sat, 20 Oct 2018 20:18:41 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 16886216C4 for ; Sat, 20 Oct 2018 20:18:41 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 16886216C4 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=rowland.harvard.edu Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726394AbeJUEaP (ORCPT ); Sun, 21 Oct 2018 00:30:15 -0400 Received: from netrider.rowland.org ([192.131.102.5]:51441 "HELO netrider.rowland.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with SMTP id S1725746AbeJUEaP (ORCPT ); Sun, 21 Oct 2018 00:30:15 -0400 Received: (qmail 29885 invoked by uid 500); 20 Oct 2018 16:18:37 -0400 Received: from localhost (sendmail-bs@127.0.0.1) by localhost with SMTP; 20 Oct 2018 16:18:37 -0400 Date: Sat, 20 Oct 2018 16:18:37 -0400 (EDT) From: Alan Stern X-X-Sender: stern@netrider.rowland.org To: "Paul E. McKenney" cc: linux-kernel@vger.kernel.org, , , , , , , , , , , , Subject: Re: Interrupts, smp_load_acquire(), smp_store_release(), etc. In-Reply-To: <20181020161049.GA13756@linux.ibm.com> Message-ID: MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat, 20 Oct 2018, Paul E. McKenney wrote: > The second (informal) litmus test has a more interesting Linux-kernel > counterpart: > > void t1_interrupt(void) > { > r0 = READ_ONCE(y); > smp_store_release(&x, 1); > } > > void t1(void) > { > smp_store_release(&y, 1); > } > > void t2(void) > { > r1 = smp_load_acquire(&x); > r2 = smp_load_acquire(&y); > } > > On store-reordering architectures that implement smp_store_release() > as a memory-barrier instruction followed by a store, the interrupt could > arrive betweentimes in t1(), so that there would be no ordering between > t1_interrupt()'s store to x and t1()'s store to y. This could (again, > in paranoid theory) result in the outcome r0==0 && r1==0 && r2==1. This is disconcerting only if we assume that t1_interrupt() has to be executed by the same CPU as t1(). If the interrupt could be fielded by a different CPU then the paranoid outcome is perfectly understandable, even in an SC context. So the question really should be limited to situations where a handler is forced to execute in the context of a particular thread. While POSIX does allow such restrictions for user programs, I'm not aware of any similar mechanism in the kernel. Alan