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=-3.9 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=no 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 6280EC41604 for ; Sat, 3 Oct 2020 16:12:05 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 1A0A6206DD for ; Sat, 3 Oct 2020 16:12:05 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=joelfernandes.org header.i=@joelfernandes.org header.b="nMJVqEVf" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1725807AbgJCQMC (ORCPT ); Sat, 3 Oct 2020 12:12:02 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:37562 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725791AbgJCQMB (ORCPT ); Sat, 3 Oct 2020 12:12:01 -0400 Received: from mail-qk1-x72e.google.com (mail-qk1-x72e.google.com [IPv6:2607:f8b0:4864:20::72e]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 7AB99C0613E7 for ; Sat, 3 Oct 2020 09:12:01 -0700 (PDT) Received: by mail-qk1-x72e.google.com with SMTP id w12so6811256qki.6 for ; Sat, 03 Oct 2020 09:12:01 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=joelfernandes.org; s=google; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to; bh=YlekjJIS5w6LOeRz6EJlfj8ygIyCZG03pej0OmsvzhU=; b=nMJVqEVfcavjaZitmb1E8dy2yIv6nXRis5PU2+sJ8AQDKHeEV4Z/Q52f8LrbpU6sG8 Hc8THdOwETxpNTyGxYgHG4luJHqPlLzTRWpD/YIznvkZ+YoBvPs+VzEkacOZ+2FZBbUq jsM28alYesEJmMd6ZrlUCLanHDwX3QhL5ZmTc= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to; bh=YlekjJIS5w6LOeRz6EJlfj8ygIyCZG03pej0OmsvzhU=; b=Ur+aZsTAr5tB54CL9gtzuv+ZNqtqUW4gzu+ESdHbdtV+omMkHWZFxTNezlIM3EbsFZ fBYsQBE1e3Z08ps8gJ0wj99PSDLlpWtg854NFo9EORcxIsvnaiIou+j74Bdt/1ld8FDL 3C/v5O+YQp/L3kHlr/IWNlMaRef7cTLbnKZAxANw47fLWe9ZvwaSPy7ZfKryqoewtIp8 bgCLbq4M0ElTUOCgQbzJKSkqD//t/SPXmYWzJCEoKV//rijIQcqWai8nlRjqsFALCErT HbkERg7lzvhCl89bdZr2Ig9nyQo/FW/J4xE86vr/iESsqsatL0g8p4B9CijD1dSkufKK Ui1g== X-Gm-Message-State: AOAM530d4dIlKFDN8Hgbs0rj/Y5Nm8sfiVyue/aMOQF2nKzAIX4VJwpa VXsDIOLp/bh78mZmlYMRbBn9NQ== X-Google-Smtp-Source: ABdhPJx/Vn4gZb2FVvaKZ4hc7v1kHqjY3wM9KDysoMHuheq8dpwUFs/mdGyXUoWR6xI16dtYsoSXCA== X-Received: by 2002:a37:62c4:: with SMTP id w187mr2178542qkb.102.1601741520280; Sat, 03 Oct 2020 09:12:00 -0700 (PDT) Received: from localhost ([2620:15c:6:12:cad3:ffff:feb3:bd59]) by smtp.gmail.com with ESMTPSA id o35sm3576581qte.23.2020.10.03.09.11.59 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sat, 03 Oct 2020 09:11:59 -0700 (PDT) Date: Sat, 3 Oct 2020 12:11:59 -0400 From: joel@joelfernandes.org To: "Paul E. McKenney" Cc: Alan Stern , parri.andrea@gmail.com, will@kernel.org, peterz@infradead.org, boqun.feng@gmail.com, npiggin@gmail.com, dhowells@redhat.com, j.alglave@ucl.ac.uk, luc.maranget@inria.fr, akiyks@gmail.com, dlustig@nvidia.com, viro@zeniv.linux.org.uk, linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org Subject: Re: Litmus test for question from Al Viro Message-ID: <20201003161159.GA3601096@google.com> References: <20201001045116.GA5014@paulmck-ThinkPad-P72> <20201001161529.GA251468@rowland.harvard.edu> <20201001213048.GF29330@paulmck-ThinkPad-P72> <20201003160846.GA3598977@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201003160846.GA3598977@google.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat, Oct 03, 2020 at 12:08:46PM -0400, joel@joelfernandes.org wrote: [...] > static void code0(struct v_struct* v,spinlock_t* l,int* out_0_r1) { > > struct v_struct *r1; /* to_free */ > > r1 = NULL; > spin_lock(l); > if (!smp_load_acquire(&v->b)) > r1 = v; > v->a = 0; > spin_unlock(l); > > *out_0_r1 = !!r1; > } > > static void code1(struct v_struct* v,spinlock_t* l,int* out_1_r1) { > > struct v_struct *r1; /* to_free */ > > r1 = v; > if (READ_ONCE(v->a)) { > spin_lock(l); > if (v->a) > r1 = NULL; > smp_store_release(&v->b, 0); > spin_unlock(l); > } > > *out_1_r1 = !!r1; > } > > Results on both arm64 and x86: > > Histogram (2 states) > 19080852:>0:r1=1; 1:r1=0; > 20919148:>0:r1=0; 1:r1=1; > No > > Witnesses > Positive: 0, Negative: 40000000 > Condition exists (0:r1=1 /\ 1:r1=1) is NOT validated > Hash=4a8c15603ffb5ab464195ea39ccd6382 > Observation AL+test Never 0 40000000 > Time AL+test 6.24 > > I guess I could do an alloc and free of v_struct. However, I just checked for > whether the to_free in Al's example could ever be NULL for both threads. Sorry, here I meant "ever be non-NULL". So basically I was trying to experimentally confirm that to_free could never be non-NULL in both code0 and code1 threads.