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=-8.1 required=3.0 tests=DKIMWL_WL_MED,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,FSL_HELO_FAKE,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1, USER_IN_DEF_DKIM_WL 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 51DFDC432C0 for ; Thu, 21 Nov 2019 19:44:29 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 257A020672 for ; Thu, 21 Nov 2019 19:44:29 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="f7hIgrLH" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726869AbfKUTo2 (ORCPT ); Thu, 21 Nov 2019 14:44:28 -0500 Received: from mail-wm1-f65.google.com ([209.85.128.65]:40392 "EHLO mail-wm1-f65.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726541AbfKUTo1 (ORCPT ); Thu, 21 Nov 2019 14:44:27 -0500 Received: by mail-wm1-f65.google.com with SMTP id y5so5083607wmi.5 for ; Thu, 21 Nov 2019 11:44:26 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20161025; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to:user-agent; bh=uTIJtrMTYzypNUnthW2Tuvb1wnw9jVilGROuq4bj1cY=; b=f7hIgrLHpIfU5/akWKo3g6Aba9HNF1z8VIF40564XgfKxm+Wqz5asZ0K3ga+NNX8JQ 25tAeHAgC3gHnkLsJRhtHbaULGoBHvRJioPF3pxZ5PHqDVudlac0GuaPUkPNcs6dhgnu p8Fts1e3CQrJfY8BcxTBaYyeCBL5JoZ+UUePdxCG00RU4Ihm8O1ByPNci1R++r/fMYGG WyUqcKr7yoqgWrXhVrgPhq84rwAQ+p8FfHp86rluJkufqsWmpDRR+y+BLSyD088TPwAA apUAECWUpHvdLRILLuj6RAizizlRH48nP5TT4DlolwOHLVP5CBXIcoZnCC7P70JV1iUg p+lQ== 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:user-agent; bh=uTIJtrMTYzypNUnthW2Tuvb1wnw9jVilGROuq4bj1cY=; b=TyKX/nYMzAb4HpPEzVPzsdH+wvUPJsS8S03zDvL8pNbyjcEiF2ibq6sqKRVXMS/UFK BZD5kT8tlitB8dedujOfSddOam+h1wPsdJQc+CgCtczFdNi7TTALu4FIvDdl4K/hS/7X qzCflWkx4hxSYMr3UCvyRa5cs1xZKLB3eSzdqP9W6ROLzQdiOLnMOr9dx25bf4ZXymoZ ithYjxbuH4ihhFAFazsLsn+mtaB+MHFNHTkeMUMtwf8bqiEyiJCe9p55PHabgrNzzSQ3 yFre5/I8VKq7iWfVsq08qRoISuJi150TIIKlY4tTh9UvXy9937jq7GVfQWfmsMeinta9 KORg== X-Gm-Message-State: APjAAAWg+6/UtwX8YiHlGh3PwogViTTtTFwUCumCJhSG4va2mCMfKn5y ef1ohanMS06FiEeAd++xN8G8uQ== X-Google-Smtp-Source: APXvYqzuAWFT3ZJU3dauo2GBA3lqNtP+wGbk4U6pFsTLxKMfy8NQ5iZzpfcEuBVioFbNF0k0Fikd5w== X-Received: by 2002:a1c:f70a:: with SMTP id v10mr6144619wmh.159.1574365465017; Thu, 21 Nov 2019 11:44:25 -0800 (PST) Received: from google.com ([100.105.32.75]) by smtp.gmail.com with ESMTPSA id m3sm4864233wrb.67.2019.11.21.11.44.24 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 21 Nov 2019 11:44:24 -0800 (PST) Date: Thu, 21 Nov 2019 20:44:18 +0100 From: Marco Elver To: Ingo Molnar Cc: peterz@infradead.org, mingo@redhat.com, will@kernel.org, linux-kernel@vger.kernel.org, Qian Cai , Thomas Gleixner Subject: Re: [PATCH] spinlock_debug: Fix various data races Message-ID: <20191121194418.GA239776@google.com> References: <20191120155715.28089-1-elver@google.com> <20191121183257.GA124760@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20191121183257.GA124760@gmail.com> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 21 Nov 2019, Ingo Molnar wrote: > > * Marco Elver wrote: > > > static inline void debug_spin_lock_after(raw_spinlock_t *lock) > > { > > - lock->owner_cpu = raw_smp_processor_id(); > > - lock->owner = current; > > + WRITE_ONCE(lock->owner_cpu, raw_smp_processor_id()); > > + WRITE_ONCE(lock->owner, current); > > } > > debug_spin_lock_after() runs inside the spinlock itself - why do these > writes have to be WRITE_ONCE()? > > > @@ -197,8 +197,8 @@ static inline void debug_write_unlock(rwlock_t *lock) > > RWLOCK_BUG_ON(lock->owner != current, lock, "wrong owner"); > > RWLOCK_BUG_ON(lock->owner_cpu != raw_smp_processor_id(), > > lock, "wrong CPU"); > > - lock->owner = SPINLOCK_OWNER_INIT; > > - lock->owner_cpu = -1; > > + WRITE_ONCE(lock->owner, SPINLOCK_OWNER_INIT); > > + WRITE_ONCE(lock->owner_cpu, -1); > > } > > This too is running inside the critical section of the spinlock - why are > the WRITE_ONCE() calls necessary? Although the writes are inside the critical section, they are read concurrently outside the critical section, e.g. in debug_spin_lock_before(). In other words, the WRITE_ONCE pair with the READ_ONCE that are *outside* the critical section. Thanks, -- Marco > Thanks, > > Ingo