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=-10.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED 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 23E80C47082 for ; Tue, 8 Jun 2021 14:49:53 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 0E86761183 for ; Tue, 8 Jun 2021 14:49:53 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233668AbhFHOvo (ORCPT ); Tue, 8 Jun 2021 10:51:44 -0400 Received: from smtp-out1.suse.de ([195.135.220.28]:47116 "EHLO smtp-out1.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233363AbhFHOvn (ORCPT ); Tue, 8 Jun 2021 10:51:43 -0400 Received: from relay2.suse.de (relay2.suse.de [149.44.160.134]) by smtp-out1.suse.de (Postfix) with ESMTP id 57F89219DC; Tue, 8 Jun 2021 14:49:49 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1623163789; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=Sp3Oqg3CSzUFZ5tDEFNZBW1Vq6oX3/Pn7E8jqzxX9UI=; b=PzTvpfSzVcPwZ1kqPT9z4EEASMn1EkG04Wjzivshe7J764DsAHMTTMpvxgmyzr8oNpERlD itF82MK7UYSDBEENQJALbaT9v9v4G897FlffSlfACCvBGJDKK98ZDNrx+Z1LqXuaW36Uc2 zaaXEtrHexEnVEcSQUAoPRTFbRftXfQ= Received: from suse.cz (unknown [10.100.216.66]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by relay2.suse.de (Postfix) with ESMTPS id 22546A3B8F; Tue, 8 Jun 2021 14:49:49 +0000 (UTC) Date: Tue, 8 Jun 2021 16:49:48 +0200 From: Petr Mladek To: John Ogness Cc: Sergey Senozhatsky , Steven Rostedt , Thomas Gleixner , linux-kernel@vger.kernel.org, Peter Zijlstra , "Paul E. McKenney" Subject: Re: [PATCH next v2 2/2] printk: fix cpu lock ordering Message-ID: References: <20210607200232.22211-1-john.ogness@linutronix.de> <20210607200232.22211-3-john.ogness@linutronix.de> <875yyoigms.fsf@jogness.linutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <875yyoigms.fsf@jogness.linutronix.de> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue 2021-06-08 16:18:51, John Ogness wrote: > On 2021-06-08, Petr Mladek wrote: > > The change makes perfect sense and the code looks correct. > > But I am not sure about the description of the memory barriers. > > OK. > > >> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c > >> index f94babb38493..8c870581cfb4 100644 > >> --- a/kernel/printk/printk.c > >> +++ b/kernel/printk/printk.c > >> @@ -3560,10 +3560,29 @@ void printk_cpu_lock_irqsave(bool *lock_flag, unsigned long *irq_flags) > >> > >> cpu = smp_processor_id(); > >> > >> - old = atomic_cmpxchg(&printk_cpulock_owner, -1, cpu); > >> + /* > >> + * Guarantee loads and stores from the previous lock owner are > >> + * visible to this CPU once it is the lock owner. This pairs > >> + * with cpu_unlock:B. > > > > These things are not easy to describe. It took me quite some time to > > understand the above description. And think that it does not say > > the full storry. > > > > IMHO, the lock should work the way that: > > > > + The new owner see all writes done or seen by the previous owner(s). > > + The previous owner(s) never see writes done by the new owner. > > You are right. I can describe those independently. > > > Honestly, I am not sure if we could describe the barriers correctly > > and effectively at the same time. > > For v3 I would describe the 2 cases separately. For lock/acquire: > > /* > * Guarantee loads and stores from this CPU when it is the lock owner > * are _not_ visible to the previous lock owner. This pairs with > * cpu_unlock:B. Sounds better to me. * > * Memory barrier involvement: > * > * If cpu_lock:A reads from cpu_unlock:B, then cpu_unlock:A can never > * read from cpu_lock:B. > * > * Relies on: > * > * RELEASE from cpu_unlock:A to cpu_unlock:B > * matching > * ACQUIRE from cpu_lock:A to cpu_lock:B > */ I can't check this so I believe you ;-) > And for unlock/release: > > /* > * Guarantee loads and stores from this CPU when it was the > * lock owner are visible to the next lock owner. This pairs > * with cpu_lock:A. Sounds good. * > * Memory barrier involvement: > * > * If cpu_lock:A reads from cpu_unlock:B, then cpu_lock:B > * reads from cpu_unlock:A. > * > * Relies on: > * > * RELEASE from cpu_unlock:A to cpu_unlock:B > * matching > * ACQUIRE from cpu_lock:A to cpu_lock:B > */ Same as for acquire ;-) > I know you are not a fan of these drawn out memory barrier comments. But > it really simplifies verification and translation to litmus > tests. Without such comments, I would be lost looking back at > printk_ringbuffer.c. > > If the previous dump_stack() cpu lock implementation had such comments, > we would know if the missing memory barriers were by design. Sure. I am fine with the comments as long as there is also some human readable description of the barrier intention. Best Regards, Petr