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=-5.2 required=3.0 tests=BAYES_00, BUG6152_INVALID_DATE_TZ_ABSURD,DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU, HEADER_FROM_DIFFERENT_DOMAINS,INVALID_DATE_TZ_ABSURD,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS 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 C9141C1B0D9 for ; Tue, 8 Dec 2020 22:31:55 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 936C9238EE for ; Tue, 8 Dec 2020 22:31:55 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1731159AbgLHWbc (ORCPT ); Tue, 8 Dec 2020 17:31:32 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:58332 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726421AbgLHWbc (ORCPT ); Tue, 8 Dec 2020 17:31:32 -0500 Received: from galois.linutronix.de (Galois.linutronix.de [IPv6:2a0a:51c0:0:12e:550::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id B5A3CC0617A7 for ; Tue, 8 Dec 2020 14:30:46 -0800 (PST) From: John Ogness DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020; t=1607466645; h=from:from:reply-to:subject:subject: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=75XIg+ZJ7vXwfOxh4mfZVbFzoFN2Q4G9QbYqnFjSG9E=; b=A1Nz0fwN0RmE7nkQCeVtAdRK9Fp5J/cc0VItqE+6Y7ssc5nfSb29Z/8sL6eNiF2I9vmFKy 0dnrrhMqEcv/tQR4USLUQrGVKit9lqQ0Ig5IckSZ2j5JTPjJo1cQ2HLA6sfYwdeYY7tJ+S 3ggfNRJfP6/R6kiGBH5Mfzs/4+E55KzFoNRME0kzLGQ0vfwgDVbze+lJYpaKp/3h/6yC/o ZxwvGUyBtaWHLQxkvOy1q/TLSrRH6qCeKjclQEW231NMhGOLuxqxQFAcGKL1hlhhMGsDrV D9XUaHzrbeWSxy9B5oXPic1voW29EtSNLmsUcLzpfrFs2sVEWEyLTBGL+KOVqQ== DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020e; t=1607466645; h=from:from:reply-to:subject:subject: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=75XIg+ZJ7vXwfOxh4mfZVbFzoFN2Q4G9QbYqnFjSG9E=; b=lxQ6PPi8VY3UiDkGHT3VY5H8IEHbdv5lVgE58r7eJjHqq8tm8fP1GrwUG9ziPkfjV+v4sC +NNZZFN62//vMrBA== To: Sergey Senozhatsky , Petr Mladek Cc: Sergey Senozhatsky , Sergey Senozhatsky , Steven Rostedt , Linus Torvalds , Greg Kroah-Hartman , Peter Zijlstra , Thomas Gleixner , linux-kernel@vger.kernel.org Subject: Re: [PATCH next v2 2/3] printk: change @clear_seq to atomic64_t In-Reply-To: References: <20201201205341.3871-1-john.ogness@linutronix.de> <20201201205341.3871-3-john.ogness@linutronix.de> Date: Tue, 08 Dec 2020 23:36:44 +0106 Message-ID: <875z5c9bhn.fsf@jogness.linutronix.de> MIME-Version: 1.0 Content-Type: text/plain Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2020-12-09, Sergey Senozhatsky wrote: >> Sigh, atomic64_read() uses a spin lock in the generic implementation >> that is used on some architectures. > > Oh... So on those archs prb is not lockless in fact, it actually > takes the spin_lock each time we read the descriptor state? > > desc_read() > atomic_long_read(state_var) > atomic64_read() > raw_spin_lock_irqsave(lock, flags) > << NMI panic >> > > Am I missing something? For the state variable we chose atomic_long_t instead of atomic64_t for this reason. atomic_long_t operations are available atomically on all architectures. However, for clear_seq we need 64-bit (even on 32-bit machines). The seqcount_latch is an excellent solution here since clear_seq does not require lockless writers. John Ogness