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_HELO_NONE,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 9D3D8C4646B for ; Wed, 26 Jun 2019 07:16:23 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 7980E2085A for ; Wed, 26 Jun 2019 07:16:23 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726872AbfFZHQV (ORCPT ); Wed, 26 Jun 2019 03:16:21 -0400 Received: from Galois.linutronix.de ([193.142.43.55]:45239 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725876AbfFZHQV (ORCPT ); Wed, 26 Jun 2019 03:16:21 -0400 Received: from localhost ([127.0.0.1] helo=vostro.local) by Galois.linutronix.de with esmtp (Exim 4.80) (envelope-from ) id 1hg2A8-0006V9-Lf; Wed, 26 Jun 2019 09:16:12 +0200 From: John Ogness To: Sergey Senozhatsky Cc: Petr Mladek , linux-kernel@vger.kernel.org, Peter Zijlstra , Steven Rostedt , Linus Torvalds , Greg Kroah-Hartman , Andrea Parri , Thomas Gleixner , Sergey Senozhatsky Subject: Re: [RFC PATCH v2 1/2] printk-rb: add a new printk ringbuffer implementation References: <20190607162349.18199-1-john.ogness@linutronix.de> <20190607162349.18199-2-john.ogness@linutronix.de> <20190618045117.GA7419@jagdpanzerIV> <87imt2bl0k.fsf@linutronix.de> <20190625064543.GA19050@jagdpanzerIV> <20190625071500.GB19050@jagdpanzerIV> <875zoujbq4.fsf@linutronix.de> <20190625090620.wufhvdxxiryumdra@pathway.suse.cz> <20190625100322.GD532@jagdpanzerIV> <87woh9hnxg.fsf@linutronix.de> <20190626020837.GA25178@jagdpanzerIV> Date: Wed, 26 Jun 2019 09:16:11 +0200 In-Reply-To: <20190626020837.GA25178@jagdpanzerIV> (Sergey Senozhatsky's message of "Wed, 26 Jun 2019 11:08:37 +0900") Message-ID: <87mui43jgk.fsf@linutronix.de> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/23.4 (gnu/linux) 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 2019-06-26, Sergey Senozhatsky wrote: > [..] >> > CPU0 CPU1 >> > printk(...) >> > sz = vscprintf(NULL, "Comm %s\n", current->comm); >> > ia64_mca_modify_comm() >> > snprintf(comm, sizeof(comm), "%s %d", current->comm, previous_current->pid); >> > memcpy(current->comm, comm, sizeof(current->comm)); >> > if ((buf = prb_reserve(... sz))) { >> > vscnprintf(buf, "Comm %s\n", current->comm); >> > ^^^^^^^^^^^^^^ ->comm has changed. >> > Nothing critical, we >> > should not corrupt >> > anything, but we will >> > truncate ->comm if its >> > new size is larger than >> > what it used to be when >> > we did vscprintf(NULL). >> > prb_commit(...); >> > } > > [..] >> In my v1 rfc series, I avoided this issue by having a separate dedicated >> ringbuffer (rb_sprintf) that was used to allocate a temporary max-size >> (2KB) buffer for sprinting to. Then _that_ was used for the real >> ringbuffer input (strlen, prb_reserve, memcpy, prb_commit). That would >> still be the approach of my choice. > > In other words per-CPU buffering, AKA printk_safe ;) Actually, no. I made use of a printk_ringbuffer (which is global). It was used for temporary memory allocation for sprintf, but the result was immediately written into the printk buffer from the same context. In contrast, printk_safe triggers a different context to handle the insertion. It is still my intention to eliminate the buffering component of printk_safe. After we get a lockless ringbuffer that we are happy with, my next series to integrate the buffer into printk will again use the sprint_rb solution to avoid the issue discussed in this thread. Perhaps it would be best to continue this discussion after I've posted that series. John Ogness