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Tue, 25 Jun 2019 19:08:42 -0700 (PDT) Received: from localhost ([175.223.45.10]) by smtp.gmail.com with ESMTPSA id x3sm319311pja.7.2019.06.25.19.08.40 (version=TLS1_3 cipher=AEAD-AES256-GCM-SHA384 bits=256/256); Tue, 25 Jun 2019 19:08:41 -0700 (PDT) Date: Wed, 26 Jun 2019 11:08:37 +0900 From: Sergey Senozhatsky To: John Ogness Cc: Sergey Senozhatsky , 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 Message-ID: <20190626020837.GA25178@jagdpanzerIV> 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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87woh9hnxg.fsf@linutronix.de> User-Agent: Mutt/1.12.1 (2019-06-15) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On (06/25/19 14:03), John Ogness 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 ;) -ss