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, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,USER_AGENT_SANE_2 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 D1A40C2B9F4 for ; Mon, 28 Jun 2021 15:30:55 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id B99D96195B for ; Mon, 28 Jun 2021 15:30:55 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234938AbhF1PdT (ORCPT ); Mon, 28 Jun 2021 11:33:19 -0400 Received: from mail.kernel.org ([198.145.29.99]:39680 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S235677AbhF1PHA (ORCPT ); Mon, 28 Jun 2021 11:07:00 -0400 Received: from oasis.local.home (cpe-66-24-58-225.stny.res.rr.com [66.24.58.225]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 57A6461457; Mon, 28 Jun 2021 14:52:42 +0000 (UTC) Date: Mon, 28 Jun 2021 10:52:40 -0400 From: Steven Rostedt To: Petr Mladek Cc: John Ogness , Sergey Senozhatsky , Thomas Gleixner , linux-kernel@vger.kernel.org Subject: Re: [PATCH printk v3 6/6] printk: syslog: close window between wait and read Message-ID: <20210628105240.4fc1be2e@oasis.local.home> In-Reply-To: References: <20210624111148.5190-1-john.ogness@linutronix.de> <20210624111148.5190-7-john.ogness@linutronix.de> <20210625093354.12384711@oasis.local.home> X-Mailer: Claws Mail 3.17.3 (GTK+ 2.24.33; x86_64-pc-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, 28 Jun 2021 16:35:48 +0200 Petr Mladek wrote: > I was double checking the code and the locking is really hard to > follow. I would if the following approach make it easier. The main > trick is that the lock is taken at the beginnig and release at > the end. It is only temporary released around a single line > when needed. > > static int syslog_print(char __user *buf, int size) > { > struct printk_info info; > struct printk_record r; > char *text; > int len = 0; > u64 seq; > > text = kmalloc(CONSOLE_LOG_MAX, GFP_KERNEL); > if (!text) > return -ENOMEM; > > prb_rec_init_rd(&r, &info, text, CONSOLE_LOG_MAX); > > mutex_lock(&syslog_lock); > > /* > * Wait for the @syslog_seq record to be vailable. @syslog_seq may > * change while waiting. > */ > do { > seq = syslog_seq; > > mutex_unlock(&syslog_lock); > len = wait_event_interruptible(log_wait, prb_read_valid(prb, seq, NULL)); > mutex_lock(&syslog_lock); > > if (len) > goto out; > } while (syslog_seq != seq); > > /* > * Copy records that fit into the buffer. The above cycle makes sure > * that the first record is always available. > */ > do { > size_t n; > size_t skip; > unsigned long err; > > if (!prb_read_valid(prb, syslog_seq, &r)) > break; > > if (r.info->seq != syslog_seq) { > /* message is gone, move to next valid one */ > syslog_seq = r.info->seq; > syslog_partial = 0; > } > > /* > * To keep reading/counting partial line consistent, > * use printk_time value as of the beginning of a line. > */ > if (!syslog_partial) > syslog_time = printk_time; > > skip = syslog_partial; > n = record_print_text(&r, true, syslog_time); > if (n - syslog_partial <= size) { > /* message fits into buffer, move forward */ > syslog_seq = r.info->seq + 1; > n -= syslog_partial; > syslog_partial = 0; > } else if (!len){ > /* partial read(), remember position */ > n = size; > syslog_partial += n; > } else > n = 0; > > if (!n) > break; > > mutex_unlock(&syslog_lock); > err = copy_to_user(buf, text + skip, n); > mutex_lock(&syslog_lock); > > if (err && !len) { > len = -EFAULT; > break; > } > > len += n; > size -= n; > buf += n; > } while(size); > out: > mutex_unlock(&syslog_lock); > kfree(text); > return len; > } That's a much more common approach to locking, that may not be as efficient, but is much easier to keep straight, and less error prone. -- Steve