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=-10.2 required=3.0 tests=BAYES_00, BUG6152_INVALID_DATE_TZ_ABSURD,DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,INVALID_DATE_TZ_ABSURD, 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 6AAA2C433E0 for ; Mon, 1 Feb 2021 13:13:02 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 34D7864DDB for ; Mon, 1 Feb 2021 13:13:02 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231687AbhBANMn (ORCPT ); Mon, 1 Feb 2021 08:12:43 -0500 Received: from Galois.linutronix.de ([193.142.43.55]:57534 "EHLO galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229707AbhBANMj (ORCPT ); Mon, 1 Feb 2021 08:12:39 -0500 From: John Ogness DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020; t=1612185116; 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=eUaCyr3F7lqUdpg4F0N9kWvTDUoAE0/ywIy7lgPol1Y=; b=LKQ9SVA7OazpDeEL++kzx1lrPvmddTdlJM7N2q+ye8vftUs5cuTPlFrZDaUaEBL3RDz3cy dqq+H0pc/B8k6CIgTU7lwYvIJnUmvRHN1Xce8GvmQN0gLhnB34lfvp0ndPDJ2n7njohe9g G+xN1vB34qwUyD7gLGSKKRftA5IGTLpD80pmOROWP5uqQI4LOOUAHjKvumgCseDwcvQSJN lSS8cpjNzKvlpk0BT/eQt+YYumUacENOhfKX5iC+QdyaGtgAC8tdhH6NBqskyqhniVK57R AzuK9Vn/aXC4qty74m474jwIsYKei2k1qQL7J2L4xJ5tzS+VRe7D0ZBiBQ5qFw== DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020e; t=1612185116; 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=eUaCyr3F7lqUdpg4F0N9kWvTDUoAE0/ywIy7lgPol1Y=; b=i9BKcFmNuVf6k65hRZktN7NvsYtG3J2GoFgLW1oaMvvTBZ2x8jIcffHob9awGz2I+ee24U rFN6qoO2/pcymyCg== To: Petr Mladek Cc: Sergey Senozhatsky , Sergey Senozhatsky , Steven Rostedt , Thomas Gleixner , linux-kernel@vger.kernel.org Subject: Re: [PATCH printk-rework 07/12] printk: add syslog_lock In-Reply-To: References: <20210126211551.26536-1-john.ogness@linutronix.de> <20210126211551.26536-8-john.ogness@linutronix.de> Date: Mon, 01 Feb 2021 14:17:55 +0106 Message-ID: <87h7mwj5k4.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 2021-02-01, Petr Mladek wrote: >> The global variables @syslog_seq, @syslog_partial, @syslog_time >> and write access to @clear_seq are protected by @logbuf_lock. >> Once @logbuf_lock is removed, these variables will need their >> own synchronization method. Introduce @syslog_lock for this >> purpose. > >> --- a/kernel/printk/printk.c >> +++ b/kernel/printk/printk.c >> @@ -390,8 +390,12 @@ DEFINE_RAW_SPINLOCK(logbuf_lock); >> printk_safe_exit_irqrestore(flags); \ >> } while (0) >> >> +/* syslog_lock protects syslog_* variables and write access to clear_seq. */ >> +static DEFINE_RAW_SPINLOCK(syslog_lock); > > I am not expert on RT code but I think that it prefers the generic > spinlocks. syslog_lock seems to be used in a normal context. > IMHO, it does not need to be a raw spinlock. > > Note that using normal spinlock would require switching the locking > order. logbuf_lock is a raw lock. Normal spinlock must not be taken > under a raw spinlock. > > Or we could switch syslog_lock to the normal spinlock later > after logbuf_lock is removed. I was planning on this last option because I think it is the simplest. There are places such as syslog_print_all() where the printk_safe_enter() and logbuf_lock locking are not at the same place as the syslog_lock locking (and syslog_lock is inside). Once the safe buffers are removed, syslog_lock can transition to a spinlock. (spinlock's must not be under local_irq_save().) >> + >> #ifdef CONFIG_PRINTK >> DECLARE_WAIT_QUEUE_HEAD(log_wait); >> +/* All 3 protected by @syslog_lock. */ >> /* the next printk record to read by syslog(READ) or /proc/kmsg */ >> static u64 syslog_seq; >> static size_t syslog_partial; >> @@ -1631,6 +1643,7 @@ int do_syslog(int type, char __user *buf, int len, int source) >> bool clear = false; >> static int saved_console_loglevel = LOGLEVEL_DEFAULT; >> int error; >> + u64 seq; > > This allows to remove definition of the same temporary variable > for case SYSLOG_ACTION_SIZE_UNREAD. Right. I missed that. >> >> error = check_syslog_permissions(type, source); >> if (error) >> @@ -1648,8 +1661,14 @@ int do_syslog(int type, char __user *buf, int len, int source) >> return 0; >> if (!access_ok(buf, len)) >> return -EFAULT; >> + >> + /* Get a consistent copy of @syslog_seq. */ >> + raw_spin_lock_irq(&syslog_lock); >> + seq = syslog_seq; >> + raw_spin_unlock_irq(&syslog_lock); >> + >> error = wait_event_interruptible(log_wait, >> - prb_read_valid(prb, syslog_seq, NULL)); >> + prb_read_valid(prb, seq, NULL)); > > Hmm, this will not detect when syslog_seq gets cleared in parallel. > I hope that nobody rely on this behavior. But who knows? > > A solution might be to have also syslog_seq latched. But I am > not sure if it is worth it. > > I am for taking the risk and use the patch as it is now. Let's keep > the code for now. We could always use the latched variable when > anyone complains. Just keep it in mind. We could add a simple helper: /* Get a consistent copy of @syslog_seq. */ static u64 syslog_seq_read(void) { unsigned long flags; raw_spin_lock_irqsave(&syslog_lock, flags); seq = syslog_seq; raw_spin_unlock_irqrestore(&syslog_lock, flags); return seq; } Then change the code to: error = wait_event_interruptible(log_wait, prb_read_valid(prb, read_syslog_seq(), NULL)); register_console() could also make use of the function. (That is why I am suggesting the flags variant.) John Ogness