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=-15.2 required=3.0 tests=BAYES_00, BUG6152_INVALID_DATE_TZ_ABSURD,DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,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 081D7C43217 for ; Fri, 5 Feb 2021 22:08:17 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id CA8A664FA9 for ; Fri, 5 Feb 2021 22:08:16 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229981AbhBEWHr (ORCPT ); Fri, 5 Feb 2021 17:07:47 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:49140 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232960AbhBEO5f (ORCPT ); Fri, 5 Feb 2021 09:57:35 -0500 Received: from galois.linutronix.de (Galois.linutronix.de [IPv6:2a0a:51c0:0:12e:550::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 55EF8C06121D for ; Fri, 5 Feb 2021 08:35:34 -0800 (PST) From: John Ogness DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020; t=1612534648; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding; bh=N8KO666twl5u14ugBpv1szWkDqruvHS7Y9dzUpQGTP0=; b=1C8T2UOISLEGbuRRwRe80ACDHSF6XD5mmn3jGC5agkBB4BU9JDCF8kSxvS0YYVnuv/0F5g J3MPl/cggNCaQwudslAkI47jxXW5C/HXDpCkuC5bUBl9OtmnAPUCJRyVcvTW7iYlnqUPOf QzYRsmIvqQJkCXPmaRV58MCMINNAf3w6B9jSIMmkqPZQcwWcGQDWTPzm5cORdGZaRNUD41 IRFczYfhqjuaNQ0GHMifr9mIJZ8YLI5L5SsAolskc0owrQ7NC8QJmSO/hJjKpBeVjIYRvM 3cKmkfG/y1Iz2HPj39b219SJopl8uOG0Whe9t8Id8hu1+gErit/IIFC+JqMIyg== DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020e; t=1612534648; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding; bh=N8KO666twl5u14ugBpv1szWkDqruvHS7Y9dzUpQGTP0=; b=BeUbJQ4GcI499wGWyYva+ayki6NdCXPRbBtgj6+U23l8BefpBbELHs3gjr+lPu++CooJOn xaubbY2VCr/mfqDQ== To: Petr Mladek Cc: Sergey Senozhatsky , Sergey Senozhatsky , Steven Rostedt , linux-kernel@vger.kernel.org, "J. Avila" Subject: [PATCH] printk: avoid prb_first_valid_seq() where possible Date: Fri, 5 Feb 2021 15:23:28 +0106 Message-Id: <20210205141728.18117-1-john.ogness@linutronix.de> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org If message sizes average larger than expected (more than 32 characters), the data_ring will wrap before the desc_ring. Once the data_ring wraps, it will start invalidating descriptors. These invalid descriptors hang around until they are eventually recycled when the desc_ring wraps. Readers do not care about invalid descriptors, but they still need to iterate past them. If the average message size is much larger than 32 characters, then there will be many invalid descriptors preceding the valid descriptors. The function prb_first_valid_seq() always begins at the oldest descriptor and searches for the first valid descriptor. This can be rather expensive for the above scenario. And, in fact, because of its heavy usage in /dev/kmsg, there have been reports of long delays and even RCU stalls. For code that does not need to search from the oldest record, replace prb_first_valid_seq() usage with prb_read_valid_*() functions, which provide a start sequence number to search from. Fixes: 896fbe20b4e2333fb55 ("printk: use the lockless ringbuffer") Reported-by: kernel test robot Reported-by: J. Avila Signed-off-by: John Ogness --- patch against next-20210205 kernel/printk/printk.c | 29 ++++++++++++++++++----------- 1 file changed, 18 insertions(+), 11 deletions(-) diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c index 5a95c688621f..035aae771ea1 100644 --- a/kernel/printk/printk.c +++ b/kernel/printk/printk.c @@ -735,9 +735,9 @@ static ssize_t devkmsg_read(struct file *file, char __user *buf, logbuf_lock_irq(); } - if (user->seq < prb_first_valid_seq(prb)) { + if (r->info->seq != user->seq) { /* our last seen message is gone, return error and reset */ - user->seq = prb_first_valid_seq(prb); + user->seq = r->info->seq; ret = -EPIPE; logbuf_unlock_irq(); goto out; @@ -812,6 +812,7 @@ static loff_t devkmsg_llseek(struct file *file, loff_t offset, int whence) static __poll_t devkmsg_poll(struct file *file, poll_table *wait) { struct devkmsg_user *user = file->private_data; + struct printk_info info; __poll_t ret = 0; if (!user) @@ -820,9 +821,9 @@ static __poll_t devkmsg_poll(struct file *file, poll_table *wait) poll_wait(file, &log_wait, wait); logbuf_lock_irq(); - if (prb_read_valid(prb, user->seq, NULL)) { + if (prb_read_valid_info(prb, user->seq, &info, NULL)) { /* return error when data has vanished underneath us */ - if (user->seq < prb_first_valid_seq(prb)) + if (info.seq != user->seq) ret = EPOLLIN|EPOLLRDNORM|EPOLLERR|EPOLLPRI; else ret = EPOLLIN|EPOLLRDNORM; @@ -1559,6 +1560,7 @@ static void syslog_clear(void) int do_syslog(int type, char __user *buf, int len, int source) { + struct printk_info info; bool clear = false; static int saved_console_loglevel = LOGLEVEL_DEFAULT; int error; @@ -1629,9 +1631,13 @@ int do_syslog(int type, char __user *buf, int len, int source) /* Number of chars in the log buffer */ case SYSLOG_ACTION_SIZE_UNREAD: logbuf_lock_irq(); - if (syslog_seq < prb_first_valid_seq(prb)) { - /* messages are gone, move to first one */ - syslog_seq = prb_first_valid_seq(prb); + if (prb_read_valid_info(prb, syslog_seq, &info, NULL)) { + if (info.seq != syslog_seq) { + /* messages are gone, move to first one */ + syslog_seq = info.seq; + syslog_partial = 0; + } + } else { syslog_partial = 0; } if (source == SYSLOG_FROM_PROC) { @@ -1643,7 +1649,6 @@ int do_syslog(int type, char __user *buf, int len, int source) error = prb_next_seq(prb) - syslog_seq; } else { bool time = syslog_partial ? syslog_time : printk_time; - struct printk_info info; unsigned int line_count; u64 seq; @@ -3429,9 +3434,11 @@ bool kmsg_dump_get_buffer(struct kmsg_dumper *dumper, bool syslog, goto out; logbuf_lock_irqsave(flags); - if (dumper->cur_seq < prb_first_valid_seq(prb)) { - /* messages are gone, move to first available one */ - dumper->cur_seq = prb_first_valid_seq(prb); + if (prb_read_valid_info(prb, dumper->cur_seq, &info, NULL)) { + if (info.seq != dumper->cur_seq) { + /* messages are gone, move to first available one */ + dumper->cur_seq = info.seq; + } } /* last entry */ -- 2.20.1