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 B64CAC433DB for ; Thu, 11 Feb 2021 18:09:08 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 1CEA360C3E for ; Thu, 11 Feb 2021 18:08:57 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231940AbhBKR75 (ORCPT ); Thu, 11 Feb 2021 12:59:57 -0500 Received: from Galois.linutronix.de ([193.142.43.55]:40266 "EHLO galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231937AbhBKRch (ORCPT ); Thu, 11 Feb 2021 12:32:37 -0500 From: John Ogness DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020; t=1613064712; 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=JMWNWeCK4yV3tnqhinoR6R2uMPrJnIXKAJ98gW1Wo7g=; b=R2qZqaowSFUrPxp5l9Jx+MNWhM5Ua19fBXl6lUu6JilAC4l2md5A26FQUXwXeirhXKyA/O fgiF7N6SMUn7TlRDf/G19ek0VXCFY4UwY4KeDFJPvcSyuYtXI/R/FG+D1l5KIcbTryi4lL voT2Qk0NwbxCmWh/mlKvXo9TIPSKZnt6Z28Qavcr9PmRrelNwZLtc+Zm+6AK1hV2wVCcJe hhx6vjcqzR8qturdn6NI23u2AOnZ287/535EJvfMLlaKeONcpcBEt+2a1glvchg41VGr4z xJjEaZ/qY3XLB/OSWVVqrloOfCpQXO5mRClHonL1K78mXNE45NPfyM9CmozR7Q== DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020e; t=1613064712; 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=JMWNWeCK4yV3tnqhinoR6R2uMPrJnIXKAJ98gW1Wo7g=; b=i7osXWI4JvakixBV3ll9Ikb0s6a3vScMqL+XMYV38bH9+xQ6qodUnUFDoNcvk2I7Z05qUN FFwhPiZGeU+78CBQ== To: Petr Mladek Cc: Sergey Senozhatsky , Sergey Senozhatsky , Steven Rostedt , "J. Avila" , kernel test robot , linux-kernel@vger.kernel.org Subject: [PATCH v2] printk: avoid prb_first_valid_seq() where possible Date: Thu, 11 Feb 2021 18:37:52 +0106 Message-Id: <20210211173152.1629-1-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-20210211 v2: Abort and report no unread messages if SYSLOG_ACTION_SIZE_UNREAD fails to read the current or any newer record. kernel/printk/printk.c | 28 ++++++++++++++++++---------- 1 file changed, 18 insertions(+), 10 deletions(-) diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c index 5a95c688621f..575a34b88936 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 +1631,14 @@ 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)) { + if (!prb_read_valid_info(prb, syslog_seq, &info, NULL)) { + /* No unread messages. */ + logbuf_unlock_irq(); + return 0; + } + if (info.seq != syslog_seq) { /* messages are gone, move to first one */ - syslog_seq = prb_first_valid_seq(prb); + syslog_seq = info.seq; syslog_partial = 0; } if (source == SYSLOG_FROM_PROC) { @@ -1643,7 +1650,6 @@ 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 +3435,11 @@ 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