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=-13.0 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, UNWANTED_LANGUAGE_BODY 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 D03E2C433E6 for ; Wed, 3 Mar 2021 14:06:38 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id A473C64ED0 for ; Wed, 3 Mar 2021 14:06:38 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1359292AbhCCOFk (ORCPT ); Wed, 3 Mar 2021 09:05:40 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:37964 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1842920AbhCCKWd (ORCPT ); Wed, 3 Mar 2021 05:22:33 -0500 Received: from galois.linutronix.de (Galois.linutronix.de [IPv6:2a0a:51c0:0:12e:550::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 844E4C08ED3F for ; Wed, 3 Mar 2021 02:15:34 -0800 (PST) From: John Ogness DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020; t=1614766533; 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: in-reply-to:in-reply-to:references:references; bh=NRhzEmfFktdT55ZMKpkoTASFREmaGJREj6FLiwVzcaE=; b=TW7qaEpNSsD9cTIlX2HtSbuNdnyYPSjge21x5JEKFhSAM5JdXFZeTRSsSPWY5+e6RIfdBG P+zfYKlDkzI2K7qGR5kYJZK7saEAtAeFcWbonL2O/27rtueaFFhjmLkm3ha4PLlAYnGS/0 7TbY4anpinLG059bPBHO52iBKjujjAjqoGRmtMmKwKPv5EtPMbPAy5YQwQTd3VSun7HgUp OpTo5XpS85WPHzXGl/b+wSS/sDYjnTwQclgz6NazDWOMYtAt0ThNimObupD5SUTRQrVco+ xn8+Uk1Q2Myo2B8/n4khcQQU6IusG4iCbSxIUMmXhaIjw2y4mFGqnRkBsRAg9w== DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020e; t=1614766533; 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: in-reply-to:in-reply-to:references:references; bh=NRhzEmfFktdT55ZMKpkoTASFREmaGJREj6FLiwVzcaE=; b=+pAj54WOhmHr1es2o0Fsj/2SyEfbc/T/o028pdoQnKNR+h1kXKrP7JPi8GAfGr1KIckWc3 HT8cllytrx6BQgAQ== To: Petr Mladek Cc: Sergey Senozhatsky , Sergey Senozhatsky , Steven Rostedt , Thomas Gleixner , linux-kernel@vger.kernel.org Subject: [PATCH next v4 08/15] printk: use seqcount_latch for clear_seq Date: Wed, 3 Mar 2021 11:15:21 +0100 Message-Id: <20210303101528.29901-9-john.ogness@linutronix.de> In-Reply-To: <20210303101528.29901-1-john.ogness@linutronix.de> References: <20210303101528.29901-1-john.ogness@linutronix.de> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org kmsg_dump_rewind_nolock() locklessly reads @clear_seq. However, this is not done atomically. Since @clear_seq is 64-bit, this cannot be an atomic operation for all platforms. Therefore, use a seqcount_latch to allow readers to always read a consistent value. Signed-off-by: John Ogness Reviewed-by: Petr Mladek --- kernel/printk/printk.c | 58 ++++++++++++++++++++++++++++++++++++------ 1 file changed, 50 insertions(+), 8 deletions(-) diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c index 2c8873fa2f29..1b4bb88c3547 100644 --- a/kernel/printk/printk.c +++ b/kernel/printk/printk.c @@ -402,8 +402,21 @@ static u64 console_seq; static u64 exclusive_console_stop_seq; static unsigned long console_dropped; -/* the next printk record to read after the last 'clear' command */ -static u64 clear_seq; +struct latched_seq { + seqcount_latch_t latch; + u64 val[2]; +}; + +/* + * The next printk record to read after the last 'clear' command. There are + * two copies (updated with seqcount_latch) so that reads can locklessly + * access a valid value. Writers are synchronized by @logbuf_lock. + */ +static struct latched_seq clear_seq = { + .latch = SEQCNT_LATCH_ZERO(clear_seq.latch), + .val[0] = 0, + .val[1] = 0, +}; #ifdef CONFIG_PRINTK_CALLER #define PREFIX_MAX 48 @@ -457,6 +470,31 @@ bool printk_percpu_data_ready(void) return __printk_percpu_data_ready; } +/* Must be called under logbuf_lock. */ +static void latched_seq_write(struct latched_seq *ls, u64 val) +{ + raw_write_seqcount_latch(&ls->latch); + ls->val[0] = val; + raw_write_seqcount_latch(&ls->latch); + ls->val[1] = val; +} + +/* Can be called from any context. */ +static u64 latched_seq_read_nolock(struct latched_seq *ls) +{ + unsigned int seq; + unsigned int idx; + u64 val; + + do { + seq = raw_read_seqcount_latch(&ls->latch); + idx = seq & 0x1; + val = ls->val[idx]; + } while (read_seqcount_latch_retry(&ls->latch, seq)); + + return val; +} + /* Return log buffer address */ char *log_buf_addr_get(void) { @@ -801,7 +839,7 @@ static loff_t devkmsg_llseek(struct file *file, loff_t offset, int whence) * like issued by 'dmesg -c'. Reading /dev/kmsg itself * changes no global state, and does not clear anything. */ - user->seq = clear_seq; + user->seq = latched_seq_read_nolock(&clear_seq); break; case SEEK_END: /* after the last record */ @@ -960,6 +998,9 @@ void log_buf_vmcoreinfo_setup(void) VMCOREINFO_SIZE(atomic_long_t); VMCOREINFO_TYPE_OFFSET(atomic_long_t, counter); + + VMCOREINFO_STRUCT_SIZE(latched_seq); + VMCOREINFO_OFFSET(latched_seq, val); } #endif @@ -1557,7 +1598,8 @@ static int syslog_print_all(char __user *buf, int size, bool clear) * Find first record that fits, including all following records, * into the user-provided buffer for this dump. */ - seq = find_first_fitting_seq(clear_seq, -1, size, true, time); + seq = find_first_fitting_seq(latched_seq_read_nolock(&clear_seq), -1, + size, true, time); prb_rec_init_rd(&r, &info, text, CONSOLE_LOG_MAX); @@ -1584,7 +1626,7 @@ static int syslog_print_all(char __user *buf, int size, bool clear) } if (clear) - clear_seq = seq; + latched_seq_write(&clear_seq, seq); logbuf_unlock_irq(); kfree(text); @@ -1594,7 +1636,7 @@ static int syslog_print_all(char __user *buf, int size, bool clear) static void syslog_clear(void) { logbuf_lock_irq(); - clear_seq = prb_next_seq(prb); + latched_seq_write(&clear_seq, prb_next_seq(prb)); logbuf_unlock_irq(); } @@ -3336,7 +3378,7 @@ void kmsg_dump(enum kmsg_dump_reason reason) dumper->active = true; logbuf_lock_irqsave(flags); - dumper->cur_seq = clear_seq; + dumper->cur_seq = latched_seq_read_nolock(&clear_seq); dumper->next_seq = prb_next_seq(prb); logbuf_unlock_irqrestore(flags); @@ -3534,7 +3576,7 @@ EXPORT_SYMBOL_GPL(kmsg_dump_get_buffer); */ void kmsg_dump_rewind_nolock(struct kmsg_dumper *dumper) { - dumper->cur_seq = clear_seq; + dumper->cur_seq = latched_seq_read_nolock(&clear_seq); dumper->next_seq = prb_next_seq(prb); } -- 2.20.1