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=-15.8 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 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 B7B6FC4338F for ; Thu, 5 Aug 2021 12:17:06 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 89CD060F02 for ; Thu, 5 Aug 2021 12:17:06 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S241246AbhHEMRT (ORCPT ); Thu, 5 Aug 2021 08:17:19 -0400 Received: from smtp-out2.suse.de ([195.135.220.29]:54012 "EHLO smtp-out2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S235505AbhHEMRR (ORCPT ); Thu, 5 Aug 2021 08:17:17 -0400 Received: from relay2.suse.de (relay2.suse.de [149.44.160.134]) by smtp-out2.suse.de (Postfix) with ESMTP id 8751220211; Thu, 5 Aug 2021 12:17:02 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1628165822; h=from:from:reply-to: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=bxW42qJCGB2Umu46WQoAh/lTQsKPbKsX5IYw5Nj12Vs=; b=OzK/3B1+nUNHT1refnTEG7vUPsxyVv1i6R5mKpzCf9zlpjABX4uhwRBmVeUOXX99PaOyTf dW3naEvJ5IHVOity5UCfixm+QL2Ro/DNZsIe2Gy21iqkznk4FTxeYy2EfZ4+NYj60gD/c+ /LppcSePhLwVFD+BOaW+IY9SCVA5KF4= Received: from suse.cz (unknown [10.100.224.162]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by relay2.suse.de (Postfix) with ESMTPS id 5046EA3B94; Thu, 5 Aug 2021 12:17:02 +0000 (UTC) Date: Thu, 5 Aug 2021 14:16:59 +0200 From: Petr Mladek To: John Ogness Cc: Sergey Senozhatsky , Steven Rostedt , Thomas Gleixner , linux-kernel@vger.kernel.org Subject: Re: [PATCH printk v1 06/10] printk: use seqcount_latch for console_seq Message-ID: References: <20210803131301.5588-1-john.ogness@linutronix.de> <20210803131301.5588-7-john.ogness@linutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210803131301.5588-7-john.ogness@linutronix.de> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue 2021-08-03 15:18:57, John Ogness wrote: > In preparation for synchronous printing, change @console_seq to use > seqcount_latch so that it can be read without requiring @console_sem. > > Signed-off-by: John Ogness > --- > kernel/printk/printk.c | 73 ++++++++++++++++++++++++------------------ > 1 file changed, 41 insertions(+), 32 deletions(-) > > diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c > index d07d98c1e846..f8f46d9fba9b 100644 > --- a/kernel/printk/printk.c > +++ b/kernel/printk/printk.c > @@ -489,9 +489,7 @@ static u64 syslog_seq; > static size_t syslog_partial; > static bool syslog_time; > > -/* All 3 protected by @console_sem. */ > -/* the next printk record to write to the console */ > -static u64 console_seq; > +/* Both protected by @console_sem. */ > static u64 exclusive_console_stop_seq; > static unsigned long console_dropped; > > @@ -500,6 +498,17 @@ struct latched_seq { > u64 val[2]; > }; > > +/* > + * The next printk record to write to the console. There are two > + * copies (updated with seqcount_latch) so that reads can locklessly > + * access a valid value. Writers are synchronized by @console_sem. > + */ > +static struct latched_seq console_seq = { > + .latch = SEQCNT_LATCH_ZERO(console_seq.latch), > + .val[0] = 0, > + .val[1] = 0, > +}; > + > /* > * The next printk record to read after the last 'clear' command. There are > * two copies (updated with seqcount_latch) so that reads can locklessly > @@ -563,7 +572,7 @@ bool printk_percpu_data_ready(void) > return __printk_percpu_data_ready; > } > > -/* Must be called under syslog_lock. */ > +/* Must be called under associated write-protection lock. */ > static void latched_seq_write(struct latched_seq *ls, u64 val) > { > raw_write_seqcount_latch(&ls->latch); > @@ -2405,9 +2414,9 @@ EXPORT_SYMBOL(_printk); > > #define prb_read_valid(rb, seq, r) false > #define prb_first_valid_seq(rb) 0 > +#define latched_seq_read_nolock(seq) 0 > +#define latched_seq_write(dst, src) > > -static u64 syslog_seq; > -static u64 console_seq; > static u64 exclusive_console_stop_seq; > static unsigned long console_dropped; > > @@ -2735,7 +2744,7 @@ void console_unlock(void) > bool do_cond_resched, retry; > struct printk_info info; > struct printk_record r; > - u64 __maybe_unused next_seq; > + u64 seq; > > if (console_suspended) { > up_console_sem(); > @@ -2779,12 +2788,14 @@ void console_unlock(void) > size_t len; > > skip: > - if (!prb_read_valid(prb, console_seq, &r)) > + seq = latched_seq_read_nolock(&console_seq); > + if (!prb_read_valid(prb, seq, &r)) > break; > > - if (console_seq != r.info->seq) { > - console_dropped += r.info->seq - console_seq; > - console_seq = r.info->seq; > + if (seq != r.info->seq) { > + console_dropped += r.info->seq - seq; > + latched_seq_write(&console_seq, r.info->seq); > + seq = r.info->seq; > } > > if (suppress_message_printing(r.info->level)) { > @@ -2793,13 +2804,13 @@ void console_unlock(void) > * directly to the console when we received it, and > * record that has level above the console loglevel. > */ > - console_seq++; > + latched_seq_write(&console_seq, seq + 1); > goto skip; > } > > /* Output to all consoles once old messages replayed. */ > if (unlikely(exclusive_console && > - console_seq >= exclusive_console_stop_seq)) { > + seq >= exclusive_console_stop_seq)) { > exclusive_console = NULL; > } > > @@ -2820,7 +2831,7 @@ void console_unlock(void) > len = record_print_text(&r, > console_msg_format & MSG_FORMAT_SYSLOG, > printk_time); > - console_seq++; > + latched_seq_write(&console_seq, seq + 1); > > /* > * While actively printing out messages, if another printk() > @@ -2848,9 +2859,6 @@ void console_unlock(void) > cond_resched(); > } > > - /* Get consistent value of the next-to-be-used sequence number. */ > - next_seq = console_seq; > - > console_locked = 0; > up_console_sem(); > > @@ -2860,7 +2868,7 @@ void console_unlock(void) > * there's a new owner and the console_unlock() from them will do the > * flush, no worries. > */ > - retry = prb_read_valid(prb, next_seq, NULL); > + retry = prb_read_valid(prb, latched_seq_read_nolock(&console_seq), NULL); > if (retry && console_trylock()) > goto again; > } > @@ -2912,18 +2920,19 @@ void console_unblank(void) > */ > void console_flush_on_panic(enum con_flush_mode mode) > { > - /* > - * If someone else is holding the console lock, trylock will fail > - * and may_schedule may be set. Ignore and proceed to unlock so > - * that messages are flushed out. As this can be called from any > - * context and we don't want to get preempted while flushing, > - * ensure may_schedule is cleared. > - */ > - console_trylock(); > - console_may_schedule = 0; > - > - if (mode == CONSOLE_REPLAY_ALL) > - console_seq = prb_first_valid_seq(prb); > + if (console_trylock()) { > + if (mode == CONSOLE_REPLAY_ALL) > + latched_seq_write(&console_seq, prb_first_valid_seq(prb)); I am scratching my head about this. Of course, latched_seq_write() does not guarantee the result when the console lock it taken by another process. But console_lock(), called below, will call latched_seq_write() anyway. Also CONSOLE_REPLAY_ALL is used by panic_print_sys_info(). It is called the following way: void panic(const char *fmt, ...) { [...] debug_locks_off(); console_flush_on_panic(CONSOLE_FLUSH_PENDING); panic_print_sys_info(); [...] } On one hand, console_flush_on_panic(CONSOLE_FLUSH_PENDING) will most likely take over the console lock even when it was taken by another CPU before. And the 2nd console_flush_on_panic() called from panic_print_sys_info() will not even notice. On the other hand, CONSOLE_REPLAY_ALL would not even try to reply the log when the console log was not available. The risk of broken console_seq is neglible. console_unlock() should be safe even with invalid console_seq. My opinion: I suggest to keep the original logic and maybe add some comment: void console_flush_on_panic(enum con_flush_mode mode) { /* * If someone else is holding the console lock, trylock will fail * and may_schedule may be set. Ignore and proceed to unlock so * that messages are flushed out. As this can be called from any * context and we don't want to get preempted while flushing, * ensure may_schedule is cleared. */ console_trylock(); console_may_schedule = 0; /* * latched_seq_write() does not guarantee consistent values * when console_trylock() failed. But this is the best effort. * console_unlock() will update anyway console_seq. prb_read_valid() * handles even invalid sequence numbers. */ if (mode == CONSOLE_REPLAY_ALL) latched_seq_write(&console_seq, prb_first_valid_seq(prb)); console_unlock(); } Best Regards, Petr