linux-kernel - Re: [PATCH next v4 1/2] lib/dump_stack: move cpu lock to printk.c

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Message-ID: <20210617093243.795b4853@gandalf.local.home>
Date:   Thu, 17 Jun 2021 09:32:43 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Stephen Boyd <swboyd@...omium.org>,
        Alexander Potapenko <glider@...gle.com>
Subject: Re: [PATCH next v4 1/2] lib/dump_stack: move cpu lock to printk.c

On Thu, 17 Jun 2021 11:56:50 +0206
John Ogness <john.ogness@...utronix.de> wrote:

> dump_stack() implements its own cpu-reentrant spinning lock to
> best-effort serialize stack traces in the printk log. However,
> there are other functions (such as show_regs()) that can also
> benefit from this serialization.
> 
> Move the cpu-reentrant spinning lock (cpu lock) into new helper
> functions printk_cpu_lock_irqsave()/printk_cpu_unlock_irqrestore()
> so that it is available for others as well. For !CONFIG_SMP the
> cpu lock is a NOP.
> 
> Note that having multiple cpu locks in the system can easily
> lead to deadlock. Code needing a cpu lock should use the
> printk cpu lock, since the printk cpu lock could be acquired
> from any code and any context.
> 
> Also note that it is not necessary for a cpu lock to disable
> interrupts. However, in upcoming work this cpu lock will be used
> for emergency tasks (for example, atomic consoles during kernel
> crashes) and any interruptions while holding the cpu lock should
> be avoided if possible.
> 
> Signed-off-by: John Ogness <john.ogness@...utronix.de>
> ---

Can we add this lock to early_printk() ?

This would make early_printk() so much more readable.

-- Steve

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 421c35571797..2b749c745c1f 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2259,6 +2259,7 @@ struct console *early_console;
 
 asmlinkage __visible void early_printk(const char *fmt, ...)
 {
+	unsigned long flags;
 	va_list ap;
 	char buf[512];
 	int n;
@@ -2270,7 +2271,9 @@ asmlinkage __visible void early_printk(const char *fmt, ...)
 	n = vscnprintf(buf, sizeof(buf), fmt, ap);
 	va_end(ap);
 
+	printk_cpu_lock_irqsave(flags);
 	early_console->write(early_console, buf, n);
+	printk_cpu_unlock_irqrestore(flags);
 }
 #endif