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Date: Mon, 1 Mar 2021 17:16:34 +0100
From: Petr Mladek <pmladek@suse.com>
Subject: Re: [PATCH next v3 01/15] um: synchronize kmsg_dumper
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References: <20210225202438.28985-1-john.ogness@linutronix.de>
 <20210225202438.28985-2-john.ogness@linutronix.de>
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To: John Ogness <john.ogness@linutronix.de>
Cc: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>, Richard Weinberger <richard@nod.at>, Jeff Dike <jdike@addtoit.com>, linux-um@lists.infradead.org, linux-kernel@vger.kernel.org, Steven Rostedt <rostedt@goodmis.org>, Sergey Senozhatsky <sergey.senozhatsky@gmail.com>, Thomas Gleixner <tglx@linutronix.de>, Andy Shevchenko <andriy.shevchenko@linux.intel.com>, Thomas Meyer <thomas@m3y3r.de>, Anton Ivanov <anton.ivanov@cambridgegreys.com>

On Thu 2021-02-25 21:24:24, John Ogness wrote:
> The kmsg_dumper can be called from any context and CPU, possibly
> from multiple CPUs simultaneously. Since a static buffer is used
> to retrieve the kernel logs, this buffer must be protected against
> simultaneous dumping. Skip dumping if another context is already
> dumping.
> 
> Signed-off-by: John Ogness <john.ogness@linutronix.de>
> Reviewed-by: Petr Mladek <pmladek@suse.com>
> ---
>  arch/um/kernel/kmsg_dump.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/arch/um/kernel/kmsg_dump.c b/arch/um/kernel/kmsg_dump.c
> index 6516ef1f8274..4869e2cc787c 100644
> --- a/arch/um/kernel/kmsg_dump.c
> +++ b/arch/um/kernel/kmsg_dump.c
> @@ -1,5 +1,6 @@
>  // SPDX-License-Identifier: GPL-2.0
>  #include <linux/kmsg_dump.h>
> +#include <linux/spinlock.h>
>  #include <linux/console.h>
>  #include <linux/string.h>
>  #include <shared/init.h>
> @@ -9,6 +10,7 @@
>  static void kmsg_dumper_stdout(struct kmsg_dumper *dumper,
>  				enum kmsg_dump_reason reason)
>  {
> +	static DEFINE_SPINLOCK(lock);
>  	static char line[1024];
>  	struct console *con;
>  	size_t len = 0;
> @@ -29,11 +31,16 @@ static void kmsg_dumper_stdout(struct kmsg_dumper *dumper,
>  	if (con)
>  		return;
>  
> +	if (!spin_trylock(&lock))

I have almost missed this. It is wrong. The last version correctly
used

	if (!spin_trylock_irqsave(&lock, flags))

kmsg_dump(KMSG_DUMP_PANIC) is called in panic() with interrupts
disabled. We have to store the flags here.

Best Regards,
Petr

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