linux-kernel - Re: [PATCH next v1 2/2] nmi_backtrace: use the printk cpu lock for show

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Message-ID: <YLZDd+Jo42yBWa9L@alley>
Date:   Tue, 1 Jun 2021 16:25:59 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org,
        "Paul E. McKenney" <paulmck@...nel.org>
Subject: Re: [PATCH next v1 2/2] nmi_backtrace: use the printk cpu lock for
 show_regs()

On Mon 2021-05-31 18:20:51, John Ogness wrote:
> dump_stack() uses the printk cpu lock to synchronize the stacktrace,
> but this can also be used for dumping the banner and regs.
> 
> Since the cpu lock allows recursive locking, it is not an issue to
> call dump_stack() with the printk cpu lock held.

This does not explain why it is serialized only in nmi_cpu_backtrace().

It would be better to serialize all show_regs() calls. But it would
require updating all the arch-specific implementations.

I know that this patch is a pre-requisite for another patchset
removing printk_safe(). Where printk_safe() is serializing
the backtraces from all CPUs at the moment.

IMHO, it is perfectly fine synchronize only nmi_cpu_backtrace() for
now. It is the most important use-case where show_regs() calls could
get messed easily. But I suggest to do it in the patchset removing
printk_safe(). Also we need to explain in the commit message the relation
to printk_safe().

Best Regards,
Petr