linux-kernel - Re: [PATCH printk v3 4/7] printk: Do not take console lock for console_flush_on

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Message-ID: <ZLZpW1PKsL8QlsxI@alley>
Date:   Tue, 18 Jul 2023 12:28:43 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Sergey Senozhatsky <senozhatsky@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH printk v3 4/7] printk: Do not take console lock for
 console_flush_on_panic()

On Mon 2023-07-17 21:52:04, John Ogness wrote:
> Currently console_flush_on_panic() will attempt to acquire the
> console lock when flushing the buffer on panic. If it fails to
> acquire the lock, it continues anyway because this is the last
> chance to get any pending records printed.
> 
> The reason why the console lock was attempted at all was to
> prevent any other CPUs from acquiring the console lock for
> printing while the panic CPU was printing. But as of the
> previous commit, non-panic CPUs will no longer attempt to
> acquire the console lock in a panic situation. Therefore it is
> no longer strictly necessary for a panic CPU to acquire the
> console lock.
> 
> Avoiding taking the console lock when flushing in panic has
> the additional benefit of avoiding possible deadlocks due to
> semaphore usage in NMI context (semaphores are not NMI-safe)
> and avoiding possible deadlocks if another CPU accesses the
> semaphore and is stopped while holding one of the semaphore's
> internal spinlocks.
> 
> Signed-off-by: John Ogness <john.ogness@...utronix.de>

Reviewed-by: Petr Mladek <pmladek@...e.com>

Best Regards,
Petr