linux-kernel - Re: [PATCH printk v6 09/17] printk: nbcon: Rely on kthreads for normal operation

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Message-ID: <ZthhHrESqU9KFXrz@pathway.suse.cz>
Date: Wed, 4 Sep 2024 15:31:10 +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 v6 09/17] printk: nbcon: Rely on kthreads for
 normal operation

On Wed 2024-09-04 14:11:28, John Ogness wrote:
> Once the kthread is running and available
> (i.e. @printk_kthreads_running is set), the kthread becomes
> responsible for flushing any pending messages which are added
> in NBCON_PRIO_NORMAL context. Namely the legacy
> console_flush_all() and device_release() no longer flush the
> console. And nbcon_atomic_flush_pending() used by
> nbcon_cpu_emergency_exit() no longer flushes messages added
> after the emergency messages.
> 
> The console context is safe when used by the kthread only when
> one of the following conditions are true:
> 
>   1. Other caller acquires the console context with
>      NBCON_PRIO_NORMAL with preemption disabled. It will
>      release the context before rescheduling.
> 
>   2. Other caller acquires the console context with
>      NBCON_PRIO_NORMAL under the device_lock.
> 
>   3. The kthread is the only context which acquires the console
>      with NBCON_PRIO_NORMAL.
> 
> This is satisfied for all atomic printing call sites:
> 
> nbcon_legacy_emit_next_record() (#1)
> 
> nbcon_atomic_flush_pending_con() (#1)
> 
> nbcon_device_release() (#2)
> 
> It is even double guaranteed when @printk_kthreads_running
> is set because then _only_ the kthread will print for
> NBCON_PRIO_NORMAL. (#3)
> 
> Signed-off-by: John Ogness <john.ogness@...utronix.de>

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

Best Regards,
Petr