linux-kernel - Re: [PATCH printk v6 19/30] printk: nbcon: Provide function to flush using write

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Date: Tue, 28 May 2024 11:30:37 +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 19/30] printk: nbcon: Provide function to flush
 using write_atomic()

On Mon 2024-05-27 08:43:38, John Ogness wrote:
> From: Thomas Gleixner <tglx@...utronix.de>
> 
> Provide nbcon_atomic_flush_pending() to perform flushing of all
> registered nbcon consoles using their write_atomic() callback.
> 
> Unlike console_flush_all(), nbcon_atomic_flush_pending() will
> only flush up through the newest record at the time of the
> call. This prevents a CPU from printing unbounded when other
> CPUs are adding records. If new records are added while
> flushing, it is expected that the dedicated printer threads
> will print those records. If the printer thread is not
> available (which is always the case at this point in the
> rework), nbcon_atomic_flush_pending() _will_ flush all records
> in the ringbuffer.
> 
> Unlike console_flush_all(), nbcon_atomic_flush_pending() will
> fully flush one console before flushing the next. This helps to
> guarantee that a block of pending records (such as a stack
> trace in an emergency situation) can be printed atomically at
> once before releasing console ownership.
> 
> nbcon_atomic_flush_pending() is safe in any context because it
> uses write_atomic() and acquires with unsafe_takeover disabled.
> 
> Use it in console_flush_on_panic() before flushing legacy
> consoles. The legacy write() callbacks are not fully safe when
> oops_in_progress is set.
> 
> Also use it in nbcon_device_release() to flush records added
> while the driver had the console locked to perform non-printing
> operations.
> 
> Co-developed-by: John Ogness <john.ogness@...utronix.de>
> Signed-off-by: John Ogness <john.ogness@...utronix.de>
> Signed-off-by: Thomas Gleixner (Intel) <tglx@...utronix.de>

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

Best Regards,
Petr