linux-kernel - Re: [RFC PATCH v1 00/25] printk: new implementation

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Message-ID: <20190213014141.GB8097@jagdpanzerIV>
Date:   Wed, 13 Feb 2019 10:41:41 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Daniel Wang <wonderfly@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Alan Cox <gnomes@...rguk.ukuu.org.uk>,
        Jiri Slaby <jslaby@...e.com>,
        Peter Feiner <pfeiner@...gle.com>,
        linux-serial@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Subject: Re: [RFC PATCH v1 00/25] printk: new implementation

On (02/12/19 15:29), John Ogness wrote:
> - console_flush_on_panic() currently is a NOP. It is pretty clear how
>   this could be implemented if atomic_write was available. But if no
>   such console is registered, it is not clear what should be done. Is
>   this function really even needed?

If you now rely on a fully preemptible printk kthread to flush
pending logbuf messages, then console_flush_on_panic() is your
only chance to see those pending logbuf messages on the serial
console when the system dies.

Non-atomic consoles should become atomic once you call bust_spinlocks(1),
this is what we currently have:

	panic()
	bust_spinlocks(1)                       // sets oops_in_progress
	console_flush_on_panic()
	 call_console_drivers()
	  -> serial_driver_write()
	      if (oops_in_progress)
	        locked = spin_trylock_irqsave(&port->lock);
	      uart_console_write();
	      if (locked)
	        spin_unlock_irqrestore(&port->lock);

	-ss