linux-kernel - Re: [PATCH tty-next v3 5/6] serial: 8250: Switch to nbcon console

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Message-ID: <ZyyM99jtCao_VmBU@pathway.suse.cz>
Date: Thu, 7 Nov 2024 10:48:39 +0100
From: Petr Mladek <pmladek@...e.com>
To: John Ogness <john.ogness@...utronix.de>
Cc: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jiri Slaby <jirislaby@...nel.org>,
	Sergey Senozhatsky <senozhatsky@...omium.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Esben Haabendal <esben@...nix.com>, linux-serial@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Geert Uytterhoeven <geert+renesas@...der.be>,
	Arnd Bergmann <arnd@...db.de>,
	Uwe Kleine-König <u.kleine-koenig@...gutronix.de>,
	Tony Lindgren <tony@...mide.com>,
	Rengarajan S <rengarajan.s@...rochip.com>,
	Peter Collingbourne <pcc@...gle.com>,
	Serge Semin <fancer.lancer@...il.com>,
	Lino Sanfilippo <l.sanfilippo@...bus.com>
Subject: Re: [PATCH tty-next v3 5/6] serial: 8250: Switch to nbcon console

On Mon 2024-10-28 14:28:35, John Ogness wrote:
> On 2024-10-25, Andy Shevchenko <andriy.shevchenko@...ux.intel.com> wrote:
> >> +/*
> >> + * Only to be used directly by the console write callbacks, which may not
> >> + * require the port lock. Use serial8250_clear_IER() instead for all other
> >> + * cases.
> >> + */
> >> +static void __serial8250_clear_IER(struct uart_8250_port *up)
> >>  {
> >>  	if (up->capabilities & UART_CAP_UUE)
> >>  		serial_out(up, UART_IER, UART_IER_UUE);
> >
> >>  		serial_out(up, UART_IER, 0);
> >>  }
> >>  
> >> +static inline void serial8250_clear_IER(struct uart_8250_port *up)
> >> +{
> >> +	__serial8250_clear_IER(up);
> >
> > Shouldn't this have a lockdep annotation to differentiate with the
> > above?
> 
> Yes, but the follow-up patch adds the annotation as a clean "revert
> patch". I can add a line about that in the commit message.
> 
> >> +static void serial8250_console_byte_write(struct uart_8250_port *up,
> >> +					  struct nbcon_write_context *wctxt)
> >> +{
> >> +	const char *s = READ_ONCE(wctxt->outbuf);
> >> +	const char *end = s + READ_ONCE(wctxt->len);
> >
> > Is there any possibility that outbuf value be changed before we get
> > the len and at the end we get the wrong pointer?
> 
> No. I was concerned about compiler optimization, since @outbuf can
> become NULL. However, it can only become NULL if ownership was
> transferred, and that is properly checked anyway. I will remove the
> READ_ONCE() usage for v4.

I agree that we do not need READ_ONCE() here.

Just to be sure that I understand it correctly.

The struct nbcon_write_context passed by *wctxt should be created on
stack of the caller. Only this process/interrupt context could change
it.

Namely, it might happen when nbcon_enter_unsafe() fails. It is done
later in this function by the code:

	while (!nbcon_enter_unsafe(wctxt))
		nbcon_reacquire_nobuf(wctxt);

and this function does not access *s or *end after this code.

Other CPUs could not change the structure in parallel

   => READ_ONCE() is not needed.


Just for completeness. The buffer could not disappear.
wctxt->outbuf always points to a static buffer.

Also the content of the buffer could not change if we read it only
after successful nbcon_enter_unsafe(). Only the panic CPU is allowed
to takeover the ownership in this case and it would use another static
buffer.

Best Regards,
Petr

PS: I do not have anything more to add for this patch. It seems to
    work work as expected.