locking API: was: [PATCH printk v1 00/18] serial: 8250: implement non-BKL console

[Petr Mladek <pmladek@suse.com>]

On Thu 2023-03-02 21:04:50, John Ogness wrote:
> Implement the necessary callbacks to allow the 8250 console driver
> to perform as a non-BKL console. Remove the implementation for the
> legacy console callback (write) and add implementations for the
> non-BKL consoles (write_atomic, write_thread, port_lock) and add
> CON_NO_BKL to the initial flags.
> 
> This is an all-in-one commit meant only for testing the new printk
> non-BKL infrastructure. It is not meant to be included mainline in
> this form. In particular, it includes mainline driver fixes that
> need to be submitted individually.
> 
> Although non-BKL consoles can coexist with legacy consoles, you
> will only receive all the benefits of the non-BKL consoles, if
> this console driver is the only console. That means no netconsole,
> no tty1, no earlyprintk, no earlycon. Just the uart8250.
> 
> For example: console=ttyS0,115200
> 
> --- a/drivers/tty/serial/8250/8250_port.c
> +++ b/drivers/tty/serial/8250/8250_port.c
> +static void atomic_console_reacquire(struct cons_write_context *wctxt,
> +				     struct cons_write_context *wctxt_init)
> +{
> +	memcpy(wctxt, wctxt_init, sizeof(*wctxt));
> +	while (!console_try_acquire(wctxt)) {
> +		cpu_relax();
> +		memcpy(wctxt, wctxt_init, sizeof(*wctxt));
> +	}
> +}
> +
>  /*
> - * Print a string to the serial port using the device FIFO
> - *
> - * It sends fifosize bytes and then waits for the fifo
> - * to get empty.
> + * It should be possible to support a hostile takeover in an unsafe
> + * section if it is write_atomic() that is being taken over. But where
> + * to put this policy?
>   */
> -static void serial8250_console_fifo_write(struct uart_8250_port *up,
> -					  const char *s, unsigned int count)
> +bool serial8250_console_write_atomic(struct uart_8250_port *up,
> +				     struct cons_write_context *wctxt)
>  {
> -	int i;
> -	const char *end = s + count;
> -	unsigned int fifosize = up->tx_loadsz;
> -	bool cr_sent = false;
> -
> -	while (s != end) {
> -		wait_for_lsr(up, UART_LSR_THRE);
> -
> -		for (i = 0; i < fifosize && s != end; ++i) {
> -			if (*s == '\n' && !cr_sent) {
> -				serial_out(up, UART_TX, '\r');
> -				cr_sent = true;
> -			} else {
> -				serial_out(up, UART_TX, *s++);
> -				cr_sent = false;
> -			}
> +	struct cons_write_context wctxt_init = {};
> +	struct cons_context *ctxt_init = &ACCESS_PRIVATE(&wctxt_init, ctxt);
> +	struct cons_context *ctxt = &ACCESS_PRIVATE(wctxt, ctxt);
> +	bool can_print = true;
> +	unsigned int ier;
> +
> +	/* With write_atomic, another context may hold the port->lock. */
> +
> +	ctxt_init->console = ctxt->console;
> +	ctxt_init->prio = ctxt->prio;
> +	ctxt_init->thread = ctxt->thread;
> +
> +	touch_nmi_watchdog();
> +
> +	/*
> +	 * Enter unsafe in order to disable interrupts. If the console is
> +	 * lost before the interrupts are disabled, bail out because another
> +	 * context took over the printing. If the console is lost after the
> +	 * interrutps are disabled, the console must be reacquired in order
> +	 * to re-enable the interrupts. However in that case no printing is
> +	 * allowed because another context took over the printing.
> +	 */
> +
> +	if (!console_enter_unsafe(wctxt))
> +		return false;
> +
> +	if (!__serial8250_clear_IER(up, wctxt, &ier))
> +		return false;
> +
> +	if (console_exit_unsafe(wctxt)) {
> +		can_print = atomic_print_line(up, wctxt);
> +		if (!can_print)
> +			atomic_console_reacquire(wctxt, &wctxt_init);

I am trying to review the 9th patch adding console_can_proceed(),
console_enter_unsafe(), console_exit_unsafe() API. And I wanted
to see how the struct cons_write_context was actually used.

I am confused now. I do not understand the motivation for the extra
@wctxt_init copy and atomic_console_reacquire().

Why do we need a copy? And why we need to reacquire it?

My feeling is that it is needed only to call
console_exit_unsafe(wctxt) later. Or do I miss anything?

> +
> +		if (can_print) {
> +			can_print = console_can_proceed(wctxt);
> +			if (can_print)
> +				wait_for_xmitr(up, UART_LSR_BOTH_EMPTY);
> +			else
> +				atomic_console_reacquire(wctxt, &wctxt_init);
> +		}
> +	} else {
> +		atomic_console_reacquire(wctxt, &wctxt_init);
> +	}
> +
> +	/*
> +	 * Enter unsafe in order to enable interrupts. If the console is
> +	 * lost before the interrupts are enabled, the console must be
> +	 * reacquired in order to re-enable the interrupts.
> +	 */
> +
> +	for (;;) {
> +		if (console_enter_unsafe(wctxt) &&
> +		    __serial8250_set_IER(up, wctxt, ier)) {
> +			break;
>  		}
> +
> +		/* HW-IRQs still disabled. Reacquire to enable them. */
> +		atomic_console_reacquire(wctxt, &wctxt_init);
>  	}
> +
> +	console_exit_unsafe(wctxt);
> +
> +	return can_print;
>  }

Best Regards,
Petr