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

[John Ogness <john.ogness@linutronix.de>]

On 2023-03-28, Petr Mladek <pmladek@suse.com> wrote:
>> +	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.

First off, I need to post the latest version of the 8250-POC patch. It
is not officially part of this series and is still going through changes
for the PREEMPT_RT tree. I will post the latest version directly after
answering this email.

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

If an atomic context loses ownership while doing certain activities, it
may need to re-acquire ownership in order to finish or cleanup what it
started.

> Why do we need a copy?

When ownership is lost, the context is cleared. In order to re-acquire,
an original copy of the context is needed. There is no technical reason
to clear the context, so maybe the context should not be cleared after a
takeover. Otherwise, many drivers will need to implement the "backup
copy" solution.

> And why we need to reacquire it?

In this particular case the context has disabled interrupts. No other
context will re-enable interrupts because the driver is implemented such
that the one who disables is the one who enables. So this context must
re-acquire ownership in order to re-enable interrupts.

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

No. It is only about re-enabling interrupts. The concept of unsafe is
not really relevant if a hostile takeover during unsafe occurs. In that
case it becomes a "hope and pray" effort at the end of panic().

John