Re: [PATCH v3] ring-buffer: Simplify reservation with try_cmpxchg() loop

[Mathieu Desnoyers]

On 2024-02-20 09:19, Steven Rostedt wrote:
> On Mon, 19 Feb 2024 18:20:32 -0500
> Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
>
> > Instead of using local_add_return() to reserve the ring buffer data,
> > Mathieu Desnoyers suggested using local_cmpxchg(). This would simplify the
> > reservation with the time keeping code.
> >
> > Although, it does not get rid of the double time stamps (before_stamp and
> > write_stamp), using cmpxchg() does get rid of the more complex case when
> > an interrupting event occurs between getting the timestamps and reserving
> > the data, as when that happens, it just tries again instead of dealing
> > with it.
> >
> > Before we had:
> >
> > 	w = local_read(&tail_page->write);
> > 	/* get time stamps */
> > 	write = local_add_return(length, &tail_page->write);
> > 	if (write - length == w) {
> > 		/* do simple case */
> > 	} else {
> > 		/* do complex case */
> > 	}
> >
> > By switching the local_add_return() to a local_try_cmpxchg() it can now be:
> >
> > 	 w = local_read(&tail_page->write);
> >   again:
> > 	/* get time stamps */
> > 	if (!local_try_cmpxchg(&tail_page->write, &w, w + length))
> > 		goto again;
> >
> > 	 /* do simple case */
>
> Something about this logic is causing __rb_next_reserve() to sometimes
> always return -EAGAIN and triggering the:
>
>      RB_WARN_ON(cpu_buffer, ++nr_loops > 1000)
>
> Which disables the ring buffer.
>
> I'm not sure what it is, but until I do, I'm removing the patch from my
> queue.

Try resetting the info->add_timestamp flags to add_ts_default on goto again
within __rb_reserve_next().

Thanks,

Mathieu


--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com