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

Steven Rostedt
2024-02-20T14:17:38Z

On Mon, 19 Feb 2024 18:20:32 -0500
Steven Rostedt <rostedt@goodmis.org> 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.

-- Steve