[lttng-dev] Quick questions about liburcu and RCU in general

[Richard Braun]

On Tue, May 07, 2013 at 07:59:14AM -0400, Mathieu Desnoyers wrote:
> > 1/ Why use poll instead of sched_yield in e.g. force_mb_all_readers ?
> > (I guess it's about portability and the effect is expected to be the same,
> > but is there another reason ?)
> 
> poll() allow us to do a millisecond-level wait (timer-based).
> sched_yield() is pretty much a scheduler hack that just says "be nice to
> other scheduled processes here". Quoting sched_yield(2):
> 
>        If the calling thread is the only thread in the highest  priority list
>        at that time, it will continue to run after a call to sched_yield().
> 
> This is a kind of behavior we don't want.

Right, I see. But then, poll enforces a minimum delay whatever happens
(which may be increased up to the scheduler time slice depending on the
kernel). I guess this isn't something we want either.

On the other hand, AIUI, poll is called only after all signals were
already sent, and if a receiving thread didn't update its need_mb yet,
so I expect it's a rare worst case that doesn't require more care.

-- 
Richard Braun