Re: NPTL mutex and the scheduling priority

Jakub Jelinek
2006-06-12T12:44:19Z

On Mon, Jun 12, 2006 at 02:23:28PM +0200, Arjan van de Ven wrote:
> On Mon, 2006-06-12 at 17:10 +0900, Atsushi Nemoto wrote:
> > # This is a copy of message posted libc-alpha ML. I want to hear from
> > # kernel people too ...
> >
> > Hi. I found that it seems NPTL's mutex does not follow the scheduling
> > parameter. If some threads were blocked by getting a single
> > mutex_lock, I expect that a thread with highest priority got the lock
> > first, but current NPTL's behaviour is different.
> \
>
> you want to use the PI futexes that are in 2.6.17-rc5-mm tree

Even for normal mutices pthread_mutex_unlock and
pthread_cond_{signal,broadcast} is supposed to honor the RT priority and
scheduling policy when waking up:
http://www.opengroup.org/onlinepubs/009695399/functions/pthread_mutex_trylock.html
"If there are threads blocked on the mutex object referenced by mutex when
pthread_mutex_unlock() is called, resulting in the mutex becoming available,
the scheduling policy shall determine which thread shall acquire the mutex."
and similarly for condvars.
"Use PI" is not a valid answer for this.
Really FUTEX_WAKE/FUTEX_REQUEUE can't use a FIFO. I think there was a patch
floating around to use a plist there instead, which is one possibility,
another one is to keep the queue sorted by priority (and adjust whenever
priority changes - one thread can be waiting on at most one futex at a
time).

Jakub