Re: NPTL mutex and the scheduling priority

Pierre Peiffer
2006-06-13T08:37:29Z

Sébastien Dugué a écrit :
> But maybe a better solution for condvars would be to implement
> something like a futex_requeue_pi() to handle the broadcast and
> only use PI futexes all along in glibc.
>
> Any ideas?

I'm currently thinking about it, and as far as I can see, it should be
technically feasible but not obvious.
In fact, PI-futex adds a rt-mutex behind each futex, when there are some
waiters. Each waiter is then queued two times: once in the chain list of
the hash-bucket, once in the (ordered) wait_list of the rt-mutex.

What we want, with a futex_requeue_pi, is a requeue of some tasks from
(futex1, rt_mutex1) to (futex2, rt_mutex2), respecting the wait_list
order of rt_mutex1.wait-list.
=> this needs something like a rt_mutex_requeue, and given an element of
rt_mutex1.wait_list, we need to retrieve its futex_q to requeue it to
the second hash-bucket chain (of futex2).

Moreover, we must take care of the case where the futex2 is not yet
locked (i.e. has no owner): there is not yet a pi_state nor a rt_mutex
associated with the futex2 ...

And during all of this, we must take care of several race conditions in
several places.

I'll continue my investigation, but I really wonder if futex_requeue_pi
will still be an "optimization" as it should be.

So comments from the experts are welcome ;-)

--
Pierre