linux-kernel - Re: NPTL mutex and the scheduling priority

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Message-ID: <20060907083244.GA12531@devserv.devel.redhat.com>
Date:	Thu, 7 Sep 2006 04:32:44 -0400
From:	Jakub Jelinek <jakub@...hat.com>
To:	Atsushi Nemoto <anemo@....ocn.ne.jp>
Cc:	sebastien.dugue@...l.net, arjan@...radead.org, mingo@...hat.com,
	linux-kernel@...r.kernel.org, pierre.peiffer@...l.net,
	Ulrich Drepper <drepper@...hat.com>
Subject: Re: NPTL mutex and the scheduling priority

On Thu, Sep 07, 2006 at 05:11:58PM +0900, Atsushi Nemoto wrote:
> Three months after, I have tried kernel 2.6.18 with recent glibc.  I
> got desired results for pthread_mutex_unlock and
> pthread_cond_broadcast, with PI-mutex.
> 
> But pthread_cond_signal and sem_post still wakeup a thread in FIFO
> order, as you can guess.
> 
> With the plist patch (applied by hand), I can get desired behavior.
> Thank you.  But It seems the patch lacks reordering on priority
> changes.

Yes, either something like the plist patch for FUTEX_WAKE etc. or, if that
proves to be too slow for the usual case (non-RT threads), FIFO wakeup
initially and conversion to plist wakeup whenever first waiter with realtime
priority is added, is still needed.  That will cure e.g. non-PI
pthread_mutex_unlock and sem_post.  For pthread_cond_{signal,broadcast} we
need further kernel changes, so that the condvar's internal lock can be
always a PI lock.

> <off_topic>
> BTW, If I tried to create a PI mutex on a kernel without PI futex
> support, pthread_mutexattr_setprotocol(PTHREAD_PRIO_INHERIT) returned
> 0 and pthread_mutex_init() returned ENOTSUP.  This is not a right
> behavior according to the manual ...
> </off_topic>

Why?
POSIX doesn't forbid ENOTSUP in pthread_mutex_init to my knowledge.

	Jakub
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