Date: Mon, 04 Oct 2004 13:34:38 -0400 From: Stephan Uphoff <ups@tree.com> To: John Baldwin <jhb@FreeBSD.org> Cc: "freebsd-arch@freebsd.org" <freebsd-arch@FreeBSD.org> Subject: Re: scheduler (sched_4bsd) questions Message-ID: <1096911278.44307.17.camel@palm.tree.com> In-Reply-To: <200410041131.35387.jhb@FreeBSD.org> References: <1095468747.31297.241.camel@palm.tree.com> <1096496057.3733.2163.camel@palm.tree.com> <1096603981.21577.195.camel@palm.tree.com> <200410041131.35387.jhb@FreeBSD.org>
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On Mon, 2004-10-04 at 11:31, John Baldwin wrote:
> On Friday 01 October 2004 12:13 am, Stephan Uphoff wrote:
> > On Wed, 2004-09-29 at 18:14, Stephan Uphoff wrote:
> > > I was looking at the MUTEX_WAKE_ALL undefined case when I used the
> > > critical section for turnstile_claim().
> > > However there are bigger problems with MUTEX_WAKE_ALL undefined
> > > so you are right - the critical section for turnstile_claim is pretty
> > > useless.
> >
> > Arghhh !!!
> >
> > MUTEX_WAKE_ALL is NOT an option in GENERIC.
> > I recall verifying that it is defined twice. Guess I must have looked at
> > the wrong source tree :-(
> > This means yes - we have bigger problems!
> >
> > Example:
> >
> > Thread A holds a mutex x contested by Thread B and C and has priority
> > pri(A).
> >
> > Thread C holds a mutex y and pri(B) < pri(C)
> >
> > Thread A releases the lock wakes thread B but lets C on the turnstile
> > wait queue.
> >
> > An interrupt thread I tries to lock mutex y owned by C.
> >
> > However priority inheritance does not work since B needs to run first to
> > take ownership of the lock.
> >
> > I is blocked :-(
>
> Ermm, if the interrupt happens after x is released then I's priority should
> propagate from I to C to B.
There is a hole after the mutex x is released by A - but before B can
claim the mutex. The turnstile for mutex x is unowned and interrupt
thread I when trying to donate its priority will run into:
if (td == NULL) {
/*
* This really isn't quite right. Really
* ought to bump priority of thread that
* next acquires the lock.
*/
return;
}
So B needs to run and acquire the mutex before priority inheritance
works again and does not get a priority boost to do so.
This is easy to fix and MUTEX_WAKE_ALL can be removed again at that time
- but my time budget is limited and Peter has an interesting bug left
that has priority.
> If the interrupt happens before x is released,
> then the final bit of propagate_priority() should handle it since it resorts
> the turnstile's thread queue so that C will be awakened rather than B.
Agreed.
Stephan
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