On Fri, 20 Dec 2013 18:42:00 +0100
Peter Zijlstra <pet...@infradead.org> wrote:
> On Fri, Dec 20, 2013 at 12:37:07PM -0500, Steven Rostedt wrote:
> > On Fri, 20 Dec 2013 18:13:43 +0100
> > Peter Zijlstra <pet...@infradead.org> wrote:
> >
> >
> > > @@ -4985,6 +4942,23 @@ migration_call(struct notifier_block *nf
> > > unsigned long flags;
> > > struct rq *rq = cpu_rq(cpu);
> > >
> > > + switch (action) {
> > > + case CPU_DOWN_PREPARE: /* explicitly allow suspend */
> > > + {
> > > + struct dl_bw *dl_b = dl_bw_of(cpu);
> > > + int cpus = dl_bw_cpus(cpu);
> > > + bool overflow;
> > > +
> > > + raw_spin_lock_irqsave(&dl_b->lock, flags);
> > > + overflow = __dl_overflow(dl_b, cpus-1, 0, 0);
> > > + raw_spin_unlock_irqrestore(&dl_b->lock, flags);
> > > +
> > > + if (overflow)
> > > + return notifier_from_errno(-EBUSY);
> >
> > Is it possible to have a race here to create a new deadline task that
> > may work with cpus but not cpus-1? That is, if a new deadline task is
> > currently being created as a CPU is going offline, this check happens
> > first while the creation is spinning on the dl_b->lock, and it sets
> > overflow to false, then once the lock is released, the new deadline
> > task makes the condition true.
> >
> > Should the system call have a get_online_cpus() somewhere?
>
> No, should be all good; the entire admission control is serialized by
> that dl_b->lock, and its a raw_spin_lock (as can be seen from the above)
> which already very much excludes hotplug.
I'm saying what stops this?
CPU 0 CPU 1
----- -----
sched_setattr()
dl_overflow()
cpus = __dl_span_weight()
cpu_down()
raw_spin_lock()
raw_spin_lock() /* blocks */
overflow = __dl_overflow(cpus-1);
raw_spin_unlock();
/* gets lock */
__dl_overflow(cpus) /* all OK! */
/* cpus goes to cpus - 1 making
__dl_overflow() not OK anymore */
-- Steve
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