Re: [Xen-devel] [PATCH v4 2/6] xen/rcu: don't use stop_machine_run() for rcu_barrier()

[Jürgen Groß <jgross@xxxxxxxx>]

On 10.03.20 17:37, Jan Beulich wrote:
> On 10.03.2020 17:34, Jürgen Groß wrote:
> > On 10.03.20 17:29, Jan Beulich wrote:
> > > On 10.03.2020 08:28, Juergen Gross wrote:
> > > > +void rcu_barrier(void)
> > > >    {
> > > > -    atomic_t cpu_count = ATOMIC_INIT(0);
> > > > -    return stop_machine_run(rcu_barrier_action, &cpu_count, NR_CPUS);
> > > > +    unsigned int n_cpus;
> > > > +
> > > > +    while ( !get_cpu_maps() )
> > > > +    {
> > > > +        process_pending_softirqs();
> > > > +        if ( !atomic_read(&cpu_count) )
> > > > +            return;
> > > > +
> > > > +        cpu_relax();
> > > > +    }
> > > > +
> > > > +    n_cpus = num_online_cpus();
> > > > +
> > > > +    if ( atomic_cmpxchg(&cpu_count, 0, n_cpus) == 0 )
> > > > +    {
> > > > +        atomic_add(n_cpus, &done_count);
> > > > +        cpumask_raise_softirq(&cpu_online_map, RCU_SOFTIRQ);
> > > > +    }
> > > > +
> > > > +    while ( atomic_read(&done_count) )
> > > Don't you leave a window for races here, in that done_count
> > > gets set to non-zero only after setting cpu_count? A CPU
> > > losing the cmpxchg attempt above may observe done_count
> > > still being zero, and hence exit without waiting for the
> > > count to actually _drop_ to zero.
> > This can only be a cpu not having joined the barrier handling, so it
> > will do that later.
> I'm afraid I don't understand - if two CPUs independently call
> rcu_barrier(), neither should fall through here without waiting
> at all, I would think?
Oh, good catch!

I have thought more about this problem and I think using counters only
for doing rendezvous accounting is rather risky. I'll have a try using
a cpumask instead.


Juergen

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