On Fri, Jan 25, 2013 at 08:11:39PM +0100, Oleg Nesterov wrote:
> On 01/25, Oleg Nesterov wrote:
> >
> > > +int percpu_ref_kill(struct percpu_ref *ref)
> > > +{
> > > ...
> > > + if (status == PCPU_REF_PTR) {
> > > + unsigned count = 0, cpu;
> > > +
> > > + synchronize_rcu();
> > > +
> > > + for_each_possible_cpu(cpu)
> > > + count += *per_cpu_ptr((unsigned __percpu *) pcpu_count,
> > > cpu);
> > > +
> > > + pr_debug("global %lli pcpu %i",
> > > + atomic64_read(&ref->count) & PCPU_COUNT_MASK,
> > > + (int) count);
> > > +
> > > + atomic64_add((int) count, &ref->count);
> > > + smp_wmb();
> > > + /* Between setting global count and setting PCPU_REF_DEAD */
> > > + ref->pcpu_count = PCPU_REF_DEAD;
> >
> > The coment explains what the code does, but not why ;)
> >
> > I guess this is for percpu_ref_put(), and this wmb() pairs with implicit
> > mb() implied by atomic64_dec_return().
>
> Hmm. Most probably I missed something, but it seems we need another
> synchronize_rcu() _after_ we set PCPU_REF_DEAD.
Yeah, correct - documentation bug.
I originally had the synchronize_rcu() there, but this is called by
exit_aio() -> kill_ioctx() when we're killing a process, and Ben LaHaise
pointed out that was less than ideal if a process had a bunch of ioctxs
- so I left the second one out there so the caller would have the option
of using call_rcu() instead.
> To simplify, suppose that percpu_ref_put() is never called directly but
> we have
>
> void put_and_dsetroy(...)
> {
> if (percpu_ref_put(...))
> destroy(...);
> }
>
> Suppose that ref->count == 2 after atomic64_add() above. IOW, we have
> a "master" reference for _kill() and someone else did _get.
>
> So the caller does
>
> percpu_ref_kill();
> put_and_dsetroy();
>
> And this can race with another holder which drops the last reference,
> its put_and_dsetroy() can see PCPU_REF_DYING and return false.
>
> Or I misunderstood the code/interface?
Nope, nailed it :) That should _definitely_ be in the documentation.
Actually - I think it'd be better to have the default percpu_ref_kill()
do the second synchronize_rcu(), and have an unsafe version that skips
it.
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