Re: [PATCH] hotplug: Optimize {get,put}_online_cpus()

[Peter Zijlstra]

On Tue, Sep 24, 2013 at 08:00:05PM +0200, Oleg Nesterov wrote:
> On 09/24, Paul E. McKenney wrote:
> >
> > On Tue, Sep 24, 2013 at 07:06:31PM +0200, Oleg Nesterov wrote:
> > >
> > > If gcc can actually do something wrong, then I suspect this barrier()
> > > should be unconditional.
> >
> > If you are saying that there should be a barrier() on all return paths
> > from get_online_cpus(), I agree.
> 
> Paul, Peter, could you provide any (even completely artificial) example
> to explain me why do we need this barrier() ? I am puzzled. And
> preempt_enable() already has barrier...
> 
> 	get_online_cpus();
> 	do_something();
> 
> Yes, we need to ensure gcc doesn't reorder this code so that
> do_something() comes before get_online_cpus(). But it can't? At least
> it should check current->cpuhp_ref != 0 first? And if it is non-zero
> we do not really care, we are already in the critical section and
> this ->cpuhp_ref has only meaning in put_online_cpus().
> 
> Confused...


So the reason I put it in was because of the inline; it could possibly
make it do:

  test  0, current->cpuhp_ref
  je	label1:
  inc	current->cpuhp_ref

label2:
  do_something();

label1:
  inc	%gs:__preempt_count
  test	0, __cpuhp_writer
  jne	label3
  inc	%gs:__cpuhp_refcount
label5
  dec	%gs:__preempt_count
  je	label4
  jmp	label2
label3:
  call	__get_online_cpus();
  jmp	label5
label4:
  call	____preempt_schedule();
  jmp	label2

In which case the recursive fast path doesn't have a barrier() between
taking the ref and starting do_something().

I wanted to make absolutely sure nothing of do_something leaked before
the label2 thing. The other labels all have barrier() from the
preempt_count ops.
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