Re: [RFC PATCH] introduce sys_membarrier(): process-wide memory barrier (v5)

[Peter Zijlstra <peterz@infradead.org>]

On Wed, 2010-01-13 at 14:36 -0500, Mathieu Desnoyers wrote:
> * Peter Zijlstra (peterz@infradead.org) wrote:
> > On Tue, 2010-01-12 at 20:37 -0500, Mathieu Desnoyers wrote:
> > > +       for_each_cpu(cpu, tmpmask) {
> > > +               spin_lock_irq(&cpu_rq(cpu)->lock);
> > > +               mm = cpu_curr(cpu)->mm;
> > > +               spin_unlock_irq(&cpu_rq(cpu)->lock);
> > > +               if (current->mm != mm)
> > > +                       cpumask_clear_cpu(cpu, tmpmask);
> > > +       } 
> > 
> > Why not:
> > 
> >   rcu_read_lock();
> >   if (current->mm != cpu_curr(cpu)->mm)
> >     cpumask_clear_cpu(cpu, tmpmask);
> >   rcu_read_unlock();
> > 
> > the RCU read lock ensures the task_struct obtained remains valid, and it
> > avoids taking the rq->lock.
> > 
> 
> If we go for a simple rcu_read_lock, I think that we need a smp_mb()
> after switch_to() updates the current task on the remote CPU, before it
> returns to user-space. Do we have this guarantee for all architectures ?
> 
> So what I'm looking for, overall, is:
> 
> schedule()
>   ...
>   switch_mm()
>     smp_mb()
>     clear mm_cpumask
>     set mm_cpumask
>   switch_to()
>     update current task
>     smp_mb()
> 
> If we have that, then the rcu_read_lock should work.
> 
> What the rq lock currently gives us is the guarantee that if the current
> thread changes on a remote CPU while we are not holding this lock, then
> a full scheduler execution is performed, which implies a memory barrier
> if we change the current thread (it does, right ?).

I'm not quite seeing it, we have 4 possibilities, switches between
threads with:

 a) our mm, another mm

   - if we observe the former, we'll send an IPI (redundant)
   - if we observe the latter, the switch_mm will have issued an mb

 b) another mm, our mm

   - if we observe the former, we're good because the cpu didn't run our
     thread when we called sys_membarrier()
   - if we observe the latter, we'll send an IPI (redundant)

 c) our mm, our mm

   - no matter which task we observe, we'll match and send an IPI

 d) another mm, another mm

   - no matter which task we observe, we'll not match and not send an
     IPI.


Or am I missing something?