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

[Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>]

* 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 ?).

Thanks,

Mathieu


-- 
Mathieu Desnoyers
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