Re: [PATCH 7/9] sched: Add migrate_disable()

[Peter Zijlstra]

On Thu, Sep 24, 2020 at 12:53:25PM +0100, Valentin Schneider wrote:
> On 21/09/20 17:36, Peter Zijlstra wrote:
> 
> [...]
> 
> > +void migrate_enable(void)
> > +{
> > +	if (--current->migration_disabled)
> > +		return;
> > +
> > +	barrier();
> > +
> > +	if (p->cpus_ptr == &p->cpus_mask)
> > +		return;
> 
> If we get to here this means we're the migrate_enable() invocation that
> marks the end of the migration_disabled region. How can cpus_ptr already
> point back to current's cpus_mask?

It might never have been changed away.


	migrate_disable()
	  ->migration_disabled++;
	|	|
	|	|
	|	v
	|	migrate_disable_switch()
	|	  if (->cpus_ptr == &->cpus_mask)
	|	    __do_set_cpus_allowed(.new_mask = cpumask_of())
	|	|
	|	|
	v	v
	migrate_enable()
	  ->migration_disabled--;


Only if we've passed through a context switch between migrate_disable()
and migrate_enable() will the mask have been changed.