Re: 2.6.16 - sys_sched_getaffinity & hotplug

[Nathan Lynch <ntl@pobox.com>]

Ingo Molnar wrote:
> 
> > Jack wrote:
> > > Should the following change be made to sched_getaffinity(). 
> > > 
> > > Index: linux/kernel/sched.c
> > > ===================================================================
> > > --- linux.orig/kernel/sched.c	2006-01-25 08:50:21.401747695 -0600
> > > +++ linux/kernel/sched.c	2006-01-27 16:57:24.504871895 -0600
> > > @@ -4031,7 +4031,7 @@ long sched_getaffinity(pid_t pid, cpumas
> > >  		goto out_unlock;
> > >  
> > >  	retval = 0;
> > > -	cpus_and(*mask, p->cpus_allowed, cpu_possible_map);
> > > +	cpus_and(*mask, p->cpus_allowed, cpu_online_map);
> > 
> In any case, Jack's change looks reasonable and obviously correct.

Are you sure?  Assuming this change is in effect, consider the
following:

Task starts with default affinity.

Task does sched_getaffinity, stashes the result in saved_mask.

Task pins itself to one cpu and does some work.

Meanwhile, more cpus are brought online.

Task finishes work and does sched_setaffinity(saved_mask).

Task will never run on the new cpus.