The attached test program fails on a dual core (and probably SMP) 
machine on x86-64.  Depending on where the thread starts, in one of the 
iterations the sched_setffinity() call succeeds but then sched_getcpu() 
fails to report the correct CPU.

In set_cpus_allowed migrate_task() is called if the new CPU set does not 
include the current CPU.  I hope that migrate_task() also works for 
p==current.

This leaves the x86-64 vgetcpu() implementation as the weak point.  Is 
the caching causing problems?  Should migrate_task() make sure the cache 
is reset?

You need a very recent glibc to compile (glibc-2.5.90-22 in rawhide). 
If this is not available replace the sched_getcpu() call.  But make sure 
you pass a pointer to a cache.

-- 
➧ Ulrich Drepper ➧ Red Hat, Inc. ➧ 444 Castro St ➧ Mountain View, CA ❖