From: Olaf Kirch <okir@suse.de>

I have been chasing a corruption of current->group_info on PPC during NFS
stress tests.  The problem seems to be that nfsd is messing with its
group_info quite a bit, while some monitoring processes look at
/proc/<pid>/status and do a get_group_info/put_group_info without any locking.

This problem can be reproduced on ppc platforms within a few seconds if you
generate some NFS load and do a "cat /proc/XXX/status" of an nfsd thread in a
tight loop.

I therefore think changes to current->group_info, and querying it from a
different process, needs to be protected using the task_lock.

(akpm: task->group_info here is safe against exit() because the task holds a
ref on group_info which is released in __put_task_struct, and the /proc file
has a ref on the task_struct).