It turns out that the setpgid() system call fails to enter an RCU
read-side critical section before doing a PID-to-task_struct translation.
This commit therefore does rcu_read_lock() before the translation, and
also does rcu_read_unlock() after the last use of the returned pointer.
Reported-by: Andrew Morton <firstname.lastname@example.org>
Signed-off-by: Paul E. McKenney <email@example.com>
Acked-by: David Howells <firstname.lastname@example.org>