signals: Fix more rcu assumptions

1) Remove the misleading comment in __sigqueue_alloc() which claims
   that holding a spinlock is equivalent to rcu_read_lock().

2) Add a rcu_read_lock/unlock around the __task_cred() access
   in __sigqueue_alloc()

This needs to be revisited to remove the remaining users of
read_lock(&tasklist_lock) but that's outside the scope of this patch.
Signed-off-by: NThomas Gleixner <tglx@linutronix.de>
LKML-Reference: <20091210004703.269843657@linutronix.de>

kernel/signal.c

@@ -218,13 +218,13 @@ __sigqueue_alloc(int sig, struct task_struct *t, gfp_t flags, int override_rlimi
 	struct user_struct *user;
 
 	/*
-	 * We won't get problems with the target's UID changing under us
-	 * because changing it requires RCU be used, and if t != current, the
-	 * caller must be holding the RCU readlock (by way of a spinlock) and
-	 * we use RCU protection here
+	 * Protect access to @t credentials. This can go away when all
+	 * callers hold rcu read lock.
 	 */
+	rcu_read_lock();
 	user = get_uid(__task_cred(t)->user);
 	atomic_inc(&user->sigpending);
+	rcu_read_unlock();
 
 	if (override_rlimit ||
 	    atomic_read(&user->sigpending) <=