signal: Exit RCU read-side critical section on each pass through loop

The kill_pid_info() can potentially loop indefinitely if tasks are created
and deleted sufficiently quickly, and if this happens, this function
will remain in a single RCU read-side critical section indefinitely.
This commit therefore exits the RCU read-side critical section on each
pass through the loop.  Because a race must happen to retry the loop,
this should have no performance impact in the common case.
Reported-by: Dave Jones <davej@redhat.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Reviewed-by: Pranith Kumar <bobby.prani@gmail.com>

kernel/signal.c

@@ -1331,23 +1331,21 @@ int kill_pid_info(int sig, struct siginfo *info, struct pid *pid)
 	int error = -ESRCH;
 	struct task_struct *p;
 
+	for (;;) {
 		rcu_read_lock();
-retry:
 		p = pid_task(pid, PIDTYPE_PID);
-	if (p) {
+		if (p)
 			error = group_send_sig_info(sig, info, p);
-		if (unlikely(error == -ESRCH))
+		rcu_read_unlock();
+		if (likely(!p || error != -ESRCH))
+			return error;
+
 		/*
-			 * The task was unhashed in between, try again.
-			 * If it is dead, pid_task() will return NULL,
-			 * if we race with de_thread() it will find the
-			 * new leader.
+		 * The task was unhashed in between, try again.  If it
+		 * is dead, pid_task() will return NULL, if we race with
+		 * de_thread() it will find the new leader.
 		 */
-			goto retry;
 	}
-	rcu_read_unlock();
-
-	return error;
 }
 
 int kill_proc_info(int sig, struct siginfo *info, pid_t pid)