zap_pid_ns_processes: clear TIF_NOTIFY_SIGNAL along with TIF_SIGPENDING

kernel_wait4() doesn't sleep and returns -EINTR if there is no
eligible child and signal_pending() is true.

That is why zap_pid_ns_processes() clears TIF_SIGPENDING but this is not
enough, it should also clear TIF_NOTIFY_SIGNAL to make signal_pending()
return false and avoid a busy-wait loop.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: 12db8b6 ("entry: Add support for TIF_NOTIFY_SIGNAL")
Signed-off-by: Oleg Nesterov <[email protected]>
Reported-by: Rachel Menge <[email protected]>
Closes: https://lore.kernel.org/all/[email protected]/
Reviewed-by: Boqun Feng <[email protected]>
Tested-by: Wei Fu <[email protected]>
Reviewed-by: Jens Axboe <[email protected]>
Cc: Allen Pais <[email protected]>
Cc: Christian Brauner <[email protected]>
Cc: Frederic Weisbecker <[email protected]>
Cc: Joel Fernandes (Google) <[email protected]>
Cc: Joel Granados <[email protected]>
Cc: Josh Triplett <[email protected]>
Cc: Lai Jiangshan <[email protected]>
Cc: Mateusz Guzik <[email protected]>
Cc: Mathieu Desnoyers <[email protected]>
Cc: Mike Christie <[email protected]>
Cc: Neeraj Upadhyay <[email protected]>
Cc: Paul E. McKenney <[email protected]>
Cc: Steven Rostedt (Google) <[email protected]>
Cc: Zqiang <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: oleg-nesterov

kernel/pid_namespace.c

@@ -218,6 +218,7 @@ void zap_pid_ns_processes(struct pid_namespace *pid_ns)
 	 */
 	do {
 		clear_thread_flag(TIF_SIGPENDING);
+		clear_thread_flag(TIF_NOTIFY_SIGNAL);
 		rc = kernel_wait4(-1, NULL, __WALL, NULL);
 	} while (rc != -ECHILD);