handle_exit_race && PF_EXITING

[Oleg Nesterov <oleg@redhat.com>]

On 11/05, Thomas Gleixner wrote:
>
> Out of curiosity, what's the race issue vs. robust list which you are
> trying to solve?

Off-topic, but this reminds me...

	#include <sched.h>
	#include <assert.h>
	#include <unistd.h>
	#include <syscall.h>

	#define FUTEX_LOCK_PI		6

	int main(void)
	{
		struct sched_param sp = {};

		sp.sched_priority = 2;
		assert(sched_setscheduler(0, SCHED_FIFO, &sp) == 0);

		int lock = vfork();
		if (!lock) {
			sp.sched_priority = 1;
			assert(sched_setscheduler(0, SCHED_FIFO, &sp) == 0);
			_exit(0);
		}

		syscall(__NR_futex, &lock, FUTEX_LOCK_PI, 0,0,0);
		return 0;
	}

this creates the unkillable RT process spinning in futex_lock_pi() on
a single CPU machine (or you can use taskset).

Probably the patch below makes sense anyway, but of course it doesn't
solve the real problem: futex_lock_pi() should not spin in this case.

It seems to me I even sent the fix a long ago, but I can't recall what
exactly it did. Probably the PF_EXITING check in attach_to_pi_owner()
must simply die, I'll try to recall...

Oleg.

--- x/kernel/futex.c
+++ x/kernel/futex.c
@@ -2842,10 +2842,12 @@ static int futex_lock_pi(u32 __user *uaddr, unsigned int flags,
 			 *   exit to complete.
 			 * - The user space value changed.
 			 */
-			queue_unlock(hb);
-			put_futex_key(&q.key);
-			cond_resched();
-			goto retry;
+			if (!fatal_signal_pending(current)) {
+				queue_unlock(hb);
+				put_futex_key(&q.key);
+				cond_resched();
+				goto retry;
+			}
 		default:
 			goto out_unlock_put_key;
 		}