timer-fd: Prevent live lock

If hrtimer_try_to_cancel() requires a retry, then depending on the
priority setting te retry loop might prevent timer callback completion
on RT. Prevent that by waiting for completion on RT, no change for a
non RT kernel.

Reported-by: Sankara Muthukrishnan <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>

fs/timerfd.c

@@ -471,7 +471,10 @@ static int do_timerfd_settime(int ufd, int flags,
 				break;
 		}
 		spin_unlock_irq(&ctx->wqh.lock);
-		cpu_relax();
+		if (isalarm(ctx))
+			hrtimer_wait_for_timer(&ctx->t.alarm.timer);
+		else
+			hrtimer_wait_for_timer(&ctx->t.tmr);
 	}
 
 	/*