sched/deadline: Fix task_struct reference leak

During the execution of the following stress test with linux-rt:

stress-ng --cyclic 30 --timeout 30 --minimize --quiet

kmemleak frequently reported a memory leak concerning the task_struct:

unreferenced object 0xffff8881305b8000 (size 16136):
  comm "stress-ng", pid 614, jiffies 4294883961 (age 286.412s)
  object hex dump (first 32 bytes):
    02 40 00 00 00 00 00 00 00 00 00 00 00 00 00 00  .@..............
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
  debug hex dump (first 16 bytes):
    53 09 00 00 00 00 00 00 00 00 00 00 00 00 00 00  S...............
  backtrace:
    [<00000000046b6790>] dup_task_struct+0x30/0x540
    [<00000000c5ca0f0b>] copy_process+0x3d9/0x50e0
    [<00000000ced59777>] kernel_clone+0xb0/0x770
    [<00000000a50befdc>] __do_sys_clone+0xb6/0xf0
    [<000000001dbf2008>] do_syscall_64+0x5d/0xf0
    [<00000000552900ff>] entry_SYSCALL_64_after_hwframe+0x6e/0x76

The issue occurs in start_dl_timer(), which increments the task_struct
reference count and sets a timer. The timer callback, dl_task_timer,
is supposed to decrement the reference count upon expiration. However,
if enqueue_task_dl() is called before the timer expires and cancels it,
the reference count is not decremented, leading to the leak.

This patch fixes the reference leak by ensuring the task_struct
reference count is properly decremented when the timer is canceled.

Fixes: feff2e65 ("sched/deadline: Unthrottle PI boosted threads while enqueuing")
Signed-off-by: Wander Lairson Costa <wander@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Juri Lelli <juri.lelli@redhat.com>
Link: https://lore.kernel.org/r/20240620125618.11419-1-wander@redhat.com

kernel/sched/deadline.c

@@ -1804,8 +1804,13 @@ static void enqueue_task_dl(struct rq *rq, struct task_struct *p, int flags)
 			 * The replenish timer needs to be canceled. No
 			 * problem if it fires concurrently: boosted threads
 			 * are ignored in dl_task_timer().
+			 *
+			 * If the timer callback was running (hrtimer_try_to_cancel == -1),
+			 * it will eventually call put_task_struct().
 			 */
-			hrtimer_try_to_cancel(&p->dl.dl_timer);
+			if (hrtimer_try_to_cancel(&p->dl.dl_timer) == 1 &&
+			    !dl_server(&p->dl))
+				put_task_struct(p);
 			p->dl.dl_throttled = 0;
 		}
 	} else if (!dl_prio(p->normal_prio)) {