Re: [PATCH] sched/deadline: fix task_struct reference leak

[Juri Lelli]

Hi Wander,

On 18/06/24 17:19, Wander Lairson Costa wrote:
> 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.
> 
> Signed-off-by: Wander Lairson Costa <wander@xxxxxxxxxx>
> ---
>  kernel/sched/deadline.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index c75d1307d86d..5953f89bfa96 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -1805,7 +1805,9 @@ static void enqueue_task_dl(struct rq *rq, struct task_struct *p, int flags)
>  			 * problem if it fires concurrently: boosted threads
>  			 * are ignored in dl_task_timer().
>  			 */
> -			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);

Wonder if this deserves a little comment saying that if the timer
callback was running (hrtimer_try_to_cancel() returns -1) it will
eventually call put_task_struct(). Other than that, the fix makes sense
to me.

Oh, another thing is that I believe we should add 'Fixes: feff2e65efd8d
("sched/deadline: Unthrottle PI boosted threads while enqueuing")' in
the changelog, shouldn't we?

Thanks,
Juri