Re: [PATCH 1/2] sched/fair: Fix panic if NEXT_BUDDY enabled

[Madadi Vineeth Reddy <vineethr@linux.ibm.com>]

On 25/11/24 07:42, Adam Li wrote:
> Enabling NEXT_BUDDY may trigger warning and kernel panic:
> 
> [  124.977300] cfs_rq->next->sched_delayed
> [  124.977310] WARNING: CPU: 51 PID: 2150 at kernel/sched/fair.c:5621 pick_task_fair+0x130/0x150
> <snip>
> [  297.371198] Unable to handle kernel NULL pointer dereference at virtual address 0000000000000051
> [  297.406112] CPU: 116 UID: 0 PID: 10328 Comm: Grizzly-worker( Tainted: G        W   E      6.12.0.adam+ #1
> [  297.597362] Call trace:
> [  297.599795]  pick_task_fair+0x50/0x150 (P)
> [  297.603879]  pick_task_fair+0x50/0x150 (L)
> [  297.607963]  pick_next_task_fair+0x30/0x3c0
> [  297.612134]  __pick_next_task+0x4c/0x220
> [  297.616045]  pick_next_task+0x44/0x980
> [  297.619782]  __schedule+0x3d0/0x628
> [  297.623259]  do_task_dead+0x50/0x60
> [  297.626736]  do_exit+0x28c/0x410
> [  297.629955]  do_group_exit+0x3c/0xa0
> [  297.633518]  get_signal+0x8c4/0x8d0
> [  297.636996]  do_signal+0x9c/0x270
> [  297.640299]  do_notify_resume+0xe0/0x198
> [  297.644212]  el0_svc+0xf4/0x170
> [  297.647342]  el0t_64_sync_handler+0x10c/0x138
> [  297.651687]  el0t_64_sync+0x1ac/0x1b0
> [  297.655339] Code: d503201f 1400002a aa1403e0 97ffda0b (39414401)
> [  297.661439] ---[ end trace 0000000000000000 ]---
> [  297.726593] Kernel panic - not syncing: Oops: Fatal exception
> 
> Do not set next buddy if sched_delayed is set.
> 
> Signed-off-by: Adam Li <adamli@os.amperecomputing.com>
> ---
>  kernel/sched/fair.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index fbdca89c677f..cd1188b7f3df 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -8748,6 +8748,8 @@ static void set_next_buddy(struct sched_entity *se)
>  			return;
>  		if (se_is_idle(se))
>  			return;
> +		if (se->sched_delayed)
> +			return;

IIUC, this happens due to a race condition between choosing the next_buddy and
dequeuing the task from the runqueue as soon as its lag becomes zero, thereby causing
task_of(se) to result in a null dereference in pick_task_fair?

Correct me if I am wrong.

Thanks,
Madadi Vineeth Reddy

>  		cfs_rq_of(se)->next = se;
>  	}
>  }