Re: [PATCH v2 2/3] sched/fair: Fix panic if pick_eevdf() returns NULL

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Nov 27, 2024 at 05:56:09AM +0000, Adam Li wrote:
> pick_eevdf() may return NULL, which triggers NULL pointer
> dereference at pick_next_entity():
> 	struct sched_entity *se = pick_eevdf(cfs_rq);
> 	if (se->sched_delayed)
> 
>     [  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

Well, if you look at pick_task_fair() you'll see we'll not get there
unless cfs_rq->nr_running is non-zero, at which point pick_eevdf()
really should NOT be returning NULL.

If it does, something else is broken.