Re: [PATCH sched_ext/for-7.1-fixes] sched_ext: Move scx_error() out of scx_link_sched()'s lock region

[Andrea Righi]

Hi Tejun,

On Wed, May 06, 2026 at 08:02:46AM -1000, Tejun Heo wrote:
> scx_link_sched() holds scx_sched_lock. The scx_error() calls inside take the
> same lock through scx_claim_exit() and deadlock. Move them out of the guard.
> 
> Fixes: 6b4576b09714 ("sched_ext: Reject sub-sched attachment to a disabled parent")
> Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>

Looks good.

Reviewed-by: Andrea Righi <arighi@xxxxxxxxxx>

Thanks,
-Andrea

> ---
>  kernel/sched/ext.c |   22 +++++++++++++++++-----
>  1 file changed, 17 insertions(+), 5 deletions(-)
> 
> --- a/kernel/sched/ext.c
> +++ b/kernel/sched/ext.c
> @@ -5661,10 +5661,12 @@ static void refresh_watchdog(void)
>  
>  static s32 scx_link_sched(struct scx_sched *sch)
>  {
> +	const char *err_msg;
> +	s32 ret = 0;
> +
>  	scoped_guard(raw_spinlock_irq, &scx_sched_lock) {
>  #ifdef CONFIG_EXT_SUB_SCHED
>  		struct scx_sched *parent = scx_parent(sch);
> -		s32 ret;
>  
>  		if (parent) {
>  			/*
> @@ -5674,15 +5676,16 @@ static s32 scx_link_sched(struct scx_sch
>  			 * parent can shoot us down.
>  			 */
>  			if (atomic_read(&parent->exit_kind) != SCX_EXIT_NONE) {
> -				scx_error(sch, "parent disabled");
> -				return -ENOENT;
> +				err_msg = "parent disabled";
> +				ret = -ENOENT;
> +				break;
>  			}
>  
>  			ret = rhashtable_lookup_insert_fast(&scx_sched_hash,
>  					&sch->hash_node, scx_sched_hash_params);
>  			if (ret) {
> -				scx_error(sch, "failed to insert into scx_sched_hash (%d)", ret);
> -				return ret;
> +				err_msg = "failed to insert into scx_sched_hash";
> +				break;
>  			}
>  
>  			list_add_tail(&sch->sibling, &parent->children);
> @@ -5692,6 +5695,15 @@ static s32 scx_link_sched(struct scx_sch
>  		list_add_tail_rcu(&sch->all, &scx_sched_all);
>  	}
>  
> +	/*
> +	 * scx_error() takes scx_sched_lock via scx_claim_exit(), so it must run after
> +	 * the guard above is released.
> +	 */
> +	if (ret) {
> +		scx_error(sch, "%s (%d)", err_msg, ret);
> +		return ret;
> +	}
> +
>  	refresh_watchdog();
>  	return 0;
>  }