Re: [PATCH] kobject: avoid blocking allocation while holding uevent_sock_mutex

[Greg KH]

On Wed, Aug 26, 2026 at 02:46:33PM +0800, Tao Yu wrote:
> uevent_net_broadcast_untagged() still holds uevent_sock_mutex across
> netlink_broadcast().  That serializes all untagged uevent senders behind
> one global mutex, and it also keeps the mutex held while the netlink
> broadcast path may perform blocking memory allocation.
> 
> This becomes visible during USB enumeration, where device_add() sends a
> KOBJ_ADD uevent from the usb_hub_wq context.  If a listener is slow or
> the netlink broadcast path runs into memory pressure, the sender can sit
> behind uevent_sock_mutex long enough to trigger hung task reports.

What has changed to cause this to happen?  USB enumeration is quite
common :)

> Keep the existing send-side ordering, but move the uevent skb allocation
> out of the critical section and use GFP_NOWAIT for the broadcast clones
> performed while uevent_sock_mutex is held.  This removes the sleeping
> allocation point from the locked region without changing uevent delivery
> semantics.
> 
> Reported-by: syzbot+c22bb42560ec86726aba@xxxxxxxxxxxxxxxxxxxxxxxxx

No link?  Was this tested?

No cc: stable?

No Fixes: tag?

> Signed-off-by: Tao Yu <tao1.yu@xxxxxxxxx>
> ---
>  lib/kobject_uevent.c | 28 +++++++++++++++++++---------
>  1 file changed, 19 insertions(+), 9 deletions(-)
> 
> diff --git a/lib/kobject_uevent.c b/lib/kobject_uevent.c
> index ddbc4d7482d24..b8832a5ca583d 100644
> --- a/lib/kobject_uevent.c
> +++ b/lib/kobject_uevent.c
> @@ -311,9 +311,26 @@ static int uevent_net_broadcast_untagged(struct kobj_uevent_env *env,
>  {
>  	struct sk_buff *skb = NULL;
>  	struct uevent_sock *ue_sk;
> +	bool has_listeners = false;
>  	int retval = 0;
>  
> -	/* send netlink message */
> +	mutex_lock(&uevent_sock_mutex);
> +	list_for_each_entry(ue_sk, &uevent_sock_list, list) {
> +		if (!netlink_has_listeners(ue_sk->sk, 1))
> +			continue;
> +
> +		has_listeners = true;
> +		break;
> +	}
> +	mutex_unlock(&uevent_sock_mutex);

guard()?

> +
> +	if (has_listeners) {

What happens if you get a listner right after the lock is released?

> +		skb = alloc_uevent_skb(env, action_string, devpath);
> +		if (!skb)
> +			return -ENOMEM;
> +	}
> +
> +	/* Keep send-side ordering, but avoid sleeping while holding the mutex. */
>  	mutex_lock(&uevent_sock_mutex);
>  	list_for_each_entry(ue_sk, &uevent_sock_list, list) {
>  		struct sock *uevent_sock = ue_sk->sk;
> @@ -321,15 +338,8 @@ static int uevent_net_broadcast_untagged(struct kobj_uevent_env *env,
>  		if (!netlink_has_listeners(uevent_sock, 1))
>  			continue;
>  
> -		if (!skb) {
> -			retval = -ENOMEM;
> -			skb = alloc_uevent_skb(env, action_string, devpath);
> -			if (!skb)
> -				continue;
> -		}
> -
>  		retval = netlink_broadcast(uevent_sock, skb_get(skb), 0, 1,
> -					   GFP_KERNEL);
> +					   GFP_NOWAIT);

This all feels very odd.  Again, what has changed to suddenly need this?

thanks,

greg k-h