netdev - Re: [PATCH net-next v2] net: let lockdep compare instance locks

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Message-ID: <20250518193313.87050-1-kuniyu@amazon.com>
Date: Sun, 18 May 2025 12:32:52 -0700
From: Kuniyuki Iwashima <kuniyu@...zon.com>
To: <kuba@...nel.org>
CC: <andrew+netdev@...n.ch>, <davem@...emloft.net>, <edumazet@...gle.com>,
	<horms@...nel.org>, <kuniyu@...zon.com>, <netdev@...r.kernel.org>,
	<pabeni@...hat.com>, <sdf@...ichev.me>
Subject: Re: [PATCH net-next v2] net: let lockdep compare instance locks

From: Jakub Kicinski <kuba@...nel.org>
Date: Sat, 17 May 2025 13:08:10 -0700
> AFAIU always returning -1 from lockdep's compare function
> basically disables checking of dependencies between given
> locks. Try to be a little more precise about what guarantees
> that instance locks won't deadlock.
> 
> Right now we only nest them under protection of rtnl_lock.
> Mostly in unregister_netdevice_many() and dev_close_many().
> 
> Signed-off-by: Jakub Kicinski <kuba@...nel.org>

Reviewed-by: Kuniyuki Iwashima <kuniyu@...zon.com>


> ---
> v2:
>  - drop the speculative small rtnl handling
> v1: https://lore.kernel.org/20250516012459.1385997-1-kuba@kernel.org
> ---
>  include/net/netdev_lock.h | 15 +++++++--------
>  1 file changed, 7 insertions(+), 8 deletions(-)
> 
> diff --git a/include/net/netdev_lock.h b/include/net/netdev_lock.h
> index 2a753813f849..c345afecd4c5 100644
> --- a/include/net/netdev_lock.h
> +++ b/include/net/netdev_lock.h
> @@ -99,16 +99,15 @@ static inline void netdev_unlock_ops_compat(struct net_device *dev)
>  static inline int netdev_lock_cmp_fn(const struct lockdep_map *a,
>  				     const struct lockdep_map *b)
>  {
> -	/* Only lower devices currently grab the instance lock, so no
> -	 * real ordering issues can occur. In the near future, only
> -	 * hardware devices will grab instance lock which also does not
> -	 * involve any ordering. Suppress lockdep ordering warnings
> -	 * until (if) we start grabbing instance lock on pure SW
> -	 * devices (bond/team/veth/etc).
> -	 */
>  	if (a == b)
>  		return 0;
> -	return -1;
> +
> +	/* Allow locking multiple devices only under rtnl_lock,
> +	 * the exact order doesn't matter.
> +	 * Note that upper devices don't lock their ops, so nesting
> +	 * mostly happens in batched device removal for now.
> +	 */
> +	return lockdep_rtnl_is_held() ? -1 : 1;
>  }
>  
>  #define netdev_lockdep_set_classes(dev)				\
> -- 
> 2.49.0
>