From: Claudio Jeker <cjeker@diehard.n-r-g.com>
Subject: Re: net lock kernel lock order
To: Alexander Bluhm <alexander.bluhm@gmx.net>
Cc: tech@openbsd.org
Date: Fri, 5 Jan 2024 15:05:44 +0100

On Fri, Jan 05, 2024 at 01:26:15PM +0100, Alexander Bluhm wrote:
> Hi,
> 
> We have code that takes kernel lock just before net lock.  As net
> lock is a rwlock that releases kernel lock while sleeping, there
> is never deadlock due to lock ordering.
> 
> The order kernel lock -> net lock is suboptimal.  If net lock is
> held by another thread the following happens:
> 
> take kernel lock -> try net lock -> release kernel lock -> sleep
> -> take net lock -> take kernel lock

this is incorrect. the kernel lock is aquired before the rwlock in a sleep
call.
 
> If we change the order we get a less expensive sequence:
> 
> try net lock -> sleep -> take net lock -> take kernel lock
> 
> ok?
> 
> bluhm
> 
> Index: net/if.c
> ===================================================================
> RCS file: /data/mirror/openbsd/cvs/src/sys/net/if.c,v
> diff -u -p -r1.714 if.c
> --- net/if.c	29 Dec 2023 11:43:04 -0000	1.714
> +++ net/if.c	5 Jan 2024 12:08:01 -0000
> @@ -1774,16 +1774,16 @@ if_linkstate_task(void *xifidx)
>  	unsigned int ifidx = (unsigned long)xifidx;
>  	struct ifnet *ifp;
>  
> -	KERNEL_LOCK();
>  	NET_LOCK();
> +	KERNEL_LOCK();
>  
>  	ifp = if_get(ifidx);
>  	if (ifp != NULL)
>  		if_linkstate(ifp);
>  	if_put(ifp);
>  
> -	NET_UNLOCK();
>  	KERNEL_UNLOCK();
> +	NET_UNLOCK();
>  }
>  
>  void
> Index: netinet/in.c
> ===================================================================
> RCS file: /data/mirror/openbsd/cvs/src/sys/netinet/in.c,v
> diff -u -p -r1.185 in.c
> --- netinet/in.c	28 Jun 2023 11:49:49 -0000	1.185
> +++ netinet/in.c	5 Jan 2024 12:10:07 -0000
> @@ -260,8 +260,8 @@ in_ioctl(u_long cmd, caddr_t data, struc
>  			return (error);
>  	}
>  
> -	KERNEL_LOCK();
>  	NET_LOCK();
> +	KERNEL_LOCK();
>  
>  	TAILQ_FOREACH(ifa, &ifp->if_addrlist, ifa_list) {
>  		if (ifa->ifa_addr->sa_family != AF_INET)
> @@ -331,8 +331,8 @@ in_ioctl(u_long cmd, caddr_t data, struc
>  		break;
>  	}
>  err:
> -	NET_UNLOCK();
>  	KERNEL_UNLOCK();
> +	NET_UNLOCK();
>  	return (error);
>  }
>  
> @@ -353,8 +353,8 @@ in_ioctl_set_ifaddr(u_long cmd, caddr_t 
>  	if (error)
>  		return (error);
>  
> -	KERNEL_LOCK();
>  	NET_LOCK();
> +	KERNEL_LOCK();
>  
>  	TAILQ_FOREACH(ifa, &ifp->if_addrlist, ifa_list) {
>  		if (ifa->ifa_addr->sa_family != AF_INET)
> @@ -387,8 +387,8 @@ in_ioctl_set_ifaddr(u_long cmd, caddr_t 
>  	if (!error)
>  		if_addrhooks_run(ifp);
>  
> -	NET_UNLOCK();
>  	KERNEL_UNLOCK();
> +	NET_UNLOCK();
>  	return error;
>  }
>  
> @@ -409,8 +409,8 @@ in_ioctl_change_ifaddr(u_long cmd, caddr
>  			return (error);
>  	}
>  
> -	KERNEL_LOCK();
>  	NET_LOCK();
> +	KERNEL_LOCK();
>  
>  	TAILQ_FOREACH(ifa, &ifp->if_addrlist, ifa_list) {
>  		if (ifa->ifa_addr->sa_family != AF_INET)
> @@ -527,8 +527,8 @@ in_ioctl_change_ifaddr(u_long cmd, caddr
>  		panic("%s: invalid ioctl %lu", __func__, cmd);
>  	}
>  
> -	NET_UNLOCK();
>  	KERNEL_UNLOCK();
> +	NET_UNLOCK();
>  	return (error);
>  }
>  
> 

-- 
:wq Claudio