netdev - Re: [PATCH bpf-next 1/8] bpf: offload: don't require rtnl for dev list manipulation

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Date:   Wed, 20 Dec 2017 18:00:32 +0300
From:   Kirill Tkhai <ktkhai@...tuozzo.com>
To:     Jakub Kicinski <jakub.kicinski@...ronome.com>,
        netdev@...r.kernel.org, alexei.starovoitov@...il.com,
        daniel@...earbox.net
Cc:     oss-drivers@...ronome.com
Subject: Re: [PATCH bpf-next 1/8] bpf: offload: don't require rtnl for dev
 list manipulation

Hi, Jakub,

thanks for looking into this.

Sadly, that __bpf_prog_offload_destroy() needs rtnl_lock() context,
but rwsem is still good as it became useful for next patches from the series.

Please, see one small minor nit near the last hunk. Everything else looks good
for me.

On 20.12.2017 07:09, Jakub Kicinski wrote:
> We only need to hold rtnl_lock() around ndo calls.  The device
> offload initialization doesn't require it.  Neither will soon-
> -to-come querying the offload info.  Use struct rw_semaphore
> because map offload will require sleeping with the semaphore
> held for read.
> 
> Suggested-by: Kirill Tkhai <ktkhai@...tuozzo.com>
> Signed-off-by: Jakub Kicinski <jakub.kicinski@...ronome.com>
> Reviewed-by: Quentin Monnet <quentin.monnet@...ronome.com>

Reviewed-by: Kirill Tkhai <ktkhai@...tuozzo.com>

> ---
>  kernel/bpf/offload.c | 21 +++++++++++++++++----
>  1 file changed, 17 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/bpf/offload.c b/kernel/bpf/offload.c
> index 8455b89d1bbf..b88e5ebdc61d 100644
> --- a/kernel/bpf/offload.c
> +++ b/kernel/bpf/offload.c
> @@ -20,8 +20,12 @@
>  #include <linux/netdevice.h>
>  #include <linux/printk.h>
>  #include <linux/rtnetlink.h>
> +#include <linux/rwsem.h>
>  
> -/* protected by RTNL */
> +/* Protects bpf_prog_offload_devs and offload members of all progs.
> + * RTNL lock cannot be taken when holding this lock.
> + */
> +static struct rw_semaphore bpf_devs_lock;
>  static LIST_HEAD(bpf_prog_offload_devs);
>  
>  int bpf_prog_offload_init(struct bpf_prog *prog, union bpf_attr *attr)
> @@ -43,17 +47,21 @@ int bpf_prog_offload_init(struct bpf_prog *prog, union bpf_attr *attr)
>  	offload->prog = prog;
>  	init_waitqueue_head(&offload->verifier_done);
>  
> -	rtnl_lock();
> +	/* Our UNREGISTER notifier will grab bpf_devs_lock, so we are safe
> +	 * to assume the netdev doesn't get unregistered as long as we hold
> +	 * bpf_devs_lock.
> +	 */
> +	down_write(&bpf_devs_lock);
>  	offload->netdev = __dev_get_by_index(net, attr->prog_ifindex);
>  	if (!offload->netdev) {
> -		rtnl_unlock();
> +		up_write(&bpf_devs_lock);
>  		kfree(offload);
>  		return -EINVAL;
>  	}
>  
>  	prog->aux->offload = offload;
>  	list_add_tail(&offload->offloads, &bpf_prog_offload_devs);
> -	rtnl_unlock();
> +	up_write(&bpf_devs_lock);
>  
>  	return 0;
>  }
> @@ -126,7 +134,9 @@ void bpf_prog_offload_destroy(struct bpf_prog *prog)
>  	wake_up(&offload->verifier_done);
>  
>  	rtnl_lock();
> +	down_write(&bpf_devs_lock);
>  	__bpf_prog_offload_destroy(prog);
> +	up_write(&bpf_devs_lock);
>  	rtnl_unlock();
>  
>  	kfree(offload);
> @@ -181,11 +191,13 @@ static int bpf_offload_notification(struct notifier_block *notifier,
>  		if (netdev->reg_state != NETREG_UNREGISTERING)
>  			break;
>  
> +		down_write(&bpf_devs_lock);
>  		list_for_each_entry_safe(offload, tmp, &bpf_prog_offload_devs,
>  					 offloads) {
>  			if (offload->netdev == netdev)
>  				__bpf_prog_offload_destroy(offload->prog);
>  		}
> +		up_write(&bpf_devs_lock);
>  		break;
>  	default:
>  		break;
> @@ -199,6 +211,7 @@ static struct notifier_block bpf_offload_notifier = {
>  
>  static int __init bpf_offload_init(void)
>  {
> +	init_rwsem(&bpf_devs_lock);

DECLARE_RWSEM() could be used instead of this.

>  	register_netdevice_notifier(&bpf_offload_notifier);
>  	return 0;
>  }
>