netdev - Re: [PATCH bpf-next v2 1/8] bpf: offload: don't require rtnl for dev list manipulation

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Message-ID: <f929bd47-5372-eeba-8be2-47155f00ee8c@virtuozzo.com>
Date:   Tue, 26 Dec 2017 19:02:50 +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 v2 1/8] bpf: offload: don't require rtnl for dev
 list manipulation

Hi, Jakub,

On 22.12.2017 00:01, Jakub Kicinski wrote:
> We don't need the RTNL lock for all operations on offload state.
> We only need to hold it around ndo calls.  The device offload
> initialization doesn't require it.  The soon-to-come querying
> of the offload info will only need it partially.  We will also
> be able to remove the waitqueue in following patches.
> 
> 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>
> ---
> v2:
>  - use dev_get_by_index_rcu() instead of implicit lock dependencies;
>  - use DECLARE_RWSEM() instead of init_rwsem() (Kirill).
> ---
>  kernel/bpf/offload.c | 33 ++++++++++++++++++++++++++-------
>  1 file changed, 26 insertions(+), 7 deletions(-)
> 
> diff --git a/kernel/bpf/offload.c b/kernel/bpf/offload.c
> index 8455b89d1bbf..f049073a37e6 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 DECLARE_RWSEM(bpf_devs_lock);
>  static LIST_HEAD(bpf_prog_offload_devs);
>  
>  int bpf_prog_offload_init(struct bpf_prog *prog, union bpf_attr *attr)
> @@ -43,19 +47,30 @@ int bpf_prog_offload_init(struct bpf_prog *prog, union bpf_attr *attr)
>  	offload->prog = prog;
>  	init_waitqueue_head(&offload->verifier_done);
>  
> -	rtnl_lock();
> -	offload->netdev = __dev_get_by_index(net, attr->prog_ifindex);
> +	rcu_read_lock();
> +	offload->netdev = dev_get_by_index_rcu(net, attr->prog_ifindex);
>  	if (!offload->netdev) {
> -		rtnl_unlock();
> -		kfree(offload);
> -		return -EINVAL;
> +		rcu_read_unlock();
> +		goto err_free;
>  	}
> +	dev_hold(offload->netdev);
> +	rcu_read_unlock();

Small remark about this. There is already dev_get_by_index() in net/core/dev.c
with the functionality above. I haven't found in next patches the reason
we have to use directly rcu_read_lock() here.

So, it seems we may replace the above block with simple dev_get_by_index().
Everything else looks good for me.

>  
> +	down_write(&bpf_devs_lock);
> +	if (offload->netdev->reg_state != NETREG_REGISTERED)
> +		goto err_unlock;
>  	prog->aux->offload = offload;
>  	list_add_tail(&offload->offloads, &bpf_prog_offload_devs);
> -	rtnl_unlock();
> +	dev_put(offload->netdev);
> +	up_write(&bpf_devs_lock);
>  
>  	return 0;
> +err_unlock:
> +	up_write(&bpf_devs_lock);
> +	dev_put(offload->netdev);
> +err_free:
> +	kfree(offload);
> +	return -EINVAL;
>  }
>  
>  static int __bpf_offload_ndo(struct bpf_prog *prog, enum bpf_netdev_command cmd,
> @@ -126,7 +141,9 @@ 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 +198,13 @@ 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;

Kirill