netdev - Re: [net PATCH v3 2/5] net: virtio: wrap rtnl_lock in test for calling with lock already held

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Date:   Fri, 13 Jan 2017 09:31:46 -0800
From:   John Fastabend <john.fastabend@...il.com>
To:     Stephen Hemminger <stephen@...workplumber.org>
Cc:     jasowang@...hat.com, mst@...hat.com, john.r.fastabend@...el.com,
        netdev@...r.kernel.org, alexei.starovoitov@...il.com,
        daniel@...earbox.net
Subject: Re: [net PATCH v3 2/5] net: virtio: wrap rtnl_lock in test for
 calling with lock already held

On 17-01-13 08:34 AM, Stephen Hemminger wrote:
> On Thu, 12 Jan 2017 18:51:00 -0800
> John Fastabend <john.fastabend@...il.com> wrote:
> 
>>  
>> -static void free_receive_bufs(struct virtnet_info *vi)
>> +static void free_receive_bufs(struct virtnet_info *vi, bool need_lock)
>>  {
>>  	struct bpf_prog *old_prog;
>>  	int i;
>>  
>> -	rtnl_lock();
>> +	if (need_lock)
>> +		rtnl_lock();
>>  	for (i = 0; i < vi->max_queue_pairs; i++) {
>>  		while (vi->rq[i].pages)
>>  			__free_pages(get_a_page(&vi->rq[i], GFP_KERNEL), 0);
>> @@ -1879,7 +1880,8 @@ static void free_receive_bufs(struct virtnet_info *vi)
>>  		if (old_prog)
>>  			bpf_prog_put(old_prog);
>>  	}
>> -	rtnl_unlock();
>> +	if (need_lock)
>> +		rtnl_unlock();
>>  }
> 
> Conditional locking is bad idea; sparse complains about it and is later source
> of bugs. The more typical way of doing this in kernel is:

OK I'll use the normal form.

> 
> void _foo(some args)
> {
> 	ASSERT_RTNL();
> 
> 	...
> }
> 
> void foo(some args)
> {
> 	rtnl_lock();
> 	_foo(some args)
> 	rtnl_unlock();
> }
> 
>