Re: [PATCH] net: check negative value for signed refcnt

[Kirill Tkhai]

On 31.01.2019 18:21, Eric Dumazet wrote:
> 
> 
> On 01/31/2019 07:15 AM, Eric Dumazet wrote:
>>
>>
>> On 01/31/2019 05:49 AM, Kirill Tkhai wrote:
>>>
>>> 2)Not related to your patch -- it looks like we have problem in existing
>>> code with this netdev_refcnt_read(). It does not imply a memory ordering
>>> or some guarantees about reading percpu values. For example, in generic
>>> code struct percpu_ref switches a counter into atomic mode before it checks
>>> for the last reference. But there is nothing in netdev_refcnt_read().
>>
>> Well, if we read an old value here, after a full and expensive synchronize_net(),
>> then we would have lot more problems than simply having a second round in
>> netdev_wait_allrefs()
>>  
>>
> 
> percpu_ref was added more recently than the netdev_refcnt stuff, and is
> interesting for users wanting a synchronous wait for the refcnt reaching 0.
> 
> netdev_wait_allrefs() was designed to be asynchronous, so that we at least release
> RTNL (and current cpu) when something is wrong and a device can not be dismantled.

Yeah, they are different, and I think we can't add more synchronize_rcu()-dependent
synchronizations in this code, since network namespaces are already destroyed very slow.