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

[Eric Dumazet]

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.