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

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Message-ID: <859d2649-31a0-0fae-7768-97bc9d754ccc@virtuozzo.com>
Date:   Thu, 31 Jan 2019 18:34:05 +0300
From:   Kirill Tkhai <ktkhai@...tuozzo.com>
To:     Eric Dumazet <eric.dumazet@...il.com>,
        alexandre.besnard@...tathome.com, davem@...emloft.net,
        ecree@...arflare.com, jiri@...lanox.com, petrm@...lanox.com,
        alexander.h.duyck@...el.com, amritha.nambiar@...el.com,
        lirongqing@...du.com
Cc:     netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] net: check negative value for signed refcnt

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.