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

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Date:   Thu, 31 Jan 2019 16:14:20 +0100 (CET)
From:   Alexandre BESNARD <alexandre.besnard@...tathome.com>
To:     Kirill Tkhai <ktkhai@...tuozzo.com>
Cc:     davem@...emloft.net, amritha nambiar <amritha.nambiar@...el.com>,
        lirongqing@...du.com, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        alexander h duyck <alexander.h.duyck@...el.com>,
        jiri@...lanox.com, petrm@...lanox.com, ecree@...arflare.com
Subject: Re: [PATCH] net: check negative value for signed refcnt

Hi Kirill, and thanks for your time,

On 31 Jan 19 14:49, Kirill Tkhai ktkhai@...tuozzo.com wrote :

> Hi, Alexandre,

> On 31.01.2019 16:20, alexandre.besnard@...tathome.com wrote:
> > From: Alexandre Besnard <alexandre.besnard@...tathome.com>

> > Device remaining references counter is get as a signed integer.

> > When unregistering network devices, the loop waiting for this counter
> > to decrement tests the 0 strict equality. Thus if an error occurs and
> > two references are given back by a protocol, we are stuck in the loop
> > forever, with a -1 value.

> > Robustness is added by checking a negative value: the device is then
> > considered free of references, and a warning is issued (it should not
> > happen, one should check that behavior)

> > Signed-off-by: Alexandre Besnard <alexandre.besnard@...tathome.com>
> > ---
> > net/core/dev.c | 5 +++++
> > 1 file changed, 5 insertions(+)

> > diff --git a/net/core/dev.c b/net/core/dev.c
> > index ddc551f..e4190ae 100644
> > --- a/net/core/dev.c
> > +++ b/net/core/dev.c
> > @@ -8687,6 +8687,11 @@ static void netdev_wait_allrefs(struct net_device *dev)
> > refcnt = netdev_refcnt_read(dev);

> > while (refcnt != 0) {
> > + if (refcnt < 0) {
> > + pr_warn("Device %s refcnt negative: device considered free, but it should not
> > happen\n",
> > + dev->name);
> > + break;
> > + }

> 1)I don't think this is a good approach. Negative value does not guarantee
> there is just a double put of device reference. Negative value is an indicator
> something goes wrong, and we definitely should not free device memory in
> this case.

> 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().

I agree with you, as it is not a full fix for a bad behavior of the refcnt: many
wrong things could happen here, and that's why I added a warning (short of a
more critical flag I could think of).

However, I think this is a good approach as a global workaround for any critical
situation caused by a negative refcnt, acting as a failsafe. What I try to avoid
here is not the bug, but a situation such as a deadlock keeping a system from
powering off, or way worse in the system life.
On the other hand, I can't think of a critical situation caused by freeing
the device memory. Processes or even systems may crash in some cases, but it
should be an expected behavior in such a case IMHO.

Actually, I think that with the current implementation, most of the systems
locked in the problem are powered off.

Do you think of any issue beyond this behavior ?