From: Alexandre BESNARD <alexandre.besnard@softathome.com>
To: Kirill Tkhai <ktkhai@virtuozzo.com>
Cc: davem@davemloft.net, amritha nambiar <amritha.nambiar@intel.com>,
lirongqing@baidu.com, netdev@vger.kernel.org,
linux-kernel@vger.kernel.org,
alexander h duyck <alexander.h.duyck@intel.com>,
jiri@mellanox.com, petrm@mellanox.com, ecree@solarflare.com
Subject: Re: [PATCH] net: check negative value for signed refcnt
Date: Thu, 31 Jan 2019 16:14:20 +0100 (CET) [thread overview]
Message-ID: <561703429.2347936.1548947660762.JavaMail.zimbra@softathome.com> (raw)
In-Reply-To: <8b654b17-f8b4-0c89-26b5-311aeb703f6d@virtuozzo.com>
Hi Kirill, and thanks for your time,
On 31 Jan 19 14:49, Kirill Tkhai ktkhai@virtuozzo.com wrote :
> Hi, Alexandre,
> On 31.01.2019 16:20, alexandre.besnard@softathome.com wrote:
> > From: Alexandre Besnard <alexandre.besnard@softathome.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@softathome.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 ?
next prev parent reply other threads:[~2019-01-31 15:14 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-01-31 13:20 [PATCH] net: check negative value for signed refcnt alexandre.besnard
2019-01-31 13:49 ` Kirill Tkhai
2019-01-31 15:14 ` Alexandre BESNARD [this message]
2019-01-31 15:31 ` Kirill Tkhai
2019-01-31 15:15 ` Eric Dumazet
2019-01-31 15:21 ` Eric Dumazet
2019-01-31 15:34 ` Kirill Tkhai
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