Re: [PATCH net] brcmfmac: clear skb head state on xmit

[Paolo Abeni <pabeni@redhat.com>]

On Wed, 2017-02-08 at 12:27 +0200, Kalle Valo wrote:
> Paolo Abeni <pabeni@redhat.com> writes:
> 
> > On Tue, 2017-02-07 at 20:23 +0100, Arend Van Spriel wrote:
> > > On 7-2-2017 17:50, Paolo Abeni wrote:
> > > > the skbs can be held by the driver for a long time, so we need
> > > > to clear any state on xmit to avoid hanging other subsystems.
> > > > The skbs are already orphaned later in cmsg code, so we just
> > > > need to clear the nf/dst/secpath.
> > > > Do it early, while the relevant entries are hopefully still
> > > > hot in the cache.
> > > 
> > > What is this about really? A bit more background about the issue
> > > might
> > > help understanding the need for this patch. Is this really specific
> > > to
> > > brcmfmac. For instance is something similar already done in mac80211?
> > 
> > The issue is apparently driver specific, as reported in:
> > 
> > https://bugzilla.redhat.com/show_bug.cgi?id=1294415
> > 
> > This is caused by xmit skbs carrying a notrack ct entry not being freed
> > by the device driver in a timely manner. Removing the ct module waits
> > for such entries refcount going to zero and hangs the kernel in busy
> > loop (for several minutes).
> > 
> > The relevant skbs are icmp6 packets (ND if I recall correctly, they
> > bcast packets at the mac level).
> > 
> > The only other known device driver suffering for the issue is the
> > infiniband ipoib driver, I send a separate patch for it.
> > 
> > I lack the broadcom h/w, but with infiniband the bug can be reproduced
> > with the following steps:
> > 
> > - ensure ipv6 is enabled on the target device, and firewalld is running
> > (e.g. the module nf_conntrack_ipv6 is loaded)
> > - assign a static ip to the device
> > - shut down the firewall (e.g. try to remove the module nf_conntrack)
> > 
> > For the brcmfmac driver most probably it is necessary being
> > disassociated from the AP before shutting down the firewall (but I
> > can't double check). This is probably why mac80211 does not suffer this
> > issue.
> > 
> > The root cause for the issue could be actually a firmware issue, any
> > better clues are more than welcome!
> 
> BTW, you should have added all this to the commit log. After reading the
> original description it left more questions open than answered them.

I'm sorry for the lack of clarity, thank you for the advice, will do
next time. 

I tried to keep the commit message short since others reviewer in other
area of the kernel really prefer shorter ones.

Cheers,

Paolo