netdev - RE: [PATCH net] hvsock: fix epollout hang from race condition

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Date:   Sat, 15 Jun 2019 07:23:29 +0000
From:   Sunil Muthuswamy <sunilmut@...rosoft.com>
To:     Dexuan Cui <decui@...rosoft.com>,
        David Miller <davem@...emloft.net>
CC:     KY Srinivasan <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        "sashal@...nel.org" <sashal@...nel.org>,
        Michael Kelley <mikelley@...rosoft.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH net] hvsock: fix epollout hang from race condition



> -----Original Message-----
> From: Dexuan Cui <decui@...rosoft.com>
> Sent: Friday, June 14, 2019 10:04 PM
> To: David Miller <davem@...emloft.net>; Sunil Muthuswamy <sunilmut@...rosoft.com>
> Cc: KY Srinivasan <kys@...rosoft.com>; Haiyang Zhang <haiyangz@...rosoft.com>; Stephen Hemminger
> <sthemmin@...rosoft.com>; sashal@...nel.org; Michael Kelley <mikelley@...rosoft.com>; netdev@...r.kernel.org; linux-
> hyperv@...r.kernel.org; linux-kernel@...r.kernel.org
> Subject: RE: [PATCH net] hvsock: fix epollout hang from race condition
> 
> > From: linux-hyperv-owner@...r.kernel.org
> > <linux-hyperv-owner@...r.kernel.org> On Behalf Of Dexuan Cui
> > Sent: Friday, June 14, 2019 8:23 PM
> > To: David Miller <davem@...emloft.net>; Sunil Muthuswamy
> > <sunilmut@...rosoft.com>
> > Cc: KY Srinivasan <kys@...rosoft.com>; Haiyang Zhang
> > <haiyangz@...rosoft.com>; Stephen Hemminger
> > <sthemmin@...rosoft.com>; sashal@...nel.org; Michael Kelley
> > <mikelley@...rosoft.com>; netdev@...r.kernel.org;
> > linux-hyperv@...r.kernel.org; linux-kernel@...r.kernel.org
> > Subject: RE: [PATCH net] hvsock: fix epollout hang from race condition
> >
> > > From: linux-hyperv-owner@...r.kernel.org
> > > <linux-hyperv-owner@...r.kernel.org> On Behalf Of David Miller
> > > Sent: Friday, June 14, 2019 7:15 PM
> > > To: Sunil Muthuswamy <sunilmut@...rosoft.com>
> > >
> > > This adds lots of new warnings:
> > >
> > > net/vmw_vsock/hyperv_transport.c: In function ‘hvs_probe’:
> > > net/vmw_vsock/hyperv_transport.c:205:20: warning: ‘vnew’ may be used
> > > uninitialized in this function [-Wmaybe-uninitialized]
> > >    remote->svm_port = host_ephemeral_port++;
> > >    ~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~
> > > net/vmw_vsock/hyperv_transport.c:332:21: note: ‘vnew’ was declared
> > here
> > >   struct vsock_sock *vnew;
> > >                      ^~~~
> > > net/vmw_vsock/hyperv_transport.c:406:22: warning: ‘hvs_new’ may be
> > > used uninitialized in this function [-Wmaybe-uninitialized]
> > >    hvs_new->vm_srv_id = *if_type;
> > >    ~~~~~~~~~~~~~~~~~~~^~~~~~~~~~
> > > net/vmw_vsock/hyperv_transport.c:333:23: note: ‘hvs_new’ was declared
> > > here
> > >   struct hvsock *hvs, *hvs_new;
> > >                        ^~~~~~~
> >
> > Hi David,
> > These warnings are not introduced by this patch from Sunil.
> 
> Well, technically speaking, the warnings are caused by Suni's patch, but I think it should
> be a bug of gcc (I'm using "gcc (Ubuntu 8.2.0-12ubuntu1) 8.2.0"). As you can see, the
> line numbers given by gcc, i.e. line 205, line 406, are not related to the warnings.
> 
> Actually, the same warnings can repro with the below one-line patch on this commit of
> today's net.git:
>     9a33629ba6b2 ("hv_netvsc: Set probe mode to sync")
> 
> --- a/net/vmw_vsock/hyperv_transport.c
> +++ b/net/vmw_vsock/hyperv_transport.c
> @@ -403,6 +403,7 @@ static void hvs_open_connection(struct vmbus_channel *chan)
> 
>         set_per_channel_state(chan, conn_from_host ? new : sk);
>         vmbus_set_chn_rescind_callback(chan, hvs_close_connection);
> +       asm ("nop");
> 
>         if (conn_from_host) {
>                 new->sk_state = TCP_ESTABLISHED;
> 
> It looks a simple inline assembly code can confuse gcc. I'm not sure if I should
> report a bug for gcc...
> 
> I posted a patch to suppress these bogus warnings just now. The Subject is:
> 
> [PATCH net] hv_sock: Suppress bogus "may be used uninitialized" warnings
> 
> Thanks,
> -- Dexuan

Yes, these warnings are not specific to this patch. And, additionally these
should already addressed in the commit 
https://git.kernel.org/pub/scm/linux/kernel/git/davem/net-next.git/commit/?id=ac383f58f3c98de37fa67452acc5bd677396e9f3
I was trying to avoid making the same changes here to avoid merge
conflicts when 'net-next' merges with 'net' next.