From: linux-hyperv-owner@vger.kernel.org
[off-list ref] On Behalf Of Dexuan Cui
Sent: Friday, June 14, 2019 8:23 PM
To: David Miller <davem@davemloft.net>; Sunil Muthuswamy
[off-list ref]
Cc: KY Srinivasan <kys@microsoft.com>; Haiyang Zhang
[off-list ref]; Stephen Hemminger
[off-list ref]; sashal@kernel.org; Michael Kelley
[off-list ref]; netdev@vger.kernel.org;
linux-hyperv@vger.kernel.org; linux-kernel@vger.kernel.org
Subject: RE: [PATCH net] hvsock: fix epollout hang from race condition

quoted

From: linux-hyperv-owner@vger.kernel.org
[off-list ref] On Behalf Of David Miller
Sent: Friday, June 14, 2019 7:15 PM
To: Sunil Muthuswamy <redacted>

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

quoted

  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