linux-kernel - RE: [patch] Staging: hv: Fix vmbus load hang caused by wrong data packing

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Date:	Fri, 16 Oct 2009 20:03:35 +0000
From:	Hank Janssen <hjanssen@...rosoft.com>
To:	Pekka Enberg <penberg@...helsinki.fi>, Greg KH <gregkh@...e.de>
CC:	Tom Hanrahan <hanrahat@...rosoft.com>,
	Hashir Abdi <habdi@...rosoft.com>,
	Haiyang Zhang <haiyangz@...rosoft.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [patch] Staging: hv: Fix vmbus load hang caused by wrong data
 packing

Greg / Pecca,

>We don't merge fixes to the kernel unless we understand _why_ they fix
>things. It's pretty likely that the problem here is that one or more
>of the structs you are putting under "#pragma pack" just need to be
>annotated with "attribute packed".

We have figured out what the problem is with it. 

It seems that the #pragma pack(push,1) doesn't change the size 
of "enum vmbus_channel_message_type", the size remains as 4 bytes, 
which is the default. I do not know if this is the result of
a gcc bug.

If you add __attribute__((packed)) here, the size becomes 1 byte, 
which couldn't be accepted by HyperV host, and causes vmbus load 
to hang.

So, the different behavior of #pragma pack(push,1) v.s.  
__attribute__((packed)) on the data struct, "enum vmbus_channel_message_type",
caused this bug. The fix is to remove __attribute__((packed)) of this data. 
We don't need to add #pragma pack(push,1) or #pragma pack(pop) here, since 
it has no effect on this structure. (BTW, seems it's a bug of gcc's 
handling of pragma pack().)

I will submit a patch later today that corrects the bug.

This patch in effect removes part of a previous patch somebody else 
Submitted that introduced this problem.

This bug masked another problem we encountered as soon as we fixed this
Problem. I am getting a NULL pointer problem on insertion of the hv_vmbus
Driver. I will write a more detailed description of that problem and send 
It to the list.

I checked the originally submitted code, and made that work with 2.6.32.
And that does run okay. 

Thanks,

Hank.

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