Re: [PATCH 1/2] staging: hv: Fix race condition in hv_utils moduleinitialization.

From: Greg KH
Date: Thu May 20 2010 - 02:37:51 EST

On Wed, May 19, 2010 at 10:12:51PM +0000, Haiyang Zhang wrote:
> > > Actually, we already assign a default callback function,
> > chn_cb_negotiate(),
> > > when the channels are opened in vmbus module. It's a real function
> > and can
> > > handle common negotiation messages.
> >
> > Then why don't you use it here?
> When vmbus is loaded and channel is offered from HyperV host, the default
> callback function, chn_cb_negotiate(), is assigned to the function ptr, and
> used to do basic responses of negotiation messages.

Great, so that works.

> After hv_utils modules is loaded the callback function ptr is overridden by
> a specialized function in hv_utils module, and handles each feature (shutdown,
> timesync, etc.) differently.

That's the problem. Provide a "correct" interface to properly change
the callback function. Just setting function pointers in a random
manner is ripe for all sorts of bad problems, don't you agree? Heck, I
don't see any locking happening here which could cause messages to be
handled when things are only half-way set up. Also, what's to say your
function pointer write is atomic in the first place :)

In short, use proper locking for something like this.

> > I still think there's a real problem somewhere else in the architecture
> > if such a sleep is necessary...
> >
> > Is the issue that the modprobe of the hv_vmbus can return before the
> > bus
> > is really all set up and ready to go? If so, just fix that, then you
> > will not need any "sleep" calls anywhere, right?
> After vmbus is loaded, the channel offering will come from the host, then
> it initializes the channel. The channel offering can happen a little later
> after vmbus_init() is done and modprobe returns. So I think we should let
> vmbus_init function wait(sleep) until all channel offerings are received before
> returning. This will ensure all channels are ready before modprobe returns.

That sounds like a good idea.


greg k-h
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