linux-kernel - Re: [PATCH 1/1] staging: hv: Fix race condition on IC channel initialization

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Date:	Fri, 21 May 2010 13:55:27 -0700
From:	Greg KH <gregkh@...e.de>
To:	Ky Srinivasan <ksrinivasan@...ell.com>
Cc:	Haiyang Zhang <haiyangz@...rosoft.com>,
	"'devel@...verdev.osuosl.org'" <devel@...verdev.osuosl.org>,
	"'virtualization@...ts.osdl.org'" <virtualization@...ts.osdl.org>,
	"'linux-kernel@...r.kernel.org'" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/1] staging: hv: Fix race condition on IC channel
 initialization

On Fri, May 21, 2010 at 02:21:46PM -0600, Ky Srinivasan wrote:
> 
> 
> >>> On 5/21/2010 at  4:12 PM, in message <20100521201228.GA6712@...e.de>, Greg KH
> <gregkh@...e.de> wrote: 
> > On Fri, May 21, 2010 at 07:58:26PM +0000, Haiyang Zhang wrote:
> >> From: Haiyang Zhang <haiyangz@...rosoft.com>
> >> 
> >> Subject: staging: hv: Fix race condition on IC channel initialization
> >> There is a possible race condition when hv_utils starts to load immediately
> >> after hv_vmbus is loading - null pointer error could happen.
> >> This patch added an atomic counter to ensure all channels are ready before
> >> vmbus_init() returns. So another module won't have any uninitialized 
> > channel.
> > 
> > Better, but not quite ready...
> > 
> >> +/* Counter of IC channels initialized */
> >> +atomic_t hv_utils_initcnt = ATOMIC_INIT(0);
> > 
> > This doesn't need to be an atomic variable, does it really?
> > 
> > Why not have a simple bool variable "vmbus_initialized" or something.
> > It starts out as false, and then turns true when you are up and ready.
> > Then provide a function that tests it:
> > 	bool hv_vmbus_ready(void)
> > 	{
> > 		return vmbus_initialized
> > 	}
> > 	EXPORT_SYMBOL_GPL(hv_vmbus_ready);
> I agree with Greg; I would go a step further and deal with this issue
> as part of loading the bus driver. After all, we already have
> dependencies established for various LIC drivers  on the bus driver.
> The fact that even after the bus driver is loaded we cannot reliably
> load other drivers implies that there is an additional dependency that
> is not currently being handled. Why can't we ensure that the bus
> driver is fully initialized before we are done with loading the bus
> driver.

Um, I think that is what this patch fixes :)

It just doesn't do it in a way that I think is very good...

thanks,

greg k-h
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