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

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Date:	Sat, 22 May 2010 09:10:44 -0600
From:	"Ky Srinivasan" <ksrinivasan@...ell.com>
To:	"Greg KH" <gregkh@...e.de>
Cc:	"'devel@...verdev.osuosl.org'" <devel@...verdev.osuosl.org>,
	"'virtualization@...ts.osdl.org'" <virtualization@...ts.osdl.org>,
	"Haiyang Zhang" <haiyangz@...rosoft.com>,
	"'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 5/21/2010 at  4:55 PM, in message <20100521205527.GB9594@...e.de>, Greg KH
<gregkh@...e.de> wrote: 
> 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...
Ok, my mistake. When I saw hv_vmbus_ready function being exported, I was under the impression each of the drivers that depend on the bus driver to check if the bus driver was properly initialized.

Regards,

K. Y
> 
> thanks,
> 
> greg k-h


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