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

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Date:	Fri, 21 May 2010 15:22:01 -0700
From:	Greg KH <gregkh@...e.de>
To:	Haiyang Zhang <haiyangz@...rosoft.com>
Cc:	"'linux-kernel@...r.kernel.org'" <linux-kernel@...r.kernel.org>,
	"'devel@...verdev.osuosl.org'" <devel@...verdev.osuosl.org>,
	"'virtualization@...ts.osdl.org'" <virtualization@...ts.osdl.org>,
	Hank Janssen <hjanssen@...rosoft.com>
Subject: Re: [PATCH 1/1] staging: hv: Fix race condition on IC channel
 initialization

On Fri, May 21, 2010 at 10:07:17PM +0000, Haiyang Zhang wrote:
> > From: Greg KH [mailto:gregkh@...e.de]
> > > +/* 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);
> > 
> > 
> > this turns into a simple function call, again, never needing to know
> > about message types or any other mess.
> 
> This looks good. I will add the hv_vmbus_ready() function. It doesn't even 
> have to be exported symbol, because it's only used in vmbus module to ensure 
> all channels are ready before vmbus_init() returns. Other modules won't get a 
> chance to see uninitialized channels after hv_vmbus is loaded.
> 
> Also, I'll cleanup the printk in hv_utils load/unload.
> 
> Regarding the atomic variable -- the channel offer processing function is 
> triggered by interrupts from host -- should we be concerned about "counter++" 
> racing with each other in two interrupts happening around the same time?

If you are, having races like this, then you should be using a lock to
protect lots of things, not just one single atomic variable, right?

thanks,

greg k-h
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