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Date:	Thu, 27 May 2010 08:16:55 +0200
From:	Jiri Slaby <jirislaby@...il.com>
To:	Haiyang Zhang <haiyangz@...rosoft.com>
CC:	Greg KH <gregkh@...e.de>,
	"'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
 (modified)

On 05/26/2010 11:25 PM, Haiyang Zhang wrote:
>> From: Greg KH [mailto:gregkh@...e.de]
>>> +			    VmbusChannelOpen(newChannel, 2 * PAGE_SIZE,
>>> +					     2 * PAGE_SIZE, NULL, 0,
>>> +					     hv_cb_utils[cnt].callback,
>>> +					     newChannel) == 0) {
>>> +				hv_cb_utils[cnt].channel = newChannel;
>>> +				mb();
>>
>> What is the mb() call for?  Why is it necessary?  (hint, if you need it,
>> something else is really wrong...)
> 
> It ensures the channel assignment happens before the wakeup call: 
> osd_WaitEventSet(ic_channel_ready), if the compiler optimization re-arrange 
> the execution order. 

wake_up() is a barrier, you don't need the mb() there.

BTW osd_WaitEventSet et al. can be easily converted to completion.

-- 
js
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