linux-kernel - RE: [PATCH 081/117] Staging: hv: vmbus: Introduce a lock to protect the ext field in hv

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Message-ID: <6E21E5352C11B742B20C142EB499E048081B238B@TK5EX14MBXC126.redmond.corp.microsoft.com>
Date:	Wed, 24 Aug 2011 00:55:12 +0000
From:	KY Srinivasan <kys@...rosoft.com>
To:	Greg KH <greg@...ah.com>
CC:	"gregkh@...e.de" <gregkh@...e.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"devel@...uxdriverproject.org" <devel@...uxdriverproject.org>,
	"virtualization@...ts.osdl.org" <virtualization@...ts.osdl.org>,
	Haiyang Zhang <haiyangz@...rosoft.com>
Subject: RE: [PATCH 081/117] Staging: hv: vmbus: Introduce a lock to protect
 the ext field in hv_device



> -----Original Message-----
> From: Greg KH [mailto:greg@...ah.com]
> Sent: Tuesday, August 23, 2011 7:08 PM
> To: KY Srinivasan
> Cc: gregkh@...e.de; linux-kernel@...r.kernel.org;
> devel@...uxdriverproject.org; virtualization@...ts.osdl.org; Haiyang Zhang
> Subject: Re: [PATCH 081/117] Staging: hv: vmbus: Introduce a lock to protect the
> ext field in hv_device
> 
> On Fri, Jul 15, 2011 at 10:47:09AM -0700, K. Y. Srinivasan wrote:
> > The current mechanism for handling references in broken.
> > Introduce a lock to protect the ext field in hv_device.
> 
> Why would that lock ever be needed?  How can things change to this
> pointer in different ways like you are thinking it could?  Doesn't the
> reference counting in the device itself handle this properly?

This is to deal with a potential race condition between the driver being
unloaded and incoming traffic from the VMBUS side. The ext pointer is 
device specific (either pointing to a storage or a network device) and what
we are protecting is the pointer being set to NULL from the unload side when
we might have a racing access from the interrupt side (on the incoming vmbus
traffic).

Regards,

K. Y 

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