linux-kernel - Re: [PATCH 1/2] PM / Usermodehelper: Introduce reference counting to solve usermodehelper

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Date:	Wed, 07 Dec 2011 17:51:44 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	Tejun Heo <tj@...nel.org>
CC:	gregkh@...e.de, dhowells@...hat.com, eparis@...hat.com,
	rjw@...k.pl, kay.sievers@...y.org, jmorris@...ei.org, bp@...64.org,
	linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] PM / Usermodehelper: Introduce reference counting
 to solve usermodehelper_disabled race

On 12/06/2011 03:08 AM, Tejun Heo wrote:

> On Tue, Dec 06, 2011 at 02:56:15AM +0530, Srivatsa S. Bhat wrote:
>> This patch adds the necessary synchronization framework to fix the race
>> condition with the 'usermodehelper_disabled' flag, by implementing a
>> refcounting solution. Specifically, it introduces the pair get_usermodehelper()
>> and put_usermodehelper(), which can be used by the readers (those who want to
>> read the value of the usermodehelper_disabled flag, such as _request_firmware()
>> in this case). The writers (those who enable/disable usermodehelpers by
>> setting/resetting that flag) can use the pair umh_control_begin() and
>> umh_control_done().
>>
>> The reason for using a refcounting solution and not just a plain mutex, is
>> that we don't want to unnecessarily serialize all users of request_firmware(),
>> which act as readers. But note that we cannot use reader-writer locks here
>> because the readers sleep (waiting for the firmware load from user-space),
>> and sleeping with spinlocks held is not allowed. So refcounting implemented
>> using mutex locks underneath, seems to be the best fit here.
> 
> I haven't really read the patch yet but, from the description, why not
> use rwsem?
> 

Hey that would be perfect! That hadn't crossed my mind.. Thanks for pointing
it out. I'll respin the patchset using rwsemaphores.

Regards,
Srivatsa S. Bhat

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