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

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Message-ID: <20111205213832.GM627@google.com>
Date:	Mon, 5 Dec 2011 13:38:32 -0800
From:	Tejun Heo <tj@...nel.org>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
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 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?

Thanks.

-- 
tejun
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