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

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Date:	Wed, 07 Dec 2011 22:59:30 +0900
From:	Namhyung Kim <namhyung@...il.com>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, gregkh@...e.de, dhowells@...hat.com,
	eparis@...hat.com, rjw@...k.pl, kay.sievers@...y.org,
	jmorris@...ei.org, tj@...nel.org, bp@...64.org,
	linux-pm@...r.kernel.org
Subject: Re: [PATCH 1/2] PM / Usermodehelper: Introduce reference counting
 to solve usermodehelper_disabled race

2011-12-07 (수), 18:00 +0530, Srivatsa S. Bhat:
> On 12/06/2011 06:29 AM, Namhyung Kim wrote:
> 
> > Hello,
> > 
> > 2011-12-06 오전 6:26, Srivatsa S. Bhat 쓴 글:
> >> The refcounting solution implemented here is adapted from the one used in
> >> the CPU hotplug infrastructure (kernel/cpu.c). If this patchset sounds
> >> reasonable, I plan to make the refcounting generic (in a later patch) and
> >> expose it via include/linux/refcount.h or something similar, and then
> >> use it
> >> at these 2 places instead of duplicating code.
> >>
> > 
> > IMHO it seems that the write path of the cpu_hotplug is protected by
> > another mutex (cpu_add_remove_lock) to guarantee that the only one
> > writer is active at a time. But I'm not sure this is the case for the
> > umhelper too.
> 
> 
> For the umhelper, I had not added anything explicit for this serialization
> because, all the users of usermodehelper_disable/enable are callers
> from hibernate/suspend code (which all take the 'pm_mutex' lock before
> doing anything) or from reboot/shutdown code.
> 

OK.


> > 
> > If more than 2 tasks call umh_control_begin() at the same time (is it
> > possible though?), it will lost tasks except for the winner and
> > active_writer AFAICS. Am I missing something?
> > 
> 
> 
> See my thoughts above about the callers of umh_control_begin(). 
> 
> Anyways, I'll use rwsemaphores as Tejun suggested, since that would be
> the most logical choice here, and it also makes the code much simpler.
> 
> Thanks a lot for your review!
> 

Yeah, I think the rwsem would be more reasonable, too.


> [Btw I was wondering why your mail didn't land in my inbox. Now I see,
> I am neither in your "To" or "Cc" list! :-)]
> 

My email client was screwed up at that time, maybe due to stupid
security stuff in my company :(

Thanks.


-- 
Regards,
Namhyung Kim


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