linux-kernel - Re: [RFC PATCH v5 2/2] Use kernfs_break_active_protection() for device online store callbacks

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Date:	Wed, 23 Apr 2014 12:52:44 -0400
From:	Tejun Heo <tj@...nel.org>
To:	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>
Cc:	Li Zhong <zhong@...ux.vnet.ibm.com>,
	LKML <linux-kernel@...r.kernel.org>, gregkh@...uxfoundation.org,
	toshi.kani@...com
Subject: Re: [RFC PATCH v5 2/2] Use kernfs_break_active_protection() for
 device online store callbacks

Hello, Rafael.

On Wed, Apr 23, 2014 at 06:12:01PM +0200, Rafael J. Wysocki wrote:
> >Why add this additional global lock across multiple subsystems?
> 
> That basically is because of how eject works when it is triggered via ACPI.
> 
> It is signaled for a device at the top of a subtree.  It may be a
> container of some sort and the eject involves everything below that
> device in the ACPI namespace.  That may involve multiple subsystem
> (CPUs, memory, PCI host bridge, etc.).
> 
> We do that in two steps, offline (which can fail) and eject proper
> (which can't fail and makes all of the involved devices go away).
> All that has to be done in one go with respect to the
> sysfs-triggered offline/online and that's why the lock is there.

Ah, okay, so it's actually synchronzing across multiple subsystems.  I
think it definitely calls for more documentation.  The requirement is
pretty unusual after all.

Thanks!

-- 
tejun
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