linux-kernel - Re: Towards eliminating the freezer

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Date:	Tue, 24 Jul 2007 10:27:24 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Oliver Neukum <oliver@...kum.org>
cc:	LKML <linux-kernel@...r.kernel.org>,
	linux-pm <linux-pm@...ts.linux-foundation.org>
Subject: Re: Towards eliminating the freezer

On Tue, 24 Jul 2007, Oliver Neukum wrote:

> Am Montag 23 Juli 2007 schrieb Alan Stern:
> > Now here's an idea which might work.  Can we require every caller of
> > device_add() to hold some existing device's semaphore?  Normally it
> > would be the semaphore of the new device's parent, but it could be a
> > higher ancestor.  There even could be a single "root" semaphore for
> > drivers registering a top-level device with no parent.
> 
> What prevents us from having a device addition semaphore?
> Adding device is not critical to performance, is it?

It would create a locking order violation.  Many drivers hold a device
semaphore while registering a child device, so they would acquire your
new semaphore while holding a device sem.  But the PM core needs to
prevent registration while calling suspend() methods, so it would need
to acquire the device sems while holding your new semaphore.

Alan Stern

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