linux-kernel - Re: [PATCH 7/7] driver-core : convert semaphore to mutex in struct class

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Message-ID: <20080118073836.GA1703@ff.dom.local>
Date:	Fri, 18 Jan 2008 08:38:36 +0100
From:	Jarek Poplawski <jarkao2@...il.com>
To:	Dave Young <hidave.darkstar@...il.com>
Cc:	Kay Sievers <kay.sievers@...y.org>,
	Alan Stern <stern@...land.harvard.edu>,
	Greg KH <gregkh@...e.de>, stefanr@...6.in-berlin.de,
	David Brownell <david-b@...bell.net>,
	Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 7/7] driver-core : convert semaphore to mutex in struct
	class

On Fri, Jan 18, 2008 at 01:31:17PM +0800, Dave Young wrote:
> On Jan 18, 2008 11:18 AM, Kay Sievers <kay.sievers@...y.org> wrote:
...
> > Yeah, might be better to wait until class_device is gone, otherwise you
> > may need to fix stuff that is just going to be removed. Your change to
> > have iterators for the class devices look like a nice preparation for
> > future changes though.
> >
> > Our rough plan is:
> >  2.6.25:
> >   - get the ~100 patches in Greg's tree (in -mm) merged :)
> >  2.6.26:
> > ???  - remove the 20 char limit in "struct device"
> >   - get rid of "struct class_device"
> 
> Fine, thanks.
> 
> Let's wait for other people's comment.

Dave, I doubt you'll ever manage to do this if you're going to wait:
probably there will be always some new changes like this around...

IMHO, it would be nice to get the real state of current lockdep
problems here to figure out if there is any chance to do this right &
without any warnings with current lockdep. If I got it right from
earlier threads it might be impossible with USB, at least.

So, since I think these nesting levels seem to be wrong in 7/7 patch,
maybe it's better to exclude it from this patchset, and to try this as
testing for some time.

My proposal is to do the annotations with mutex_lock_nested(),
everywhere in this patch, according to 'real' relations between these
classes from thread POV, so e.g.:

mutex_lock_nested(&some_class->mutex, CLASS_PARENT);
mutex_lock_nested(&some_class->mutex, CLASS_CLASS);
...or more if needed:
mutex_lock_nested(&some_class->mutex, CLASS_CHILD);
mutex_lock_nested(&some_class->mutex, CLASS_ROOT);
...etc.

using adequate names for these enums, and only after this check what
lockdep thinks about it. Then, if there are no obvious mistakes, but
lockdep doesn't like it, send the patch and report without trying to
'silence' lockdep, so we could see what's going on, and if there are
any chances to make it right.

Thanks,
Jarek P.
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