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

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Date:	Thu, 17 Jan 2008 16:56:26 +0800
From:	"Dave Young" <hidave.darkstar@...il.com>
To:	"Jarek Poplawski" <jarkao2@...il.com>
Cc:	"Greg KH" <gregkh@...e.de>, stefanr@...6.in-berlin.de,
	david-b@...bell.net, stern@...land.harvard.edu,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 7/7] driver-core : convert semaphore to mutex in struct class

On Jan 17, 2008 4:38 PM, Jarek Poplawski <jarkao2@...il.com> wrote:
> On 17-01-2008 02:17, Dave Young wrote:
> > On Jan 16, 2008 4:34 PM, Jarek Poplawski <jarkao2@...il.com> wrote:
> >> On Wed, Jan 16, 2008 at 09:03:03AM +0800, Dave Young wrote:
> >> ...
> >>> The lockdep warining was posted in the below thread, actually, I have
> >>> built and run this patced kernel for several days, there's no more
> >>> warnings.
> >>> http://lkml.org/lkml/2008/1/3/2
> >> Right... But, with something like this:
> >>
> >> ... have_some_fun(... cls)
> >> {
> >>         mutex_lock_nested(&cls->mutex, SINGLE_DEPTH_NESTING);
> >>         have_other_fun(cls);
> >>         mutex_unlock(&cls->mutex);
> >>
> >> }
> >>
> >> ... have_more_fun(...)
> >> {
> >>         ...
> >>
> >>         mutex_init(&cls->mutex);
> >>
> >>         mutex_lock(&cls->mutex);
> >>         have_some_fun(cls);
> >>         mutex_unlock(&cls->mutex);
> >> }
> >>
> >> probably you wouldn't get any lockdep warning too...
> >
> > Sorry for late reply.
> > Actually, I don't know much about lockdep. Could you tell how to use
> > it properly in this scenario?
>
> As you have noticed while working on this patch, if two different
> instances of the same structure containig some lock are created in
> the same place, lockdep will treat this one (the same) lock. It looks
> strange, but actually it's a feature which enables to track
> dependencies between different locks on 'class' level instead of
> instance 'level'. The downside is that lockdep is very often too
> sensitive by default, so you have to 'annotate' when instancess are
> actually on different level (e.g. parents and children here) and
> could be locked at the same time or in some order.
>
> You can use e.g. mutex_lock_nested() or lockdep_set_class*() for this.
> Then lockdep simply trusts you, and starts to think they are different
> locks. If you do it wrong there will be simply no more warnings, but
> undercover lockups still possible (and diagnosing a bit harder then).
> So, since in your patch there are two levels of locking, and you
> started to annotate lockdep about a child taking parent's class lock
> with:
> mutex_lock_nested(&parent_class->mutex, SINGLE_DEPTH_NESTING);
>
> you should do the same everywhere in a situation like this.
> lockdep will treat this simply as lock B vs. A (mutex_lock(&cls))
> dependencies.

Thanks for the explain, let me check the lockings again.

>
> Regards,
> Jarek P.
>
> PS: BTW, it seems after this patch 1/1 the locking was changed a bit,
> so these previous tests could be not enough.
>
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