linux-kernel - Re: check && lockdep_no_validate (Was: lockdep: Introduce wait-type checks)

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Date:	Fri, 17 Jan 2014 17:31:11 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: check && lockdep_no_validate (Was: lockdep: Introduce
	wait-type checks)

On 01/16, Alan Stern wrote:
>
> On Thu, 16 Jan 2014, Peter Zijlstra wrote:
>
> > On Thu, Jan 16, 2014 at 06:43:48PM +0100, Oleg Nesterov wrote:
>
> > > Perhaps we should change the meaning of lockdep_set_novalidate_class?
> > > (perhaps with rename). What do you think about the patch below?
> > >
> > > With this patch __lockdep_no_validate__ means "automatically nested",
> >
> > Yes, I suppose that might work, it would allow some validation.
>
> I haven't seen the patch, but I'm not so sure it will work.  Suppose we
> have two devices, D1 and D2, and some other mutex, M.  Then the locking
> pattern:
>
> 	lock(D1);
> 	lock(M);
> 	unlock(M);
> 	unlock(D1);
>
> generally should not conflict with:
>
> 	lock(M);
> 	lock(D2);
> 	unlock(D2);
> 	unlock(M);

Yes, sure. This change assumes that the only problem in drivers/base is
dev->parent->mutex / dev->mutex dependency. If the locking is even more
"broken" (wrt lockdep), we can't replace lockdep_set_novalidate_class()
with lockdep_set_auto_nested().

And, otoh, with this change lockdep can miss the real problems too, for
example:

	func1(dev)
	{
		device_lock(dev->parent);
		mutex_lock(MUTEX);
		device_lock(dev);
		...
	}

	func2(dev)
	{
		device_lock(dev);
		mutex_lock(MUTEX);
		...
	}

lockdep will only notice dev -> MUTEX dependency.

I booted the kernel (under kvm) with this change and there is nothing
in dmesg, but of course this is not the real testing.

So do you think that dev->mutex should not be validated at all ?


Just in case... Of course, if we actually add auto_nested we should not
use a single class unless dev->mutex will be the only user.

Oleg.

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