Re: [PATCH RFC] drivers/core: Replace lockdep_set_novalidate_class() with unique class keys

[Boqun Feng <boqun.feng@gmail.com>]

On Tue, Feb 14, 2023 at 11:48:07AM +0100, Peter Zijlstra wrote:
> On Mon, Feb 13, 2023 at 05:51:11PM -0800, Boqun Feng wrote:
> > On Mon, Feb 13, 2023 at 05:29:49PM +0100, Peter Zijlstra wrote:
> > > On Mon, Feb 13, 2023 at 10:25:59AM -0500, Alan Stern wrote:
> > > > On Mon, Feb 13, 2023 at 10:24:13AM +0100, Peter Zijlstra wrote:
> > > > > On Sun, Feb 12, 2023 at 10:23:44AM -0500, Alan Stern wrote:
> > > > > > Provided it acquires the parent device's lock first, this is 
> > > > > > utterly safe no matter what order the children are locked in.  Try 
> > > > > > telling that to lockdep! 
> > > > > 
> > > > > mutex_lock_next_lock(child->lock, parent->lock) is there to express this
> > > > > exact pattern, it allows taking multiple child->lock class locks (in any
> > > > > order) provided parent->lock is held.
> > > > 
> > > > Ah, this is news to me.  Is this sort of thing documented somewhere?
> > 
> > Basically if you have two lock instances A and B with the same class,
> > and you know that locking ordering is always A -> B, then you can do
> > 
> > 	mutex_lock(A);
> > 	mutex_lock_nest_lock(B, A); // lock B.
> > 
> 
> No, this isn't quite right, You need at least 3 locks and 2 classes.
> 
> P, C1, C2
> 
> Then:
> 
> 	mutex_lock(P)
> 	mutex_lock_next_lock(C1, P)
> 	mutex_lock_next_lock(C2, P)
> 
> And it will accept any order of Cn -- since it assumes that any
> multi-lock of Cn will always hold P, therefore the ordering fully given
> by P.

Ah, right, I was missing the fact that it works with 2 classes...

But I think with only one class, the nest_lock() still works, right?
In other words, if P and Cn are the same lock class in your example.

Also seems I gave a wrong answer to Alan, just to clarify, the following
is not a deadlock to lockdep:

T1:
	mutex_lock(P)
	mutex_lock_next_lock(C1, P)
	mutex_lock_next_lock(C2, P)
	mutex_lock(B)

T2:
	mutex_lock(P)
	mutex_lock(B)
	mutex_lock_next_lock(C1, P)
	mutex_lock_next_lock(C2, P)

Because of any pair of

	mutex_lock(L);
	... // other locks maybe
	mutex_lock_nest_lock(M, L);

lockdep will not add M into the dependency graph, since it's nested and
should be serialized by L.

Regards,
Boqun