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

[Alan Stern]

On Mon, Feb 13, 2023 at 05:51:11PM -0800, Boqun Feng wrote:
> 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.
> 
> to tell the lockdep this is not deadlock, plus lockdep will treat the
> acquisition of A and the precondition of acquisition B, so the following
> is not a deadlock as well:
> 
> T1:
> 	mutex_lock(A);
> 	mutex_lock(C);
> 	mutex_lock_nest_lock(B, A);
> 
> T2:
> 	mutex_lock(A);
> 	mutex_lock_nest_lock(B, A);
> 	mutex_lock(C);

Why isn't this treated as a deadlock?  It sure looks like a deadlock to 
me.  Is this an example where lockdep just doesn't get the right answer?

Alan Stern