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

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Feb 14, 2023 at 08:22:28AM -0800, Boqun Feng wrote:

> 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.

I don't think so, but I don't think I've carefully considered that case.

> 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)
> 

This should in fact complain about a CB-BC deadlock, (but I've not
tested it, just going on memories of how I implemented it).

> 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.

We do enter M into the dependency graph, but instead ignore M-M
recursion. Specifically so that we might catch the above deadlock vs B.