Re: [PATCH rcu 5/8] slab: Explain why SLAB_DESTROY_BY_RCU reference before locking

[Christoph Lameter]

On Wed, 19 Oct 2022, Paul E. McKenney wrote:

> It is not obvious to the casual user why it is absolutely necessary to
> acquire a reference to a SLAB_DESTROY_BY_RCU structure before acquiring
> a lock in that structure.  Therefore, add a comment explaining this point.

Sorry but this is not correct and difficult to comprehend.

1. You do not need a reference to a slab object after it was allocated.
   Objects must be properly protected by rcu_locks.

2. Locks are initialized once on slab allocation via a constructor (*not* on object allocation via kmem_cache_alloc)

3. Modifying locks at allocation/free is not possible since references to
   these objects may still persist after free and before alloc.

4. The old term SLAB_DESTROY_BY_RCU is used here.