linux-kernel - Re: [PATCH rcu 5/8] slab: Explain why SLAB_DESTROY_BY

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Date:   Fri, 21 Oct 2022 09:44:23 +0200 (CEST)
From:   Christoph Lameter <cl@...two.de>
To:     "Paul E. McKenney" <paulmck@...nel.org>
cc:     rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...com, rostedt@...dmis.org,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Hyeonggon Yoo <42.hyeyoo@...il.com>, linux-mm@...ck.org
Subject: Re: [PATCH rcu 5/8] slab: Explain why SLAB_DESTROY_BY_RCU reference
 before locking

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.