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

[Vlastimil Babka <vbabka@suse.cz>]

On 10/21/22 17:42, Paul E. McKenney wrote:
> On Fri, Oct 21, 2022 at 03:50:17PM +0200, Vlastimil Babka wrote:
>> On 10/21/22 15:43, Paul E. McKenney wrote:
>> > On Fri, Oct 21, 2022 at 09:44:23AM +0200, Christoph Lameter wrote:
>> >> 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.
>> > 
>> > Thank you for looking this over, but Vlastimil beat you to it.  How does
>> > the update below look?
>> 
>> LGTM.
> 
> May I please have your ack?
> 
> 							Thanx, Paul
> 
>> > ------------------------------------------------------------------------
>> > 
>> > commit ff4c536e6b44e2e185e38c3653851f92e07139da
>> > Author: Paul E. McKenney <paulmck@kernel.org>
>> > Date:   Mon Sep 26 08:57:56 2022 -0700
>> > 
>> >     slab: Explain why SLAB_TYPESAFE_BY_RCU reference before locking
>> >     
>> >     It is not obvious to the casual user why it is absolutely necessary to
>> >     acquire a reference to a SLAB_TYPESAFE_BY_RCU structure before acquiring
>> >     a lock in that structure.  Therefore, add a comment explaining this point.
>> >     
>> >     [ paulmck: Apply Vlastimil Babka feedback. ]
>> >     
>> >     Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
>> 
>> Acked-by: Vlastimil Babka <vbabka@suse.cz>

It was there :)

>> >     Cc: Christoph Lameter <cl@linux.com>
>> >     Cc: Pekka Enberg <penberg@kernel.org>
>> >     Cc: David Rientjes <rientjes@google.com>
>> >     Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
>> >     Cc: Andrew Morton <akpm@linux-foundation.org>
>> >     Cc: Vlastimil Babka <vbabka@suse.cz>
>> >     Cc: Roman Gushchin <roman.gushchin@linux.dev>
>> >     Cc: Hyeonggon Yoo <42.hyeyoo@gmail.com>
>> >     Cc: <linux-mm@kvack.org>
>> > 
>> > diff --git a/include/linux/slab.h b/include/linux/slab.h
>> > index 90877fcde70bd..487418c7ea8cd 100644
>> > --- a/include/linux/slab.h
>> > +++ b/include/linux/slab.h
>> > @@ -76,6 +76,17 @@
>> >   * rcu_read_lock before reading the address, then rcu_read_unlock after
>> >   * taking the spinlock within the structure expected at that address.
>> >   *
>> > + * Note that it is not possible to acquire a lock within a structure
>> > + * allocated with SLAB_TYPESAFE_BY_RCU without first acquiring a reference
>> > + * as described above.  The reason is that SLAB_TYPESAFE_BY_RCU pages
>> > + * are not zeroed before being given to the slab, which means that any
>> > + * locks must be initialized after each and every kmem_struct_alloc().
>> > + * Alternatively, make the ctor passed to kmem_cache_create() initialize
>> > + * the locks at page-allocation time, as is done in __i915_request_ctor(),
>> > + * sighand_ctor(), and anon_vma_ctor().  Such a ctor permits readers
>> > + * to safely acquire those ctor-initialized locks under rcu_read_lock()
>> > + * protection.
>> > + *
>> >   * Note that SLAB_TYPESAFE_BY_RCU was originally named SLAB_DESTROY_BY_RCU.
>> >   */
>> >  /* Defer freeing slabs to RCU */
>>