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

[Paul E. McKenney]

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@xxxxxxxxxx>
> > 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@xxxxxxxxxx>
> 
> Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
> 
> >     Cc: Christoph Lameter <cl@xxxxxxxxx>
> >     Cc: Pekka Enberg <penberg@xxxxxxxxxx>
> >     Cc: David Rientjes <rientjes@xxxxxxxxxx>
> >     Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> >     Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> >     Cc: Vlastimil Babka <vbabka@xxxxxxx>
> >     Cc: Roman Gushchin <roman.gushchin@xxxxxxxxx>
> >     Cc: Hyeonggon Yoo <42.hyeyoo@xxxxxxxxx>
> >     Cc: <linux-mm@xxxxxxxxx>
> > 
> > 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 */
>