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. > 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 */
