On 10/20/22 00:46, 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. s/SLAB_DESTROY_BY_RCU/SLAB_TYPESAFE_BY_RCU/ in subject, commit log and the added comment? :) > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx> > 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> > --- > include/linux/slab.h | 6 ++++++ > 1 file changed, 6 insertions(+) > > diff --git a/include/linux/slab.h b/include/linux/slab.h > index 90877fcde70bd..446303e385265 100644 > --- a/include/linux/slab.h > +++ b/include/linux/slab.h > @@ -76,6 +76,12 @@ > * 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_DESTROY_BY_RCU without first acquiring a reference > + * as described above. The reason is that SLAB_DESTROY_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(). > + * Wonder if slab caches with a constructor should be OK here as AFAIK it should mean the object has to be in the initialized state both when allocated and freed? > * Note that SLAB_TYPESAFE_BY_RCU was originally named SLAB_DESTROY_BY_RCU. > */ > /* Defer freeing slabs to RCU */
