On Thu, Oct 20, 2022 at 09:10:49AM +0200, Vlastimil Babka wrote: > 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? :) Boy, I was certainly living in the past when I did this patch, wasn't I? Thank you, will fix on next rebase. > > 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? It does look that way, thank you! And __i915_request_ctor(), sighand_ctor(), and anon_vma_ctor() actually do this, initializing a lock in the process. The ctor function could just initialize the locks, and all would be well. In addition, this makes sequence-lock-like approaches a bit easier, as in "just use a sequence lock". I will update with attribution. Thanx, Paul > > * Note that SLAB_TYPESAFE_BY_RCU was originally named SLAB_DESTROY_BY_RCU. > > */ > > /* Defer freeing slabs to RCU */ >
