Messages in this thread | | | Date | Thu, 20 Oct 2022 09:31:58 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH rcu 5/8] slab: Explain why SLAB_DESTROY_BY_RCU reference before locking |
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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@kernel.org> > > 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> > > --- > > 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 */ >
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