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

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

[Paul E. McKenney]

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.

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().
+ *
  * Note that SLAB_TYPESAFE_BY_RCU was originally named SLAB_DESTROY_BY_RCU.
  */
 /* Defer freeing slabs to RCU */
-- 
2.31.1.189.g2e36527f23

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

[Vlastimil Babka]

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@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?

>   * Note that SLAB_TYPESAFE_BY_RCU was originally named SLAB_DESTROY_BY_RCU.
>   */
>  /* Defer freeing slabs to RCU */

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

[Paul E. McKenney]

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 */
> 

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

[Christoph Lameter]

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.

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

[Paul E. McKenney]

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?

							Thanx, Paul

------------------------------------------------------------------------

commit ff4c536e6b44e2e185e38c3653851f92e07139da
Author: Paul E. McKenney <paulmck@kernel.org>
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@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>

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 */

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

[Vlastimil Babka]

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@kernel.org>
> 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@kernel.org>

Acked-by: Vlastimil Babka <vbabka@suse.cz>

>     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>
> 
> 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 */

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@kernel.org>
> > 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@kernel.org>
> 
> Acked-by: Vlastimil Babka <vbabka@suse.cz>
> 
> >     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>
> > 
> > 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 */
> 

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

[Vlastimil Babka]

On 10/21/22 17:42, Paul E. McKenney wrote:
> 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@kernel.org>
>> > 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@kernel.org>
>> 
>> Acked-by: Vlastimil Babka <vbabka@suse.cz>

It was there :)

>> >     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>
>> > 
>> > 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 */
>> 

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

[Paul E. McKenney]

On Fri, Oct 21, 2022 at 05:50:39PM +0200, Vlastimil Babka wrote:
> On 10/21/22 17:42, Paul E. McKenney wrote:
> > 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@kernel.org>
> >> > 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@kernel.org>
> >> 
> >> Acked-by: Vlastimil Babka <vbabka@suse.cz>
> 
> It was there :)

One of those mornings, I guess...

Thank you very much!!!

							Thanx, Paul

> >> >     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>
> >> > 
> >> > 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 */
> >> 
>