linux-kernel - Re: [PATCH rcu 5/8] slab: Explain why SLAB_DESTROY_BY

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Date:   Fri, 21 Oct 2022 09:10:03 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Christoph Lameter <cl@...two.de>, rcu@...r.kernel.org,
        linux-kernel@...r.kernel.org, kernel-team@...com,
        rostedt@...dmis.org, Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Hyeonggon Yoo <42.hyeyoo@...il.com>, linux-mm@...ck.org
Subject: Re: [PATCH rcu 5/8] slab: Explain why SLAB_DESTROY_BY_RCU reference
 before locking

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@...nel.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@...nel.org>
> >> 
> >> Acked-by: Vlastimil Babka <vbabka@...e.cz>
> 
> It was there :)

One of those mornings, I guess...

Thank you very much!!!

							Thanx, Paul

> >> >     Cc: Christoph Lameter <cl@...ux.com>
> >> >     Cc: Pekka Enberg <penberg@...nel.org>
> >> >     Cc: David Rientjes <rientjes@...gle.com>
> >> >     Cc: Joonsoo Kim <iamjoonsoo.kim@....com>
> >> >     Cc: Andrew Morton <akpm@...ux-foundation.org>
> >> >     Cc: Vlastimil Babka <vbabka@...e.cz>
> >> >     Cc: Roman Gushchin <roman.gushchin@...ux.dev>
> >> >     Cc: Hyeonggon Yoo <42.hyeyoo@...il.com>
> >> >     Cc: <linux-mm@...ck.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 */
> >> 
>