linux-kernel - Re: [PATCH] slub: fix slab_pad

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Date:	Thu, 3 Sep 2009 22:00:04 +0300
From:	Pekka Enberg <penberg@...helsinki.fi>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Christoph Lameter <cl@...ux-foundation.org>,
	Zdenek Kabelac <zdenek.kabelac@...il.com>,
	Patrick McHardy <kaber@...sh.net>, Robin Holt <holt@....com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Jesper Dangaard Brouer <hawk@...x.dk>,
	Linux Netdev List <netdev@...r.kernel.org>,
	Netfilter Developers <netfilter-devel@...r.kernel.org>,
	paulmck@...ux.vnet.ibm.com
Subject: Re: [PATCH] slub: fix slab_pad_check()

On Thu, Sep 3, 2009 at 8:59 PM, Eric Dumazet<eric.dumazet@...il.com> wrote:
> Christoph Lameter a écrit :
>> On Thu, 3 Sep 2009, Eric Dumazet wrote:
>>
>>> Point is we cannot deal with RCU quietness before disposing the slab cache,
>>> (if SLAB_DESTROY_BY_RCU was set on the cache) since this disposing *will*
>>> make call_rcu() calls when a full slab is freed/purged.
>>
>> There is no need to do call_rcu calls for frees at that point since
>> objects are no longer in use. We could simply disable SLAB_DESTROY_BY_RCU
>> for the final clearing of caches.
>>
>>> And when RCU grace period is elapsed, the callback *will* need access to
>>> the cache we want to dismantle. Better to not have kfreed()/poisoned it...
>>
>> But going through the RCU period is pointless since no user of the cache
>> remains.
>>
>>> I believe you mix two RCU uses here.
>>>
>>> 1) The one we all know, is use normal caches (!SLAB_DESTROY_BY_RCU)
>>> (or kmalloc()), and use call_rcu(... kfree_something)
>>>
>>>    In this case, you are 100% right that the subsystem itself has
>>>    to call rcu_barrier() (or respect whatever self-synchro) itself,
>>>    before calling kmem_cache_destroy()
>>>
>>> 2) The SLAB_DESTROY_BY_RCU one.
>>>
>>>    Part of cache dismantle needs to call rcu_barrier() itself.
>>>    Caller doesnt have to use rcu_barrier(). It would be a waste of time,
>>>    as kmem_cache_destroy() will refill rcu wait queues with its own stuff.
>>
>> The dismantling does not need RCU since there are no operations on the
>> objects in progress. So simply switch DESTROY_BY_RCU off for close.
>>
>>
>> ---
>>  mm/slub.c |    4 ++--
>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> Index: linux-2.6/mm/slub.c
>> ===================================================================
>> --- linux-2.6.orig/mm/slub.c  2009-09-03 10:14:51.000000000 -0500
>> +++ linux-2.6/mm/slub.c       2009-09-03 10:18:32.000000000 -0500
>> @@ -2594,9 +2594,9 @@ static inline int kmem_cache_close(struc
>>   */
>>  void kmem_cache_destroy(struct kmem_cache *s)
>>  {
>> -     if (s->flags & SLAB_DESTROY_BY_RCU)
>> -             rcu_barrier();
>>       down_write(&slub_lock);
>> +     /* Stop deferring frees so that we can immediately free structures */
>> +     s->flags &= ~SLAB_DESTROY_BY_RCU;
>>       s->refcount--;
>>       if (!s->refcount) {
>>               list_del(&s->list);
>
> It seems very smart, but needs review of all callers to make sure no slabs
> are waiting for final freeing in call_rcu queue on some cpu.
>
> I suspect most of them will then have to use rcu_barrier() before calling
> kmem_cache_destroy(), so why not factorizing code in one place ?

.[snip]

Can someone please explain what's the upside in Christoph's approach?
Performance? Correctness? Something else entirely? We're looking at a
tested bug fix here and I don't understand why I shouldn't just go
ahead and merge it. Hmm?

                        Pekka
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