Re: [PATCH] slub: fix slab_pad_check()

From: Christoph Lameter
Date: Thu Sep 03 2009 - 13:07:55 EST

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

> 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()
> 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();
+ /* Stop deferring frees so that we can immediately free structures */
+ s->flags &= ~SLAB_DESTROY_BY_RCU;
if (!s->refcount) {
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