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 ?
net/dccp/ipv6.c:1145: .slab_flags = SLAB_DESTROY_BY_RCU,
net/dccp/ipv4.c:941: .slab_flags = SLAB_DESTROY_BY_RCU,
net/ipv4/udp.c:1593: .slab_flags = SLAB_DESTROY_BY_RCU,
net/ipv4/udplite.c:54: .slab_flags = SLAB_DESTROY_BY_RCU,
net/ipv4/tcp_ipv4.c:2446: .slab_flags = SLAB_DESTROY_BY_RCU,
net/ipv4/udp.c.orig:1587: .slab_flags = SLAB_DESTROY_BY_RCU,
net/ipv6/udp.c:1274: .slab_flags = SLAB_DESTROY_BY_RCU,
net/ipv6/udplite.c:52: .slab_flags = SLAB_DESTROY_BY_RCU,
net/ipv6/tcp_ipv6.c:2085: .slab_flags = SLAB_DESTROY_BY_RCU,
net/netfilter/nf_conntrack_core.c:1269: 0, SLAB_DESTROY_BY_RCU, NULL);
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