Hi Eric, On Thu, Sep 3, 2009 at 10:38 AM, Eric Dumazet<eric.dumazet@xxxxxxxxx> wrote: >> The rcu_barrier() call was added by this commit: >> >> http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commitdiff;h=7ed9f7e5db58c6e8c2b4b738a75d5dcd8e17aad5 >> >> I guess we should CC Paul as well. > > Sure ! > > rcu_barrier() is definitly better than synchronize_rcu() in > kmem_cache_destroy() > > But its location was not really right (for SLUB at least) > > SLAB_DESTROY_BY_RCU means subsystem will call kfree(elems) without waiting RCU > grace period. > > By the time subsystem calls kmem_cache_destroy(), all previously allocated > elems must have already be kfreed() by this subsystem. > > We must however wait that all slabs, queued for freeing by rcu_free_slab(), > are indeed freed, since this freeing needs access to kmem_cache pointer. > > As kmem_cache_close() might clean/purge the cache and call rcu_free_slab(), > we must call rcu_barrier() *after* kmem_cache_close(), and before kfree(kmem_cache *s) > > Alternatively we could delay this final kfree(s) (with call_rcu()) but would > have to copy s->name in kmem_cache_create() instead of keeping a pointer to > a string that might be in a module, and freed at rmmod time. > > Given that there is few uses in current tree that call kmem_cache_destroy() > on a SLAB_DESTROY_BY_RCU cache, there is no need to try to optimize this > rcu_barrier() call, unless we want superfast reboot/halt sequences... Oh, sure, the fix looks sane to me. It's just that I am a complete coward when it comes to merging RCU related patches so I always try to fish an Acked-by from Paul or Christoph ;). Pekka -- To unsubscribe from this list: send the line "unsubscribe netfilter-devel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
