Re: [PATCH] slub: fix slab_pad_check() and SLAB_DESTROY_BY_RCU

[Pekka Enberg <penberg@cs.helsinki.fi>]

Hi Eric,

On Thu, Sep 3, 2009 at 10:38 AM, Eric Dumazet<eric.dumazet@gmail.com> 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