Re: [PATCH] slab: document SLAB_DESTROY_BY_RCU

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Thu, Nov 13, 2008 at 03:28:29PM +0100, Peter Zijlstra wrote:
> Subject: slab: document SLAB_DESTROY_BY_RCU
> From: Peter Zijlstra <a.p.zijlstra@chello.nl>
> Date: Wed, 02 Apr 2008 14:13:42 +0200
> 
> Explain this SLAB_DESTROY_BY_RCU thing...

With Hugh's suggested change,

Re-acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

> Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
> Acked-by: Jens Axboe <jens.axboe@oracle.com>
> Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> ---
>  include/linux/slab.h |   26 ++++++++++++++++++++++++++
>  1 file changed, 26 insertions(+)
> 
> Index: linux-2.6/include/linux/slab.h
> ===================================================================
> --- linux-2.6.orig/include/linux/slab.h
> +++ linux-2.6/include/linux/slab.h
> @@ -23,6 +23,32 @@
>  #define SLAB_CACHE_DMA		0x00004000UL	/* Use GFP_DMA memory */
>  #define SLAB_STORE_USER		0x00010000UL	/* DEBUG: Store the last owner for bug hunting */
>  #define SLAB_PANIC		0x00040000UL	/* Panic if kmem_cache_create() fails */
> +/*
> + * SLAB_DESTROY_BY_RCU - **WARNING** READ THIS!
> + *
> + * This delays freeing the SLAB page by a grace period, it does _NOT_
> + * delay object freeing. This means that if you do kmem_cache_free()
> + * that memory location is free to be reused at any time. Thus it may
> + * be possible to see another object there in the same RCU grace period.
> + *
> + * This feature only ensures the memory location backing the object
> + * stays valid, the trick to using this is relying on an independent
> + * object validation pass. Something like:
> + *
> + *  rcu_read_lock()
> + * again:
> + *  obj = lockless_lookup(key);
> + *  if (obj) {
> + *    if (!try_get_ref(obj)) // might fail for free objects
> + *      goto again;
> + *
> + *    if (obj->key != key) { // not the object we expected
> + *      put_ref(obj);
> + *      goto again;
> + *    }
> + *  }
> + *  rcu_read_unlock();
> + */
>  #define SLAB_DESTROY_BY_RCU	0x00080000UL	/* Defer freeing slabs to RCU */
>  #define SLAB_MEM_SPREAD		0x00100000UL	/* Spread some memory over cpuset */
>  #define SLAB_TRACE		0x00200000UL	/* Trace allocations and frees */
>