slab: document SLAB_DESTROY_BY_RCU

#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();

 *

 * See also the comment on struct slab_rcu in mm/slab.c.

 */

#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 */