Re: [PATCH] slab: document SLAB_DESTROY_BY_RCU

[Pekka Enberg]

On Thu, Nov 13, 2008 at 4:28 PM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> Subject: slab: document SLAB_DESTROY_BY_RCU
> From: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
> Date: Wed, 02 Apr 2008 14:13:42 +0200
>
> Explain this SLAB_DESTROY_BY_RCU thing...
>
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
> Acked-by: Jens Axboe <jens.axboe@xxxxxxxxxx>
> Acked-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>

Acked-by: Pekka Enberg <penberg@xxxxxxxxxxxxxx>

> ---
>  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 */
>
> --
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