Re: kmemcheck caught read from freed memory (cfq_free_io_context)

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

On Wed, Apr 02, 2008 at 02:13:42PM +0200, Peter Zijlstra wrote:
> On Wed, 2008-04-02 at 13:53 +0200, Jens Axboe wrote:
> 
> > > Yeah, SLAB_DESTROY_BY_RCU should have a _HUGE_ comment explaining it,
> > > I'm sure this is not the first (nor the last) time people get that
> > > wrong.
> > 
> > It should, SLAB_DESTROY_BY_RCU is definitely useful, but it is expected
> > to be an 'easier' way of doing the call_rcu() manually. So it definitely
> > needs more documentation.
> > 
> 
> Ok I gave it a go, how bad is this text?

Looks good to me!

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

> Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
> ---
> diff --git a/include/linux/slab.h b/include/linux/slab.h
> index f950a89..e049ddc 100644
> --- a/include/linux/slab.h
> +++ b/include/linux/slab.h
> @@ -25,6 +25,32 @@
>  #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 */
> 
>