Messages in this thread Patch in this message | | | Subject | Re: kmemcheck caught read from freed memory (cfq_free_io_context) | From | Peter Zijlstra <> | Date | Wed, 02 Apr 2008 14:13:42 +0200 |
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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?
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 */
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