Messages in this thread | | | Date | Thu, 13 Nov 2008 09:32:49 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH] slab: document SLAB_DESTROY_BY_RCU |
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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 */ >
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