Re: [PATCH] slab: document SLAB_DESTROY_BY_RCU

[Hugh Dickins]

On Thu, 13 Nov 2008, Peter Zijlstra wrote:
> Subject: slab: document SLAB_DESTROY_BY_RCU

Yes, that's good, thanks Peter.  But I still think it's even
better with at least a pointer to the original description in
mm/slab.c: I've added one or two lines to the comment below.


From: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>

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>
Signed-off-by: Hugh Dickins <hugh@xxxxxxxxxxx>
---
 include/linux/slab.h |   28 ++++++++++++++++++++++++++++
 1 file changed, 28 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 +23,34 @@
 #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 */
--
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