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authorPeter Zijlstra <a.p.zijlstra@chello.nl>2008-11-13 20:40:12 +0200
committerPekka Enberg <penberg@cs.helsinki.fi>2008-11-13 20:49:02 +0200
commitd7de4c1dc3a2faca0bf05d9e342f885cb2696766 (patch)
treec7ea498a84b9319443af8cafba5deb6602dfb3a5
parent02f5621042e3f7e2fb6c741cbe5ee7c1f3caf354 (diff)
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slab: document SLAB_DESTROY_BY_RCU
Explain this SLAB_DESTROY_BY_RCU thing... [hugh@veritas.com: add a pointer to comment in mm/slab.c] 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> Acked-by: Christoph Lameter <cl@linux-foundation.org> Signed-off-by: Hugh Dickins <hugh@veritas.com> Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>
-rw-r--r--include/linux/slab.h28
1 files changed, 28 insertions, 0 deletions
diff --git a/include/linux/slab.h b/include/linux/slab.h
index ba965c8..000da12 100644
--- a/include/linux/slab.h
+++ b/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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