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Subject[GIT PULL] SLAB updates for 2.6.28-rc4
Hi Linus,

Please pull the latest SLAB git tree from:

git://git.kernel.org/pub/scm/linux/kernel/git/penberg/slab-2.6.git for-linus

The branch contains a small Kconfig help text update from Simon Arlott and a
documentation update for SLAB_DESTROY_BY_RCU from Peter Zijlstra.

Pekka

Peter Zijlstra (1):
slab: document SLAB_DESTROY_BY_RCU

Simon Arlott (1):
Kconfig: SLUB is the default slab allocator

include/linux/slab.h | 28 ++++++++++++++++++++++++++++
init/Kconfig | 6 +++---
2 files changed, 31 insertions(+), 3 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 */
diff --git a/init/Kconfig b/init/Kconfig
index 86b00c5..226da27 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -771,8 +771,7 @@ config SLAB
help
The regular slab allocator that is established and known to work
well in all environments. It organizes cache hot objects in
- per cpu and per node queues. SLAB is the default choice for
- a slab allocator.
+ per cpu and per node queues.

config SLUB
bool "SLUB (Unqueued Allocator)"
@@ -781,7 +780,8 @@ config SLUB
instead of managing queues of cached objects (SLAB approach).
Per cpu caching is realized using slabs of objects instead
of queues of objects. SLUB can use memory efficiently
- and has enhanced diagnostics.
+ and has enhanced diagnostics. SLUB is the default choice for
+ a slab allocator.

config SLOB
depends on EMBEDDED

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