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authorLinus Torvalds <torvalds@linux-foundation.org>2008-11-13 11:30:46 -0800
committerLinus Torvalds <torvalds@linux-foundation.org>2008-11-13 11:30:46 -0800
commit2d1595ad8766c9cbab81d259168c00261d382ac5 (patch)
tree28fba62cf218ef484113ab4335e3f1f3fbecd484
parent90aaa53c5a5af33a061313681d8f3234712b866b (diff)
parentd7de4c1dc3a2faca0bf05d9e342f885cb2696766 (diff)
downloadop-kernel-dev-2d1595ad8766c9cbab81d259168c00261d382ac5.zip
op-kernel-dev-2d1595ad8766c9cbab81d259168c00261d382ac5.tar.gz
Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/penberg/slab-2.6
* 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/penberg/slab-2.6: slab: document SLAB_DESTROY_BY_RCU Kconfig: SLUB is the default slab allocator
-rw-r--r--include/linux/slab.h28
-rw-r--r--init/Kconfig6
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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