[PATCH] local_t protection (critical section)

Mathieu Desnoyers
2007-09-04T20:09:19Z

local_t protection (critical section)

Adds local_enter(flags) and local_exit(flags) as primitives to surround critical
sections using local_t types.

On architectures providing fast atomic primitives, this turns into a preempt
disable/enable().
However, on architectures not providing such fast primitives, such as ia64, it
turns into a local irq disable/enable so that we can use *_local primitives that
are non atomic.

This is only the primary work here: made for testing ia64 with cmpxchg_local
(other local_* primitives still use atomic_long_t operations as fallback).

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Christoph Lameter <clameter@sgi.com>
---
include/asm-generic/local.h | 3 +++
include/asm-i386/local.h | 3 +++
include/asm-ia64/intrinsics.h | 14 ++++++++++++--
3 files changed, 18 insertions(+), 2 deletions(-)

Index: linux-2.6-lttng/include/asm-generic/local.h
===================================================================
--- linux-2.6-lttng.orig/include/asm-generic/local.h 2007-09-04 15:32:02.000000000 -0400
+++ linux-2.6-lttng/include/asm-generic/local.h 2007-09-04 15:36:41.000000000 -0400
@@ -46,6 +46,9 @@ typedef struct
#define local_add_unless(l, a, u) atomic_long_add_unless((&(l)->a), (a), (u))
#define local_inc_not_zero(l) atomic_long_inc_not_zero(&(l)->a)

+#define local_enter(flags) local_irq_save(flags)
+#define local_exit(flags) local_irq_restore(flags)
+
/* Non-atomic variants, ie. preemption disabled and won't be touched
* in interrupt, etc. Some archs can optimize this case well. */
#define __local_inc(l) local_set((l), local_read(l) + 1)
Index: linux-2.6-lttng/include/asm-i386/local.h
===================================================================
--- linux-2.6-lttng.orig/include/asm-i386/local.h 2007-09-04 15:28:52.000000000 -0400
+++ linux-2.6-lttng/include/asm-i386/local.h 2007-09-04 15:31:54.000000000 -0400
@@ -194,6 +194,9 @@ static __inline__ long local_sub_return(
})
#define local_inc_not_zero(l) local_add_unless((l), 1, 0)

+#define local_enter(flags) preempt_disable()
+#define local_exit(flags) preempt_enable()
+
/* On x86, these are no better than the atomic variants. */
#define __local_inc(l) local_inc(l)
#define __local_dec(l) local_dec(l)
Index: linux-2.6-lttng/include/asm-ia64/intrinsics.h
===================================================================
--- linux-2.6-lttng.orig/include/asm-ia64/intrinsics.h 2007-09-04 15:47:24.000000000 -0400
+++ linux-2.6-lttng/include/asm-ia64/intrinsics.h 2007-09-04 15:49:41.000000000 -0400
@@ -160,8 +160,18 @@ extern long ia64_cmpxchg_called_with_bad
#define cmpxchg(ptr,o,n) cmpxchg_acq(ptr,o,n)
#define cmpxchg64(ptr,o,n) cmpxchg_acq(ptr,o,n)

-#define cmpxchg_local cmpxchg
-#define cmpxchg64_local cmpxchg64
+/* Must be executed between local_enter/local_exit. */
+static inline void *cmpxchg_local(void **p, void *old, void *new)
+{
+ unsigned long flags;
+ void *before;
+
+ before = *p;
+ if (likely(before == old))
+ *p = new;
+ return before;
+}
+#define cmpxchg64_local cmpxchg_local

#ifdef CONFIG_IA64_DEBUG_CMPXCHG
# define CMPXCHG_BUGCHECK_DECL int _cmpxchg_bugcheck_count = 128;