I missed an obvious one!
x86 CPUs are defined not to reorder stores past earlier loads, so there is
no hardware memory barrier required to implement a release-consistent store
(all stores are, by definition).
So ditch the generic lock bitops, and implement optimised versions for x86,
which removes the mfence from __clear_bit_unlock (which is already a useful
primitive for SLUB).
Signed-off-by: Nick Piggin <[email protected]>
---
Index: linux-2.6/include/asm-x86/bitops_32.h
===================================================================
--- linux-2.6.orig/include/asm-x86/bitops_32.h
+++ linux-2.6/include/asm-x86/bitops_32.h
@@ -76,6 +76,20 @@ static inline void clear_bit(int nr, vol
:"Ir" (nr));
}
+/*
+ * clear_bit_unlock - Clears a bit in memory
+ * @nr: Bit to clear
+ * @addr: Address to start counting from
+ *
+ * clear_bit() is atomic and implies release semantics before the memory
+ * operation. It can be used for an unlock.
+ */
+static inline void clear_bit_unlock(unsigned long nr, volatile unsigned long *addr)
+{
+ barrier();
+ clear_bit(nr, addr);
+}
+
static inline void __clear_bit(int nr, volatile unsigned long * addr)
{
__asm__ __volatile__(
@@ -83,6 +97,25 @@ static inline void __clear_bit(int nr, v
:"+m" (ADDR)
:"Ir" (nr));
}
+
+/*
+ * __clear_bit_unlock - Clears a bit in memory
+ * @nr: Bit to clear
+ * @addr: Address to start counting from
+ *
+ * __clear_bit() is non-atomic and implies release semantics before the memory
+ * operation. It can be used for an unlock if no other CPUs can concurrently
+ * modify other bits in the word.
+ *
+ * No memory barrier is required here, because x86 cannot reorder stores past
+ * older loads. Same principle as spin_unlock.
+ */
+static inline void __clear_bit_unlock(unsigned long nr, volatile unsigned long *addr)
+{
+ barrier();
+ __clear_bit(nr, addr);
+}
+
#define smp_mb__before_clear_bit() barrier()
#define smp_mb__after_clear_bit() barrier()
@@ -142,6 +175,15 @@ static inline int test_and_set_bit(int n
}
/**
+ * test_and_set_bit_lock - Set a bit and return its old value for lock
+ * @nr: Bit to set
+ * @addr: Address to count from
+ *
+ * This is the same as test_and_set_bit on x86
+ */
+#define test_and_set_bit_lock test_and_set_bit
+
+/**
* __test_and_set_bit - Set a bit and return its old value
* @nr: Bit to set
* @addr: Address to count from
@@ -402,7 +444,6 @@ static inline int fls(int x)
}
#include <asm-generic/bitops/hweight.h>
-#include <asm-generic/bitops/lock.h>
#endif /* __KERNEL__ */
Index: linux-2.6/include/asm-x86/bitops_64.h
===================================================================
--- linux-2.6.orig/include/asm-x86/bitops_64.h
+++ linux-2.6/include/asm-x86/bitops_64.h
@@ -68,6 +68,20 @@ static __inline__ void clear_bit(int nr,
:"dIr" (nr));
}
+/*
+ * clear_bit_unlock - Clears a bit in memory
+ * @nr: Bit to clear
+ * @addr: Address to start counting from
+ *
+ * clear_bit() is atomic and implies release semantics before the memory
+ * operation. It can be used for an unlock.
+ */
+static inline void clear_bit_unlock(unsigned long nr, volatile unsigned long *addr)
+{
+ barrier();
+ clear_bit(nr, addr);
+}
+
static __inline__ void __clear_bit(int nr, volatile void * addr)
{
__asm__ __volatile__(
@@ -76,6 +90,24 @@ static __inline__ void __clear_bit(int n
:"dIr" (nr));
}
+/*
+ * __clear_bit_unlock - Clears a bit in memory
+ * @nr: Bit to clear
+ * @addr: Address to start counting from
+ *
+ * __clear_bit() is non-atomic and implies release semantics before the memory
+ * operation. It can be used for an unlock if no other CPUs can concurrently
+ * modify other bits in the word.
+ *
+ * No memory barrier is required here, because x86 cannot reorder stores past
+ * older loads. Same principle as spin_unlock.
+ */
+static inline void __clear_bit_unlock(unsigned long nr, volatile unsigned long *addr)
+{
+ barrier();
+ __clear_bit(nr, addr);
+}
+
#define smp_mb__before_clear_bit() barrier()
#define smp_mb__after_clear_bit() barrier()
@@ -133,6 +165,15 @@ static __inline__ int test_and_set_bit(i
}
/**
+ * test_and_set_bit_lock - Set a bit and return its old value for lock
+ * @nr: Bit to set
+ * @addr: Address to count from
+ *
+ * This is the same as test_and_set_bit on x86
+ */
+#define test_and_set_bit_lock test_and_set_bit
+
+/**
* __test_and_set_bit - Set a bit and return its old value
* @nr: Bit to set
* @addr: Address to count from
@@ -408,7 +449,6 @@ static __inline__ int fls(int x)
#define ARCH_HAS_FAST_MULTIPLIER 1
#include <asm-generic/bitops/hweight.h>
-#include <asm-generic/bitops/lock.h>
#endif /* __KERNEL__ */
-
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