From: Nick Piggin <nickpiggin@yahoo.com.au>
Date: Mon, 14 Nov 2005 00:07:25 +0000 (-0800)
Subject: [PATCH] atomic: inc_not_zero
X-Git-Tag: v2.6.15-rc2~158
X-Git-Url: https://err.no/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=8426e1f6af0fd7f44d040af7263750c5a52f3cc3;p=linux-2.6

[PATCH] atomic: inc_not_zero

Introduce an atomic_inc_not_zero operation.  Make this a special case of
atomic_add_unless because lockless pagecache actually wants
atomic_inc_not_negativeone due to its offset refcount.

Signed-off-by: Nick Piggin <npiggin@suse.de>
Cc: "Paul E. McKenney" <paulmck@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
---

diff --git a/Documentation/atomic_ops.txt b/Documentation/atomic_ops.txt
index f1744161ef..23a1c2402b 100644
--- a/Documentation/atomic_ops.txt
+++ b/Documentation/atomic_ops.txt
@@ -115,7 +115,7 @@ boolean is return which indicates whether the resulting counter value
 is negative.  It requires explicit memory barrier semantics around the
 operation.
 
-Finally:
+Then:
 
 	int atomic_cmpxchg(atomic_t *v, int old, int new);
 
@@ -129,6 +129,18 @@ atomic_cmpxchg requires explicit memory barriers around the operation.
 The semantics for atomic_cmpxchg are the same as those defined for 'cas'
 below.
 
+Finally:
+
+	int atomic_add_unless(atomic_t *v, int a, int u);
+
+If the atomic value v is not equal to u, this function adds a to v, and
+returns non zero. If v is equal to u then it returns zero. This is done as
+an atomic operation.
+
+atomic_add_unless requires explicit memory barriers around the operation.
+
+atomic_inc_not_zero, equivalent to atomic_add_unless(v, 1, 0)
+
 
 If a caller requires memory barrier semantics around an atomic_t
 operation which does not return a value, a set of interfaces are
diff --git a/arch/sparc/lib/atomic32.c b/arch/sparc/lib/atomic32.c
index be46f65451..cb3cf0f228 100644
--- a/arch/sparc/lib/atomic32.c
+++ b/arch/sparc/lib/atomic32.c
@@ -53,6 +53,21 @@ int atomic_cmpxchg(atomic_t *v, int old, int new)
 	return ret;
 }
 
+int atomic_add_unless(atomic_t *v, int a, int u)
+{
+	int ret;
+	unsigned long flags;
+
+	spin_lock_irqsave(ATOMIC_HASH(v), flags);
+	ret = v->counter;
+	if (ret != u)
+		v->counter += a;
+	spin_unlock_irqrestore(ATOMIC_HASH(v), flags);
+	return ret != u;
+}
+
+static inline void atomic_clear_mask(unsigned long mask, unsigned long *addr)
+/* Atomic operations are already serializing */
 void atomic_set(atomic_t *v, int i)
 {
 	unsigned long flags;
diff --git a/include/asm-alpha/atomic.h b/include/asm-alpha/atomic.h
index a6660809a8..36505bb4e8 100644
--- a/include/asm-alpha/atomic.h
+++ b/include/asm-alpha/atomic.h
@@ -179,6 +179,16 @@ static __inline__ long atomic64_sub_return(long i, atomic64_t * v)
 
 #define atomic_cmpxchg(v, o, n) ((int)cmpxchg(&((v)->counter), (o), (n)))
 
+#define atomic_add_unless(v, a, u)				\
+({								\
+	int c, old;						\
+	c = atomic_read(v);					\
+	while (c != (u) && (old = atomic_cmpxchg((v), c, c + (a))) != c) \
+		c = old;					\
+	c != (u);						\
+})
+#define atomic_inc_not_zero(v) atomic_add_unless((v), 1, 0)
+
 #define atomic_dec_return(v) atomic_sub_return(1,(v))
 #define atomic64_dec_return(v) atomic64_sub_return(1,(v))
 
diff --git a/include/asm-arm/atomic.h b/include/asm-arm/atomic.h
index 8ab1689ef5..75b8027197 100644
--- a/include/asm-arm/atomic.h
+++ b/include/asm-arm/atomic.h
@@ -173,6 +173,17 @@ static inline void atomic_clear_mask(unsigned long mask, unsigned long *addr)
 
 #endif /* __LINUX_ARM_ARCH__ */
 
+static inline int atomic_add_unless(atomic_t *v, int a, int u)
+{
+	int c, old;
+
+	c = atomic_read(v);
+	while (c != u && (old = atomic_cmpxchg((v), c, c + a)) != c)
+		c = old;
+	return c != u;
+}
+#define atomic_inc_not_zero(v) atomic_add_unless((v), 1, 0)
+
 #define atomic_add(i, v)	(void) atomic_add_return(i, v)
 #define atomic_inc(v)		(void) atomic_add_return(1, v)
 #define atomic_sub(i, v)	(void) atomic_sub_return(i, v)
diff --git a/include/asm-arm26/atomic.h b/include/asm-arm26/atomic.h
index 54b24ead71..a47cadc596 100644
--- a/include/asm-arm26/atomic.h
+++ b/include/asm-arm26/atomic.h
@@ -76,6 +76,21 @@ static inline int atomic_cmpxchg(atomic_t *v, int old, int new)
 	return ret;
 }
 
+static inline int atomic_add_unless(atomic_t *v, int a, int u)
+{
+	int ret;
+	unsigned long flags;
+
+	local_irq_save(flags);
+	ret = v->counter;
+	if (ret != u)
+		v->counter += a;
+	local_irq_restore(flags);
+
+	return ret != u;
+}
+#define atomic_inc_not_zero(v) atomic_add_unless((v), 1, 0)
+
 static inline void atomic_clear_mask(unsigned long mask, unsigned long *addr)
 {
         unsigned long flags;
diff --git a/include/asm-cris/atomic.h b/include/asm-cris/atomic.h
index 45891f7de0..683b05a57d 100644
--- a/include/asm-cris/atomic.h
+++ b/include/asm-cris/atomic.h
@@ -136,6 +136,20 @@ static inline int atomic_cmpxchg(atomic_t *v, int old, int new)
 	return ret;
 }
 
+static inline int atomic_add_unless(atomic_t *v, int a, int u)
+{
+	int ret;
+	unsigned long flags;
+
+	cris_atomic_save(v, flags);
+	ret = v->counter;
+	if (ret != u)
+		v->counter += a;
+	cris_atomic_restore(v, flags);
+	return ret != u;
+}
+#define atomic_inc_not_zero(v) atomic_add_unless((v), 1, 0)
+
 /* Atomic operations are already serializing */
 #define smp_mb__before_atomic_dec()    barrier()
 #define smp_mb__after_atomic_dec()     barrier()
diff --git a/include/asm-frv/atomic.h b/include/asm-frv/atomic.h
index 55f06a0e94..f6539ff569 100644
--- a/include/asm-frv/atomic.h
+++ b/include/asm-frv/atomic.h
@@ -416,4 +416,14 @@ extern uint32_t __cmpxchg_32(uint32_t *v, uint32_t test, uint32_t new);
 
 #define atomic_cmpxchg(v, old, new) ((int)cmpxchg(&((v)->counter), old, new))
 
+#define atomic_add_unless(v, a, u)				\
+({								\
+	int c, old;						\
+	c = atomic_read(v);					\
+	while (c != (u) && (old = atomic_cmpxchg((v), c, c + (a))) != c) \
+		c = old;					\
+	c != (u);						\
+})
+#define atomic_inc_not_zero(v) atomic_add_unless((v), 1, 0)
+
 #endif /* _ASM_ATOMIC_H */
diff --git a/include/asm-h8300/atomic.h b/include/asm-h8300/atomic.h
index d504392594..f23d86819e 100644
--- a/include/asm-h8300/atomic.h
+++ b/include/asm-h8300/atomic.h
@@ -95,6 +95,20 @@ static inline int atomic_cmpxchg(atomic_t *v, int old, int new)
 	return ret;
 }
 
+static inline int atomic_add_unless(atomic_t *v, int a, int u)
+{
+	int ret;
+	unsigned long flags;
+
+	local_irq_save(flags);
+	ret = v->counter;
+	if (ret != u)
+		v->counter += a;
+	local_irq_restore(flags);
+	return ret != u;
+}
+#define atomic_inc_not_zero(v) atomic_add_unless((v), 1, 0)
+
 static __inline__ void atomic_clear_mask(unsigned long mask, unsigned long *v)
 {
 	__asm__ __volatile__("stc ccr,r1l\n\t"
diff --git a/include/asm-i386/atomic.h b/include/asm-i386/atomic.h
index 5ff698e9d2..c68557aa04 100644
--- a/include/asm-i386/atomic.h
+++ b/include/asm-i386/atomic.h
@@ -217,6 +217,25 @@ static __inline__ int atomic_sub_return(int i, atomic_t *v)
 
 #define atomic_cmpxchg(v, old, new) ((int)cmpxchg(&((v)->counter), old, new))
 
+/**
+ * atomic_add_unless - add unless the number is a given value
+ * @v: pointer of type atomic_t
+ * @a: the amount to add to v...
+ * @u: ...unless v is equal to u.
+ *
+ * Atomically adds @a to @v, so long as it was not @u.
+ * Returns non-zero if @v was not @u, and zero otherwise.
+ */
+#define atomic_add_unless(v, a, u)				\
+({								\
+	int c, old;						\
+	c = atomic_read(v);					\
+	while (c != (u) && (old = atomic_cmpxchg((v), c, c + (a))) != c) \
+		c = old;					\
+	c != (u);						\
+})
+#define atomic_inc_not_zero(v) atomic_add_unless((v), 1, 0)
+
 #define atomic_inc_return(v)  (atomic_add_return(1,v))
 #define atomic_dec_return(v)  (atomic_sub_return(1,v))
 
diff --git a/include/asm-ia64/atomic.h b/include/asm-ia64/atomic.h
index 593d3da9f3..2fbebf85c3 100644
--- a/include/asm-ia64/atomic.h
+++ b/include/asm-ia64/atomic.h
@@ -90,6 +90,16 @@ ia64_atomic64_sub (__s64 i, atomic64_t *v)
 
 #define atomic_cmpxchg(v, old, new) ((int)cmpxchg(&((v)->counter), old, new))
 
+#define atomic_add_unless(v, a, u)				\
+({								\
+	int c, old;						\
+	c = atomic_read(v);					\
+	while (c != (u) && (old = atomic_cmpxchg((v), c, c + (a))) != c) \
+		c = old;					\
+	c != (u);						\
+})
+#define atomic_inc_not_zero(v) atomic_add_unless((v), 1, 0)
+
 #define atomic_add_return(i,v)						\
 ({									\
 	int __ia64_aar_i = (i);						\
diff --git a/include/asm-m68k/atomic.h b/include/asm-m68k/atomic.h
index b821975a36..e3c962eeab 100644
--- a/include/asm-m68k/atomic.h
+++ b/include/asm-m68k/atomic.h
@@ -141,6 +141,16 @@ static inline void atomic_set_mask(unsigned long mask, unsigned long *v)
 
 #define atomic_cmpxchg(v, o, n) ((int)cmpxchg(&((v)->counter), (o), (n)))
 
+#define atomic_add_unless(v, a, u)				\
+({								\
+	int c, old;						\
+	c = atomic_read(v);					\
+	while (c != (u) && (old = atomic_cmpxchg((v), c, c + (a))) != c) \
+		c = old;					\
+	c != (u);						\
+})
+#define atomic_inc_not_zero(v) atomic_add_unless((v), 1, 0)
+
 /* Atomic operations are already serializing */
 #define smp_mb__before_atomic_dec()	barrier()
 #define smp_mb__after_atomic_dec()	barrier()
diff --git a/include/asm-m68knommu/atomic.h b/include/asm-m68knommu/atomic.h
index 2fd33a56b6..3c1cc153c4 100644
--- a/include/asm-m68knommu/atomic.h
+++ b/include/asm-m68knommu/atomic.h
@@ -130,6 +130,16 @@ static inline int atomic_sub_return(int i, atomic_t * v)
 
 #define atomic_cmpxchg(v, o, n) ((int)cmpxchg(&((v)->counter), (o), (n)))
 
+#define atomic_add_unless(v, a, u)				\
+({								\
+	int c, old;						\
+	c = atomic_read(v);					\
+	while (c != (u) && (old = atomic_cmpxchg((v), c, c + (a))) != c) \
+		c = old;					\
+	c != (u);						\
+})
+#define atomic_inc_not_zero(v) atomic_add_unless((v), 1, 0)
+
 #define atomic_dec_return(v) atomic_sub_return(1,(v))
 #define atomic_inc_return(v) atomic_add_return(1,(v))
 
diff --git a/include/asm-mips/atomic.h b/include/asm-mips/atomic.h
index 4fba0d003c..2c87b41e69 100644
--- a/include/asm-mips/atomic.h
+++ b/include/asm-mips/atomic.h
@@ -289,6 +289,25 @@ static __inline__ int atomic_sub_if_positive(int i, atomic_t * v)
 
 #define atomic_cmpxchg(v, o, n) ((int)cmpxchg(&((v)->counter), (o), (n)))
 
+/**
+ * atomic_add_unless - add unless the number is a given value
+ * @v: pointer of type atomic_t
+ * @a: the amount to add to v...
+ * @u: ...unless v is equal to u.
+ *
+ * Atomically adds @a to @v, so long as it was not @u.
+ * Returns non-zero if @v was not @u, and zero otherwise.
+ */
+#define atomic_add_unless(v, a, u)				\
+({								\
+	int c, old;						\
+	c = atomic_read(v);					\
+	while (c != (u) && (old = atomic_cmpxchg((v), c, c + (a))) != c) \
+		c = old;					\
+	c != (u);						\
+})
+#define atomic_inc_not_zero(v) atomic_add_unless((v), 1, 0)
+
 #define atomic_dec_return(v) atomic_sub_return(1,(v))
 #define atomic_inc_return(v) atomic_add_return(1,(v))
 
diff --git a/include/asm-parisc/atomic.h b/include/asm-parisc/atomic.h
index 52c9a45b5f..983e9a2b60 100644
--- a/include/asm-parisc/atomic.h
+++ b/include/asm-parisc/atomic.h
@@ -166,6 +166,25 @@ static __inline__ int atomic_read(const atomic_t *v)
 /* exported interface */
 #define atomic_cmpxchg(v, o, n) ((int)cmpxchg(&((v)->counter), (o), (n)))
 
+/**
+ * atomic_add_unless - add unless the number is a given value
+ * @v: pointer of type atomic_t
+ * @a: the amount to add to v...
+ * @u: ...unless v is equal to u.
+ *
+ * Atomically adds @a to @v, so long as it was not @u.
+ * Returns non-zero if @v was not @u, and zero otherwise.
+ */
+#define atomic_add_unless(v, a, u)				\
+({								\
+	int c, old;						\
+	c = atomic_read(v);					\
+	while (c != (u) && (old = atomic_cmpxchg((v), c, c + (a))) != c) \
+		c = old;					\
+	c != (u);						\
+})
+#define atomic_inc_not_zero(v) atomic_add_unless((v), 1, 0)
+
 #define atomic_add(i,v)	((void)(__atomic_add_return( ((int)i),(v))))
 #define atomic_sub(i,v)	((void)(__atomic_add_return(-((int)i),(v))))
 #define atomic_inc(v)	((void)(__atomic_add_return(   1,(v))))
diff --git a/include/asm-powerpc/atomic.h b/include/asm-powerpc/atomic.h
index 37205faa9d..ec4b144689 100644
--- a/include/asm-powerpc/atomic.h
+++ b/include/asm-powerpc/atomic.h
@@ -166,6 +166,31 @@ static __inline__ int atomic_dec_return(atomic_t *v)
 
 #define atomic_cmpxchg(v, o, n) ((int)cmpxchg(&((v)->counter), (o), (n)))
 
+/**
+ * atomic_add_unless - add unless the number is a given value
+ * @v: pointer of type atomic_t
+ * @a: the amount to add to v...
+ * @u: ...unless v is equal to u.
+ *
+ * Atomically adds @a to @v, so long as it was not @u.
+ * Returns non-zero if @v was not @u, and zero otherwise.
+ */
+#define atomic_add_unless(v, a, u)							 \
+({															 \
+	   int c, old;											 \
+	   c = atomic_read(v);									 \
+	   for (;;) {											  \
+			   if (unlikely(c == (u)))						 \
+					   break;								  \
+			   old = atomic_cmpxchg((v), c, c + (a));		  \
+			   if (likely(old == c))						   \
+					   break;								  \
+			   c = old;										\
+	   }													   \
+	   c != (u);											   \
+})
+#define atomic_inc_not_zero(v) atomic_add_unless((v), 1, 0)
+
 #define atomic_sub_and_test(a, v)	(atomic_sub_return((a), (v)) == 0)
 #define atomic_dec_and_test(v)		(atomic_dec_return((v)) == 0)
 
diff --git a/include/asm-s390/atomic.h b/include/asm-s390/atomic.h
index 631014d5de..b3bd4f679f 100644
--- a/include/asm-s390/atomic.h
+++ b/include/asm-s390/atomic.h
@@ -200,6 +200,16 @@ atomic_compare_and_swap(int expected_oldval,int new_val,atomic_t *v)
 
 #define atomic_cmpxchg(v, o, n) (atomic_compare_and_swap((o), (n), &((v)->counter)))
 
+#define atomic_add_unless(v, a, u)				\
+({								\
+	int c, old;						\
+	c = atomic_read(v);					\
+	while (c != (u) && (old = atomic_cmpxchg((v), c, c + (a))) != c) \
+		c = old;					\
+	c != (u);						\
+})
+#define atomic_inc_not_zero(v) atomic_add_unless((v), 1, 0)
+
 #define smp_mb__before_atomic_dec()	smp_mb()
 #define smp_mb__after_atomic_dec()	smp_mb()
 #define smp_mb__before_atomic_inc()	smp_mb()
diff --git a/include/asm-sh/atomic.h b/include/asm-sh/atomic.h
index a148c762d3..aabfd33446 100644
--- a/include/asm-sh/atomic.h
+++ b/include/asm-sh/atomic.h
@@ -101,6 +101,21 @@ static inline int atomic_cmpxchg(atomic_t *v, int old, int new)
 	return ret;
 }
 
+static inline int atomic_add_unless(atomic_t *v, int a, int u)
+{
+	int ret;
+	unsigned long flags;
+
+	local_irq_save(flags);
+	ret = v->counter;
+	if (ret != u)
+		v->counter += a;
+	local_irq_restore(flags);
+
+	return ret != u;
+}
+#define atomic_inc_not_zero(v) atomic_add_unless((v), 1, 0)
+
 static __inline__ void atomic_clear_mask(unsigned int mask, atomic_t *v)
 {
 	unsigned long flags;
diff --git a/include/asm-sh64/atomic.h b/include/asm-sh64/atomic.h
index 6eeb57b015..927a2bc27b 100644
--- a/include/asm-sh64/atomic.h
+++ b/include/asm-sh64/atomic.h
@@ -113,6 +113,21 @@ static inline int atomic_cmpxchg(atomic_t *v, int old, int new)
 	return ret;
 }
 
+static inline int atomic_add_unless(atomic_t *v, int a, int u)
+{
+	int ret;
+	unsigned long flags;
+
+	local_irq_save(flags);
+	ret = v->counter;
+	if (ret != u)
+		v->counter += a;
+	local_irq_restore(flags);
+
+	return ret != u;
+}
+#define atomic_inc_not_zero(v) atomic_add_unless((v), 1, 0)
+
 static __inline__ void atomic_clear_mask(unsigned int mask, atomic_t *v)
 {
 	unsigned long flags;
diff --git a/include/asm-sparc/atomic.h b/include/asm-sparc/atomic.h
index 52bdd1a895..62bec7ad27 100644
--- a/include/asm-sparc/atomic.h
+++ b/include/asm-sparc/atomic.h
@@ -20,6 +20,7 @@ typedef struct { volatile int counter; } atomic_t;
 
 extern int __atomic_add_return(int, atomic_t *);
 extern int atomic_cmpxchg(atomic_t *, int, int);
+extern int atomic_add_unless(atomic_t *, int, int);
 extern void atomic_set(atomic_t *, int);
 
 #define atomic_read(v)          ((v)->counter)
@@ -49,6 +50,8 @@ extern void atomic_set(atomic_t *, int);
 #define atomic_dec_and_test(v) (atomic_dec_return(v) == 0)
 #define atomic_sub_and_test(i, v) (atomic_sub_return(i, v) == 0)
 
+#define atomic_inc_not_zero(v) atomic_add_unless((v), 1, 0)
+
 /* This is the old 24-bit implementation.  It's still used internally
  * by some sparc-specific code, notably the semaphore implementation.
  */
diff --git a/include/asm-sparc64/atomic.h b/include/asm-sparc64/atomic.h
index 3a0b4383bb..8198c3d0d0 100644
--- a/include/asm-sparc64/atomic.h
+++ b/include/asm-sparc64/atomic.h
@@ -72,6 +72,16 @@ extern int atomic64_sub_ret(int, atomic64_t *);
 
 #define atomic_cmpxchg(v, o, n) ((int)cmpxchg(&((v)->counter), (o), (n)))
 
+#define atomic_add_unless(v, a, u)				\
+({								\
+	int c, old;						\
+	c = atomic_read(v);					\
+	while (c != (u) && (old = atomic_cmpxchg((v), c, c + (a))) != c) \
+		c = old;					\
+	c != (u);						\
+})
+#define atomic_inc_not_zero(v) atomic_add_unless((v), 1, 0)
+
 /* Atomic operations are already serializing */
 #ifdef CONFIG_SMP
 #define smp_mb__before_atomic_dec()	membar_storeload_loadload();
diff --git a/include/asm-v850/atomic.h b/include/asm-v850/atomic.h
index e497166ca4..bede3172ce 100644
--- a/include/asm-v850/atomic.h
+++ b/include/asm-v850/atomic.h
@@ -104,6 +104,22 @@ static inline int atomic_cmpxchg(atomic_t *v, int old, int new)
 	return ret;
 }
 
+static inline int atomic_add_unless(atomic_t *v, int a, int u)
+{
+	int ret;
+	unsigned long flags;
+
+	local_irq_save(flags);
+	ret = v->counter;
+	if (ret != u)
+		v->counter += a;
+	local_irq_restore(flags);
+
+	return ret != u;
+}
+
+#define atomic_inc_not_zero(v) atomic_add_unless((v), 1, 0)
+
 /* Atomic operations are already serializing on ARM */
 #define smp_mb__before_atomic_dec()	barrier()
 #define smp_mb__after_atomic_dec()	barrier()
diff --git a/include/asm-x86_64/atomic.h b/include/asm-x86_64/atomic.h
index 75c8a1e967..0866ef67f1 100644
--- a/include/asm-x86_64/atomic.h
+++ b/include/asm-x86_64/atomic.h
@@ -362,6 +362,25 @@ static __inline__ int atomic_sub_return(int i, atomic_t *v)
 
 #define atomic_cmpxchg(v, old, new) ((int)cmpxchg(&((v)->counter), old, new))
 
+/**
+ * atomic_add_unless - add unless the number is a given value
+ * @v: pointer of type atomic_t
+ * @a: the amount to add to v...
+ * @u: ...unless v is equal to u.
+ *
+ * Atomically adds @a to @v, so long as it was not @u.
+ * Returns non-zero if @v was not @u, and zero otherwise.
+ */
+#define atomic_add_unless(v, a, u)				\
+({								\
+	int c, old;						\
+	c = atomic_read(v);					\
+	while (c != (u) && (old = atomic_cmpxchg((v), c, c + (a))) != c) \
+		c = old;					\
+	c != (u);						\
+})
+#define atomic_inc_not_zero(v) atomic_add_unless((v), 1, 0)
+
 #define atomic_inc_return(v)  (atomic_add_return(1,v))
 #define atomic_dec_return(v)  (atomic_sub_return(1,v))
 
diff --git a/include/asm-xtensa/atomic.h b/include/asm-xtensa/atomic.h
index cd40c5e751..3670cc7695 100644
--- a/include/asm-xtensa/atomic.h
+++ b/include/asm-xtensa/atomic.h
@@ -225,6 +225,25 @@ static inline int atomic_sub_return(int i, atomic_t * v)
 
 #define atomic_cmpxchg(v, o, n) ((int)cmpxchg(&((v)->counter), (o), (n)))
 
+/**
+ * atomic_add_unless - add unless the number is a given value
+ * @v: pointer of type atomic_t
+ * @a: the amount to add to v...
+ * @u: ...unless v is equal to u.
+ *
+ * Atomically adds @a to @v, so long as it was not @u.
+ * Returns non-zero if @v was not @u, and zero otherwise.
+ */
+#define atomic_add_unless(v, a, u)				\
+({								\
+	int c, old;						\
+	c = atomic_read(v);					\
+	while (c != (u) && (old = atomic_cmpxchg((v), c, c + (a))) != c) \
+		c = old;					\
+	c != (u);						\
+})
+#define atomic_inc_not_zero(v) atomic_add_unless((v), 1, 0)
+
 static inline void atomic_clear_mask(unsigned int mask, atomic_t *v)
 {
     unsigned int all_f = -1;