[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>

23 files changed, 321 insertions, 1 deletions

diff --git a/Documentation/atomic_ops.txt b/Documentation/atomic_ops.txt
index f1744161ef06..23a1c2402bcc 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 be46f6545184..cb3cf0f22822 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 a6660809a879..36505bb4e8cb 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 8ab1689ef56a..75b802719723 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 54b24ead7132..a47cadc59686 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 45891f7de00f..683b05a57d88 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 55f06a0e949f..f6539ff569c5 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 d50439259491..f23d86819ea8 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 5ff698e9d2c2..c68557aa04b2 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 593d3da9f3c2..2fbebf85c31d 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 b821975a361a..e3c962eeabf3 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 2fd33a56b603..3c1cc153c415 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 4fba0d003c99..2c87b41e69ba 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 52c9a45b5f87..983e9a2b6042 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 37205faa9d7c..ec4b14468959 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 631014d5de90..b3bd4f679f72 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 a148c762d366..aabfd334462c 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 6eeb57b015ce..927a2bc27b30 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 52bdd1a895fa..62bec7ad271c 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 3a0b4383bbac..8198c3d0d007 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 e497166ca42b..bede3172ce7f 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 75c8a1e96737..0866ef67f198 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 cd40c5e75160..3670cc7695da 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;