From 84d0c4d7eca5a558115f6417889459d676c3ebec Mon Sep 17 00:00:00 2001
From: Mateusz Guzik <mjguzik@gmail.com>
Date: Sat, 28 Oct 2023 05:36:15 +0000
Subject: [PATCH] Add atomic_dec_not_last_64

Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
---
 include/os/freebsd/spl/sys/atomic.h | 17 +++++++++++++++++
 include/os/linux/spl/sys/atomic.h   |  1 +
 lib/libspl/atomic.c                 | 18 ++++++++++++++++++
 lib/libspl/include/atomic.h         |  1 +
 4 files changed, 37 insertions(+)

diff --git a/include/os/freebsd/spl/sys/atomic.h b/include/os/freebsd/spl/sys/atomic.h
index 40a67704fd..2dbb372ca2 100644
--- a/include/os/freebsd/spl/sys/atomic.h
+++ b/include/os/freebsd/spl/sys/atomic.h
@@ -149,6 +149,23 @@ atomic_cas_64(volatile uint64_t *target, uint64_t cmp, uint64_t newval)
 #endif
 #endif
 
+static inline int
+atomic_dec_not_last_64(volatile uint64_t *target)
+{
+	uint64_t val, oldval;
+
+	val = *target;
+	for (;;) {
+		if (val <= 1) {
+			return (0);
+		}
+		oldval = atomic_cas_64(target, val, val - 1);
+		if (oldval == val)
+			return (1);
+		val = oldval;
+	}
+}
+
 static __inline void
 atomic_inc_64(volatile uint64_t *target)
 {
diff --git a/include/os/linux/spl/sys/atomic.h b/include/os/linux/spl/sys/atomic.h
index 8f7fa5aeda..141ea94841 100644
--- a/include/os/linux/spl/sys/atomic.h
+++ b/include/os/linux/spl/sys/atomic.h
@@ -62,6 +62,7 @@
 #define	atomic_swap_64(v, x)	atomic64_xchg((atomic64_t *)(v), x)
 #define	atomic_load_64(v)	atomic64_read((atomic64_t *)(v))
 #define	atomic_store_64(v, x)	atomic64_set((atomic64_t *)(v), x)
+#define	atomic_dec_not_last_64(v) atomic64_dec_if_positive((atomic64_t *)(v))
 
 #ifdef _LP64
 static __inline__ void *
diff --git a/lib/libspl/atomic.c b/lib/libspl/atomic.c
index 8cc350710b..a37feb25a2 100644
--- a/lib/libspl/atomic.c
+++ b/lib/libspl/atomic.c
@@ -62,6 +62,24 @@ ATOMIC_DEC(uchar, uchar_t)
 ATOMIC_DEC(ushort, ushort_t)
 ATOMIC_DEC(uint, uint_t)
 ATOMIC_DEC(ulong, ulong_t)
+
+int
+atomic_dec_not_last_64(volatile uint64_t *target)
+{
+	uint64_t val, oldval;
+
+	val = *target;
+	for (;;) {
+		if (val <= 1) {
+			return (0);
+		}
+		oldval = atomic_cas_64(target, val, val - 1);
+		if (oldval == val)
+			return (1);
+		val = oldval;
+	}
+}
+
 /* END CSTYLED */
 
 
diff --git a/lib/libspl/include/atomic.h b/lib/libspl/include/atomic.h
index 4ebdbbda98..c234cecadc 100644
--- a/lib/libspl/include/atomic.h
+++ b/lib/libspl/include/atomic.h
@@ -155,6 +155,7 @@ extern uint_t atomic_dec_uint_nv(volatile uint_t *);
 extern ulong_t atomic_dec_ulong_nv(volatile ulong_t *);
 #if defined(_INT64_TYPE)
 extern uint64_t atomic_dec_64_nv(volatile uint64_t *);
+extern int atomic_dec_not_last_64(volatile uint64_t *);
 #endif
 
 /*