[RFC PATCH] lib: Introduce generic __cmpxchg_u64() and use it where needed

[Guenter Roeck]

Some architectures do not or not always support cmpxchg64(). This results
in on/off problems when the function is used in common code. The latest
example is

net/sunrpc/auth_gss/gss_krb5_seal.c: In function 'gss_seq_send64_fetch_and_inc':
net/sunrpc/auth_gss/gss_krb5_seal.c:145:14: error:
	implicit declaration of function 'cmpxchg64'

which is seen with some powerpc and mips builds.

Introduce a generic version of __cmpxchg_u64() and use it for affected
architectures.

Fixes: 21924765862a
	("SUNRPC: use cmpxchg64() in gss_seq_send64_fetch_and_inc()")
Cc: Arnd Bergmann <arnd@xxxxxxxx>
Cc: Trond Myklebust <trond.myklebust@xxxxxxxxxxxxxxx>
Signed-off-by: Guenter Roeck <linux@xxxxxxxxxxxx>
---
Couple of questions:
- Is this the right (or an acceptable) approach to fix the problem ?
- Should I split the patch into three, one to introduce __cmpxchg_u64()
  and one per architecture ?
- Who should take the patch (series) ?

 arch/mips/Kconfig                  |  1 +
 arch/mips/include/asm/cmpxchg.h    |  3 ++
 arch/powerpc/Kconfig               |  1 +
 arch/powerpc/include/asm/cmpxchg.h |  3 ++
 lib/Kconfig                        |  3 ++
 lib/Makefile                       |  2 ++
 lib/cmpxchg64.c                    | 60 ++++++++++++++++++++++++++++++++++++++
 7 files changed, 73 insertions(+)
 create mode 100644 lib/cmpxchg64.c

diff --git a/arch/mips/Kconfig b/arch/mips/Kconfig
index 80778b40f8fa..7392a5f4e517 100644
--- a/arch/mips/Kconfig
+++ b/arch/mips/Kconfig
@@ -18,6 +18,7 @@ config MIPS
 	select CPU_PM if CPU_IDLE
 	select DMA_DIRECT_OPS
 	select GENERIC_ATOMIC64 if !64BIT
+	select GENERIC_CMPXCHG64 if !64BIT && SMP
 	select GENERIC_CLOCKEVENTS
 	select GENERIC_CMOS_UPDATE
 	select GENERIC_CPU_AUTOPROBE
diff --git a/arch/mips/include/asm/cmpxchg.h b/arch/mips/include/asm/cmpxchg.h
index 89e9fb7976fe..ca837b05bf3d 100644
--- a/arch/mips/include/asm/cmpxchg.h
+++ b/arch/mips/include/asm/cmpxchg.h
@@ -206,6 +206,9 @@ static inline unsigned long __cmpxchg(volatile void *ptr, unsigned long old,
 #define cmpxchg64_local(ptr, o, n) __cmpxchg64_local_generic((ptr), (o), (n))
 #ifndef CONFIG_SMP
 #define cmpxchg64(ptr, o, n) cmpxchg64_local((ptr), (o), (n))
+#else
+extern u64 __cmpxchg_u64(u64 *p, u64 old, u64 new);
+#define cmpxchg64(ptr, o, n) __cmpxchg_u64((ptr), (o), (n))
 #endif
 #endif
 
diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index e84943d24e5c..bd1d99c664c4 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig
@@ -161,6 +161,7 @@ config PPC
 	select EDAC_ATOMIC_SCRUB
 	select EDAC_SUPPORT
 	select GENERIC_ATOMIC64			if PPC32
+	select GENERIC_CMPXCHG64		if PPC32
 	select GENERIC_CLOCKEVENTS
 	select GENERIC_CLOCKEVENTS_BROADCAST	if SMP
 	select GENERIC_CMOS_UPDATE
diff --git a/arch/powerpc/include/asm/cmpxchg.h b/arch/powerpc/include/asm/cmpxchg.h
index 27183871eb3b..da8be4189731 100644
--- a/arch/powerpc/include/asm/cmpxchg.h
+++ b/arch/powerpc/include/asm/cmpxchg.h
@@ -534,8 +534,11 @@ __cmpxchg_acquire(void *ptr, unsigned long old, unsigned long new,
 	cmpxchg_acquire((ptr), (o), (n));				\
 })
 #else
+extern u64 __cmpxchg_u64(u64 *p, u64 old, u64 new);
+
 #include <asm-generic/cmpxchg-local.h>
 #define cmpxchg64_local(ptr, o, n) __cmpxchg64_local_generic((ptr), (o), (n))
+#define cmpxchg64(ptr, o, n) __cmpxchg_u64((ptr), (o), (n))
 #endif
 
 #endif /* __KERNEL__ */
diff --git a/lib/Kconfig b/lib/Kconfig
index d1573a16aa92..2b581a70ded2 100644
--- a/lib/Kconfig
+++ b/lib/Kconfig
@@ -500,6 +500,9 @@ config NLATTR
 config GENERIC_ATOMIC64
        bool
 
+config GENERIC_CMPXCHG64
+	bool
+
 config LRU_CACHE
 	tristate
 
diff --git a/lib/Makefile b/lib/Makefile
index 988949c4fd3a..4646a06ed418 100644
--- a/lib/Makefile
+++ b/lib/Makefile
@@ -172,6 +172,8 @@ obj-$(CONFIG_GENERIC_CSUM) += checksum.o
 
 obj-$(CONFIG_GENERIC_ATOMIC64) += atomic64.o
 
+obj-$(CONFIG_GENERIC_CMPXCHG64) += cmpxchg64.o
+
 obj-$(CONFIG_ATOMIC64_SELFTEST) += atomic64_test.o
 
 obj-$(CONFIG_CPU_RMAP) += cpu_rmap.o
diff --git a/lib/cmpxchg64.c b/lib/cmpxchg64.c
new file mode 100644
index 000000000000..239c43d05d00
--- /dev/null
+++ b/lib/cmpxchg64.c
@@ -0,0 +1,60 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Generic implementation of cmpxchg64().
+ * Derived from implementation in arch/sparc/lib/atomic32.c
+ * and from locking code implemented in lib/atomic32.c.
+ */
+
+#include <linux/cache.h>
+#include <linux/export.h>
+#include <linux/irqflags.h>
+#include <linux/spinlock.h>
+#include <linux/types.h>
+
+/*
+ * We use a hashed array of spinlocks to provide exclusive access
+ * to each variable. Since this is expected to used on systems
+ * with small numbers of CPUs (<= 4 or so), we use a relatively
+ * small array of 16 spinlocks to avoid wasting too much memory
+ * on the spinlock array.
+ */
+#define NR_LOCKS	16
+
+/* Ensure that each lock is in a separate cacheline */
+static union {
+	raw_spinlock_t lock;
+	char pad[L1_CACHE_BYTES];
+} cmpxchg_lock[NR_LOCKS] __cacheline_aligned_in_smp = {
+	[0 ... (NR_LOCKS - 1)] = {
+		.lock =  __RAW_SPIN_LOCK_UNLOCKED(cmpxchg_lock.lock),
+	},
+};
+
+static inline raw_spinlock_t *lock_addr(const u64 *v)
+{
+	unsigned long addr = (unsigned long) v;
+
+	addr >>= L1_CACHE_SHIFT;
+	addr ^= (addr >> 8) ^ (addr >> 16);
+	return &cmpxchg_lock[addr & (NR_LOCKS - 1)].lock;
+}
+
+/*
+ * Generic version of __cmpxchg_u64, to be used for cmpxchg64().
+ * Takes u64 parameters.
+ */
+u64 __cmpxchg_u64(u64 *ptr, u64 old, u64 new)
+{
+	raw_spinlock_t *lock = lock_addr(ptr);
+	unsigned long flags;
+	u64 prev;
+
+	raw_spin_lock_irqsave(lock, flags);
+	prev = READ_ONCE(*ptr);
+	if (prev == old)
+		*ptr = new;
+	raw_spin_unlock_irqrestore(lock, flags);
+
+	return prev;
+}
+EXPORT_SYMBOL(__cmpxchg_u64);
-- 
2.7.4