[PATCH] riscv: locks: introduce ticket-based spinlock implementation

[guoren@kernel.org]

From: Peter Zijlstra <peterz@infradead.org>

This patch introduces a ticket lock implementation for riscv, and the
implementation is from Peter Zijlstra generic ticket-lock patch:

    locking: Generic ticket-lock

    On Wed, Apr 14, 2021 at 11:05:24AM +0200, Peter Zijlstra wrote:

    > That made me look at the qspinlock code, and queued_spin_*lock() uses
    > atomic_try_cmpxchg_acquire(), which means any arch that uses qspinlock
    > and has RCpc atomics will give us massive pain.
    >
    > Current archs using qspinlock are: x86, arm64, power, sparc64, mips and
    > openrisc (WTF?!).
    >
    > Of those, x86 and sparc are TSO archs with SC atomics, arm64 has RCsc
    > atomics, power has RCtso atomics (and is the arch we all hate for having
    > RCtso locks).
    >
    > Now MIPS has all sorts of ill specified barriers, but last time looked
    > at it it didn't actually use any of that and stuck to using smp_mb(), so
    > it will have RCsc atomics.
    >
    > /me goes look at wth openrisc is..  doesn't even appear to have
    > asm/barrier.h :-/ Looking at wikipedia it also doesn't appear to
    > actually have hardware ...

    FWIW this is broken, anything SMP *MUST* define mb(), at the very least.

    > I'm thinking openrisc is a prime candidate for this ticket_lock.h we're
    > all talking about.

    How's this then? Compile tested only on openrisc/simple_smp_defconfig.

Cc: Peter Zijlstra <peterz@infradead.org>
Co-developed-by: Guo Ren <guoren@linux.alibaba.com>
Tested-by: Guo Ren <guoren@linux.alibaba.com>
Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
Cc: Christoph Müllner <christophm30@gmail.com>
Cc: Stafford Horne <shorne@gmail.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Palmer Dabbelt <palmerdabbelt@google.com>
Cc: Anup Patel <anup@brainfault.org>
Cc: Arnd Bergmann <arnd@arndb.de>
---
 arch/riscv/Kconfig                      |   1 +
 arch/riscv/include/asm/Kbuild           |   1 +
 arch/riscv/include/asm/spinlock.h       | 130 ++++++------------------
 arch/riscv/include/asm/spinlock_types.h |  13 +--
 4 files changed, 36 insertions(+), 109 deletions(-)

diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index d18a59ea10e5..a2f745715614 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -36,6 +36,7 @@ config RISCV
 	select ARCH_OPTIONAL_KERNEL_RWX_DEFAULT
 	select ARCH_SUPPORTS_HUGETLBFS if MMU
 	select ARCH_USE_MEMTEST
+	select ARCH_USE_QUEUED_RWLOCKS
 	select ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT if MMU
 	select ARCH_WANT_FRAME_POINTERS
 	select ARCH_WANT_HUGE_PMD_SHARE if 64BIT
diff --git a/arch/riscv/include/asm/Kbuild b/arch/riscv/include/asm/Kbuild
index 445ccc97305a..e57ef80a796e 100644
--- a/arch/riscv/include/asm/Kbuild
+++ b/arch/riscv/include/asm/Kbuild
@@ -3,5 +3,6 @@ generic-y += early_ioremap.h
 generic-y += extable.h
 generic-y += flat.h
 generic-y += kvm_para.h
+generic-y += qrwlock.h
 generic-y += user.h
 generic-y += vmlinux.lds.h
diff --git a/arch/riscv/include/asm/spinlock.h b/arch/riscv/include/asm/spinlock.h
index f4f7fa1b7ca8..61287b3ff4a7 100644
--- a/arch/riscv/include/asm/spinlock.h
+++ b/arch/riscv/include/asm/spinlock.h
@@ -7,129 +7,59 @@
 #ifndef _ASM_RISCV_SPINLOCK_H
 #define _ASM_RISCV_SPINLOCK_H
 
-#include <linux/kernel.h>
-#include <asm/current.h>
-#include <asm/fence.h>
-
-/*
- * Simple spin lock operations.  These provide no fairness guarantees.
- */
-
-/* FIXME: Replace this with a ticket lock, like MIPS. */
+static __always_inline void ticket_lock(arch_spinlock_t *lock)
+{
+	u32 val = atomic_fetch_add(1<<16, lock); /* SC, gives us RCsc */
+	u16 ticket = val >> 16;
 
-#define arch_spin_is_locked(x)	(READ_ONCE((x)->lock) != 0)
+	if (ticket == (u16)val)
+		return;
 
-static inline void arch_spin_unlock(arch_spinlock_t *lock)
-{
-	smp_store_release(&lock->lock, 0);
+	atomic_cond_read_acquire(lock, ticket == (u16)VAL);
 }
 
-static inline int arch_spin_trylock(arch_spinlock_t *lock)
+static __always_inline bool ticket_trylock(arch_spinlock_t *lock)
 {
-	int tmp = 1, busy;
+	u32 old = atomic_read(lock);
 
-	__asm__ __volatile__ (
-		"	amoswap.w %0, %2, %1\n"
-		RISCV_ACQUIRE_BARRIER
-		: "=r" (busy), "+A" (lock->lock)
-		: "r" (tmp)
-		: "memory");
+	if ((old >> 16) != (old & 0xffff))
+		return false;
 
-	return !busy;
+	return atomic_try_cmpxchg(lock, &old, old + (1<<16)); /* SC, for RCsc */
 }
 
-static inline void arch_spin_lock(arch_spinlock_t *lock)
+static __always_inline void ticket_unlock(arch_spinlock_t *lock)
 {
-	while (1) {
-		if (arch_spin_is_locked(lock))
-			continue;
+	u16 *ptr = (u16 *)lock;
+	u32 val = atomic_read(lock);
 
-		if (arch_spin_trylock(lock))
-			break;
-	}
+	smp_store_release(ptr, (u16)val + 1);
 }
 
-/***********************************************************/
-
-static inline void arch_read_lock(arch_rwlock_t *lock)
+static __always_inline int ticket_value_unlocked(arch_spinlock_t lock)
 {
-	int tmp;
-
-	__asm__ __volatile__(
-		"1:	lr.w	%1, %0\n"
-		"	bltz	%1, 1b\n"
-		"	addi	%1, %1, 1\n"
-		"	sc.w	%1, %1, %0\n"
-		"	bnez	%1, 1b\n"
-		RISCV_ACQUIRE_BARRIER
-		: "+A" (lock->lock), "=&r" (tmp)
-		:: "memory");
+	return (((u32)lock.counter >> 16) == ((u32)lock.counter & 0xffff));
 }
 
-static inline void arch_write_lock(arch_rwlock_t *lock)
+static __always_inline int ticket_is_locked(arch_spinlock_t *lock)
 {
-	int tmp;
-
-	__asm__ __volatile__(
-		"1:	lr.w	%1, %0\n"
-		"	bnez	%1, 1b\n"
-		"	li	%1, -1\n"
-		"	sc.w	%1, %1, %0\n"
-		"	bnez	%1, 1b\n"
-		RISCV_ACQUIRE_BARRIER
-		: "+A" (lock->lock), "=&r" (tmp)
-		:: "memory");
+	return !ticket_value_unlocked(READ_ONCE(*lock));
 }
 
-static inline int arch_read_trylock(arch_rwlock_t *lock)
+static __always_inline int ticket_is_contended(arch_spinlock_t *lock)
 {
-	int busy;
-
-	__asm__ __volatile__(
-		"1:	lr.w	%1, %0\n"
-		"	bltz	%1, 1f\n"
-		"	addi	%1, %1, 1\n"
-		"	sc.w	%1, %1, %0\n"
-		"	bnez	%1, 1b\n"
-		RISCV_ACQUIRE_BARRIER
-		"1:\n"
-		: "+A" (lock->lock), "=&r" (busy)
-		:: "memory");
-
-	return !busy;
-}
+	u32 val = atomic_read(lock);
 
-static inline int arch_write_trylock(arch_rwlock_t *lock)
-{
-	int busy;
-
-	__asm__ __volatile__(
-		"1:	lr.w	%1, %0\n"
-		"	bnez	%1, 1f\n"
-		"	li	%1, -1\n"
-		"	sc.w	%1, %1, %0\n"
-		"	bnez	%1, 1b\n"
-		RISCV_ACQUIRE_BARRIER
-		"1:\n"
-		: "+A" (lock->lock), "=&r" (busy)
-		:: "memory");
-
-	return !busy;
+	return (s16)((val >> 16) - (val & 0xffff)) > 1;
 }
 
-static inline void arch_read_unlock(arch_rwlock_t *lock)
-{
-	__asm__ __volatile__(
-		RISCV_RELEASE_BARRIER
-		"	amoadd.w x0, %1, %0\n"
-		: "+A" (lock->lock)
-		: "r" (-1)
-		: "memory");
-}
+#define arch_spin_lock(l)		ticket_lock(l)
+#define arch_spin_trylock(l)		ticket_trylock(l)
+#define arch_spin_unlock(l)		ticket_unlock(l)
+#define arch_spin_value_unlocked(l)	ticket_value_unlocked(l)
+#define arch_spin_is_locked(l)		ticket_is_locked(l)
+#define arch_spin_is_contended(l)	ticket_is_contended(l)
 
-static inline void arch_write_unlock(arch_rwlock_t *lock)
-{
-	smp_store_release(&lock->lock, 0);
-}
+#include <asm/qrwlock.h>
 
 #endif /* _ASM_RISCV_SPINLOCK_H */
diff --git a/arch/riscv/include/asm/spinlock_types.h b/arch/riscv/include/asm/spinlock_types.h
index f398e7638dd6..4d7cbb3d62e8 100644
--- a/arch/riscv/include/asm/spinlock_types.h
+++ b/arch/riscv/include/asm/spinlock_types.h
@@ -10,16 +10,11 @@
 # error "please don't include this file directly"
 #endif
 
-typedef struct {
-	volatile unsigned int lock;
-} arch_spinlock_t;
+#include <linux/types.h>
+typedef atomic_t arch_spinlock_t;
 
-#define __ARCH_SPIN_LOCK_UNLOCKED	{ 0 }
+#define __ARCH_SPIN_LOCK_UNLOCKED	ATOMIC_INIT(0)
 
-typedef struct {
-	volatile unsigned int lock;
-} arch_rwlock_t;
-
-#define __ARCH_RW_LOCK_UNLOCKED		{ 0 }
+#include <asm-generic/qrwlock_types.h>
 
 #endif /* _ASM_RISCV_SPINLOCK_TYPES_H */
-- 
2.25.1