Re: [PATCH v6 3/9] riscv: locks: Introduce ticket-based spinlock implementation

[Guo Ren <guoren@kernel.org>]

On Wed, Mar 31, 2021 at 10:32 PM <guoren@kernel.org> wrote:
>
> From: Guo Ren <guoren@linux.alibaba.com>
>
> This patch introduces a ticket lock implementation for riscv, along the
> same lines as the implementation for arch/arm & arch/csky.
>
> We still use qspinlock as default.
>
> Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Anup Patel <anup@brainfault.org>
> Cc: Arnd Bergmann <arnd@arndb.de>
> ---
>  arch/riscv/Kconfig                      |  7 ++-
>  arch/riscv/include/asm/spinlock.h       | 84 +++++++++++++++++++++++++
>  arch/riscv/include/asm/spinlock_types.h | 17 +++++
>  3 files changed, 107 insertions(+), 1 deletion(-)
>
> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> index 67cc65ba1ea1..34d0276f01d5 100644
> --- a/arch/riscv/Kconfig
> +++ b/arch/riscv/Kconfig
> @@ -34,7 +34,7 @@ config RISCV
>         select ARCH_WANT_FRAME_POINTERS
>         select ARCH_WANT_HUGE_PMD_SHARE if 64BIT
>         select ARCH_USE_QUEUED_RWLOCKS
> -       select ARCH_USE_QUEUED_SPINLOCKS
> +       select ARCH_USE_QUEUED_SPINLOCKS        if !RISCV_TICKET_LOCK
>         select ARCH_USE_QUEUED_SPINLOCKS_XCHG32
>         select CLONE_BACKWARDS
>         select CLINT_TIMER if !MMU
> @@ -344,6 +344,11 @@ config NEED_PER_CPU_EMBED_FIRST_CHUNK
>         def_bool y
>         depends on NUMA
>
> +config RISCV_TICKET_LOCK
> +       bool "Ticket-based spin-locking"
> +       help
> +         Say Y here to use ticket-based spin-locking.
> +
>  config RISCV_ISA_C
>         bool "Emit compressed instructions when building Linux"
>         default y
> diff --git a/arch/riscv/include/asm/spinlock.h b/arch/riscv/include/asm/spinlock.h
> index a557de67a425..90b7eaa950cf 100644
> --- a/arch/riscv/include/asm/spinlock.h
> +++ b/arch/riscv/include/asm/spinlock.h
> @@ -7,7 +7,91 @@
>  #ifndef _ASM_RISCV_SPINLOCK_H
>  #define _ASM_RISCV_SPINLOCK_H
>
> +#ifdef CONFIG_RISCV_TICKET_LOCK
> +#ifdef CONFIG_32BIT
> +#define __ASM_SLLIW "slli\t"
> +#define __ASM_SRLIW "srli\t"
> +#else
> +#define __ASM_SLLIW "slliw\t"
> +#define __ASM_SRLIW "srliw\t"
> +#endif
> +
> +/*
> + * Ticket-based spin-locking.
> + */
> +static inline void arch_spin_lock(arch_spinlock_t *lock)
> +{
> +       arch_spinlock_t lockval;
> +       u32 tmp;
> +
> +       asm volatile (
> +               "1:     lr.w    %0, %2          \n"
> +               "       mv      %1, %0          \n"
> +               "       addw    %0, %0, %3      \n"
> +               "       sc.w    %0, %0, %2      \n"
> +               "       bnez    %0, 1b          \n"
> +               : "=&r" (tmp), "=&r" (lockval), "+A" (lock->lock)
> +               : "r" (1 << TICKET_NEXT)
> +               : "memory");
It's could be optimized by amoadd.w with Anup advice, and I'll update
it in the next patchset version:
diff --git a/arch/riscv/include/asm/spinlock.h
b/arch/riscv/include/asm/spinlock.h
index 90b7eaa950cf..435286ad342b 100644
--- a/arch/riscv/include/asm/spinlock.h
+++ b/arch/riscv/include/asm/spinlock.h
@@ -22,15 +22,10 @@
 static inline void arch_spin_lock(arch_spinlock_t *lock)
 {
        arch_spinlock_t lockval;
-       u32 tmp;

        asm volatile (
-               "1:     lr.w    %0, %2          \n"
-               "       mv      %1, %0          \n"
-               "       addw    %0, %0, %3      \n"
-               "       sc.w    %0, %0, %2      \n"
-               "       bnez    %0, 1b          \n"
-               : "=&r" (tmp), "=&r" (lockval), "+A" (lock->lock)
+               "   amoadd.w    %0, %2, %1      \n"
+               : "=&r" (lockval), "+A" (lock->lock)
                : "r" (1 << TICKET_NEXT)
                : "memory");




> +
> +       smp_cond_load_acquire(&lock->tickets.owner,
> +                                       VAL == lockval.tickets.next);
> +}
> +
> +static inline int arch_spin_trylock(arch_spinlock_t *lock)
> +{
> +       u32 tmp, contended, res;
> +
> +       do {
> +               asm volatile (
> +               "       lr.w    %0, %3          \n"
> +               __ASM_SRLIW    "%1, %0, %5      \n"
> +               __ASM_SLLIW    "%2, %0, %5      \n"
> +               "       or      %1, %2, %1      \n"
> +               "       li      %2, 0           \n"
> +               "       sub     %1, %1, %0      \n"
> +               "       bnez    %1, 1f          \n"
> +               "       addw    %0, %0, %4      \n"
> +               "       sc.w    %2, %0, %3      \n"
> +               "1:                             \n"
> +               : "=&r" (tmp), "=&r" (contended), "=&r" (res),
> +                 "+A" (lock->lock)
> +               : "r" (1 << TICKET_NEXT), "I" (TICKET_NEXT)
> +               : "memory");
> +       } while (res);
> +
> +       if (!contended)
> +               __atomic_acquire_fence();
> +
> +       return !contended;
> +}
> +
> +static inline void arch_spin_unlock(arch_spinlock_t *lock)
> +{
> +       smp_store_release(&lock->tickets.owner, lock->tickets.owner + 1);
> +}
> +
> +static inline int arch_spin_value_unlocked(arch_spinlock_t lock)
> +{
> +       return lock.tickets.owner == lock.tickets.next;
> +}
> +
> +static inline int arch_spin_is_locked(arch_spinlock_t *lock)
> +{
> +       return !arch_spin_value_unlocked(READ_ONCE(*lock));
> +}
> +
> +static inline int arch_spin_is_contended(arch_spinlock_t *lock)
> +{
> +       struct __raw_tickets tickets = READ_ONCE(lock->tickets);
> +
> +       return (tickets.next - tickets.owner) > 1;
> +}
> +#define arch_spin_is_contended arch_spin_is_contended
> +#else /* CONFIG_RISCV_TICKET_LOCK */
>  #include <asm/qspinlock.h>
> +#endif /* CONFIG_RISCV_TICKET_LOCK */
> +
>  #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 d033a973f287..afbb19841d0f 100644
> --- a/arch/riscv/include/asm/spinlock_types.h
> +++ b/arch/riscv/include/asm/spinlock_types.h
> @@ -10,7 +10,24 @@
>  # error "please don't include this file directly"
>  #endif
>
> +#ifdef CONFIG_RISCV_TICKET_LOCK
> +#define TICKET_NEXT    16
> +
> +typedef struct {
> +       union {
> +               u32 lock;
> +               struct __raw_tickets {
> +                       /* little endian */
> +                       u16 owner;
> +                       u16 next;
> +               } tickets;
> +       };
> +} arch_spinlock_t;
> +
> +#define __ARCH_SPIN_LOCK_UNLOCKED      { { 0 } }
> +#else
>  #include <asm-generic/qspinlock_types.h>
> +#endif
>  #include <asm-generic/qrwlock_types.h>
>
>  #endif /* _ASM_RISCV_SPINLOCK_TYPES_H */
> --
> 2.17.1
>


--
Best Regards
 Guo Ren

ML: https://lore.kernel.org/linux-csky/