linux-kernel - Re: [PATCH V7 1/5] asm-generic: ticket-lock: Remove unnecessary atomic

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Date:   Mon, 4 Jul 2022 11:52:39 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     guoren@...nel.org
Cc:     palmer@...osinc.com, arnd@...db.de, mingo@...hat.com,
        will@...nel.org, longman@...hat.com, boqun.feng@...il.com,
        linux-riscv@...ts.infradead.org, linux-arch@...r.kernel.org,
        linux-kernel@...r.kernel.org, Guo Ren <guoren@...ux.alibaba.com>
Subject: Re: [PATCH V7 1/5] asm-generic: ticket-lock: Remove unnecessary
 atomic_read

On Tue, Jun 28, 2022 at 04:17:03AM -0400, guoren@...nel.org wrote:
> From: Guo Ren <guoren@...ux.alibaba.com>
> 
> Remove unnecessary atomic_read in arch_spin_value_unlocked(lock),
> because the value has been in lock. This patch could prevent
> arch_spin_value_unlocked contend spin_lock data again.
> 
> Signed-off-by: Guo Ren <guoren@...ux.alibaba.com>
> Signed-off-by: Guo Ren <guoren@...nel.org>
> Cc: Peter Zijlstra (Intel) <peterz@...radead.org>
> Cc: Arnd Bergmann <arnd@...db.de>
> Cc: Palmer Dabbelt <palmer@...osinc.com>
> ---
>  include/asm-generic/spinlock.h | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/include/asm-generic/spinlock.h b/include/asm-generic/spinlock.h
> index fdfebcb050f4..f1e4fa100f5a 100644
> --- a/include/asm-generic/spinlock.h
> +++ b/include/asm-generic/spinlock.h
> @@ -84,7 +84,9 @@ static __always_inline int arch_spin_is_contended(arch_spinlock_t *lock)
>  
>  static __always_inline int arch_spin_value_unlocked(arch_spinlock_t lock)
>  {
> -	return !arch_spin_is_locked(&lock);
> +	u32 val = lock.counter;
> +
> +	return ((val >> 16) == (val & 0xffff));
>  }

Wouldn't the right thing be to flip arch_spin_is_locked() and
arch_spin_value_is_unlocked() ?


diff --git a/include/asm-generic/spinlock.h b/include/asm-generic/spinlock.h
index fdfebcb050f4..63ab4da262f2 100644
--- a/include/asm-generic/spinlock.h
+++ b/include/asm-generic/spinlock.h
@@ -68,23 +68,25 @@ static __always_inline void arch_spin_unlock(arch_spinlock_t *lock)
 	smp_store_release(ptr, (u16)val + 1);
 }
 
-static __always_inline int arch_spin_is_locked(arch_spinlock_t *lock)
+static __always_inline int arch_spin_is_contended(arch_spinlock_t *lock)
 {
 	u32 val = atomic_read(lock);
 
-	return ((val >> 16) != (val & 0xffff));
+	return (s16)((val >> 16) - (val & 0xffff)) > 1;
 }
 
-static __always_inline int arch_spin_is_contended(arch_spinlock_t *lock)
+static __always_inline int arch_spin_value_unlocked(arch_spinlock_t lock)
 {
-	u32 val = atomic_read(lock);
+	u32 val = lock.counter;
 
-	return (s16)((val >> 16) - (val & 0xffff)) > 1;
+	return ((val >> 16) == (val & 0xffff));
 }
 
-static __always_inline int arch_spin_value_unlocked(arch_spinlock_t lock)
+static __always_inline int arch_spin_is_locked(arch_spinlock_t *lock)
 {
-	return !arch_spin_is_locked(&lock);
+	arch_spinlock_t val = READ_ONCE(*lock);
+
+	return !arch_spin_value_unlocked(val);
 }
 
 #include <asm/qrwlock.h>