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

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Date:   Wed, 29 Jun 2022 08:27:19 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     "'guoren@...nel.org'" <guoren@...nel.org>,
        "palmer@...osinc.com" <palmer@...osinc.com>,
        "arnd@...db.de" <arnd@...db.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "will@...nel.org" <will@...nel.org>,
        "longman@...hat.com" <longman@...hat.com>,
        "boqun.feng@...il.com" <boqun.feng@...il.com>
CC:     "linux-riscv@...ts.infradead.org" <linux-riscv@...ts.infradead.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Guo Ren <guoren@...ux.alibaba.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: RE: [PATCH V7 1/5] asm-generic: ticket-lock: Remove unnecessary
 atomic_read

From: guoren@...nel.org
> Sent: 28 June 2022 09:17
> 
> 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.

I'm confused (as usual).
Isn't atomic_read() pretty much free?

..
> 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));

That almost certainly needs a READ_ONCE().

The result is also inherently stale.
So the uses must be pretty limited.

	David

>  }
> 
>  #include <asm/qrwlock.h>
> --
> 2.36.1

-
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