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

David Laight
2022-06-29T08:27:46Z

From: guoren@kernel.org
> Sent: 28 June 2022 09:17
>
> From: Guo Ren <guoren@linux.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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