linux-kernel - Re: [PATCH v2] bit_spinlock: introduce smp_cond_load

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Date:   Tue, 6 Nov 2018 10:06:42 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Will Deacon <will.deacon@....com>
Cc:     Gao Xiang <gaoxiang25@...wei.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Philippe Ombredanne <pombredanne@...b.com>,
        Kate Stewart <kstewart@...uxfoundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org, Miao Xie <miaoxie@...wei.com>,
        Chao Yu <chao@...nel.org>
Subject: Re: [PATCH v2] bit_spinlock: introduce smp_cond_load_relaxed

On Mon, Nov 05, 2018 at 10:49:21PM +0000, Will Deacon wrote:
> diff --git a/include/asm-generic/bitops/lock.h b/include/asm-generic/bitops/lock.h
> index 3ae021368f48..9de8d3544630 100644
> --- a/include/asm-generic/bitops/lock.h
> +++ b/include/asm-generic/bitops/lock.h
> @@ -6,6 +6,15 @@
>  #include <linux/compiler.h>
>  #include <asm/barrier.h>
>  
> +static inline void spin_until_bit_unlock(unsigned int nr,
> +					 volatile unsigned long *p)
> +{
> +	unsigned long mask = BIT_MASK(bitnum);
> +
> +	p += BIT_WORD(nr);
> +	smp_cond_load_relaxed(p, VAL & mask);
> +}
> +
>  /**
>   * test_and_set_bit_lock - Set a bit and return its old value, for lock
>   * @nr: Bit to set
> diff --git a/include/linux/bit_spinlock.h b/include/linux/bit_spinlock.h
> index bbc4730a6505..d711c62e718c 100644
> --- a/include/linux/bit_spinlock.h
> +++ b/include/linux/bit_spinlock.h
> @@ -26,9 +26,7 @@ static inline void bit_spin_lock(int bitnum, unsigned long *addr)
>  #if defined(CONFIG_SMP) || defined(CONFIG_DEBUG_SPINLOCK)
>  	while (unlikely(test_and_set_bit_lock(bitnum, addr))) {
>  		preempt_enable();
> -		do {
> -			cpu_relax();
> -		} while (test_bit(bitnum, addr));
> +		spin_until_bit_unlock(bitnum, addr);
>  		preempt_disable();
>  	}
>  #endif

Yes, that's much better. Ideally though, we'd get rid of bit spinlocks
that have significant enough contention for this to matter.