Re: [PATCH v2] bit_spinlock: introduce smp_cond_load_relaxed

From: Peter Zijlstra <peterz@infradead.org>
To: Will Deacon <will.deacon@arm.com>
Cc: Gao Xiang <gaoxiang25@huawei.com>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Philippe Ombredanne <pombredanne@nexb.com>,
	Kate Stewart <kstewart@linuxfoundation.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	linux-kernel@vger.kernel.org, Miao Xie <miaoxie@huawei.com>,
	Chao Yu <chao@kernel.org>
Subject: Re: [PATCH v2] bit_spinlock: introduce smp_cond_load_relaxed
Date: Tue, 6 Nov 2018 10:06:42 +0100	[thread overview]
Message-ID: <20181106090642.GH22431@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20181105224654.GA25864@brain-police>

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.