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

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Date:   Tue, 6 Nov 2018 09:45:42 +0800
From:   Gao Xiang <gaoxiang25@...wei.com>
To:     Will Deacon <will.deacon@....com>
CC:     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>, <peterz@...radead.org>
Subject: Re: [PATCH v2] bit_spinlock: introduce smp_cond_load_relaxed

Hi Will,

On 2018/11/6 6:49, Will Deacon wrote:
> Hi Gao,
> 
> On Tue, Oct 30, 2018 at 02:04:41PM +0800, Gao Xiang wrote:
>> It is better to use wrapped smp_cond_load_relaxed
>> instead of open-coded busy waiting for bit_spinlock.
>>
>> Signed-off-by: Gao Xiang <gaoxiang25@...wei.com>
>> ---
>>
>> change log v2:
>>  - fix the incorrect expression !(VAL >> (bitnum & (BITS_PER_LONG-1)))
>>  - the test result is described in the following reply.
> Please include the results in the commit message, so that this change is
> justified.

Will add in the next version...

> 
> This appears to introduce a bunch of overhead for the uncontended fastpath.
> How about the much-simpler-but-completely-untested (tm) patch below?

Actually I thought to do like the following (much simpler indeed) at first...

But the current implementation of smp_cond_load_relaxed will do a judgement immediately
which seems unnecessary (right after the test_and_set_bit_lock rather than after
__cmpwait_relaxed...)
        for (;;) {							\
		VAL = READ_ONCE(*__PTR);				\
		if (cond_expr)						\
			break;						\
		__cmpwait_relaxed(__PTR, VAL);				\
	}								\

p.s. I have no idea the original uncontended fastpath really works effectively...
some idea about this? Thanks in advance...


Thanks,
Gao Xiang


> 
> Will
> 
> --->8
> 
> 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