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

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Date:   Tue, 6 Nov 2018 18:22:31 +0800
From:   Gao Xiang <gaoxiang25@...wei.com>
To:     Peter Zijlstra <peterz@...radead.org>
CC:     Will Deacon <will.deacon@....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

Hi Peter,

On 2018/11/6 17:06, Peter Zijlstra wrote:
> 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.

OK, I will send v3 to fix like the above.

Thanks,
Gao Xiang

> 
>