Received-SPF: softfail (google.com: domain of transitioning linux-kernel-owner@vger.kernel.org does not designate 23.128.96.19 as permitted sender) client-ip=23.128.96.19;
Message-ID: <5033835b-195f-2712-d621-c3c06cc61a08@arm.com>
Date:   Fri, 22 Apr 2022 12:10:36 +0100
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:91.0)
 Gecko/20100101 Thunderbird/91.8.0
Subject: Re: [PATCH v4 1/7] sched/fair: Provide u64 read for 32-bits arch
 helper
Content-Language: en-US
To:     Tao Zhou <tao.zhou@linux.dev>
Cc:     peterz@infradead.org, mingo@redhat.com, vincent.guittot@linaro.org,
        linux-kernel@vger.kernel.org, dietmar.eggemann@arm.com,
        morten.rasmussen@arm.com, chris.redpath@arm.com, qperret@google.com
References: <20220412134220.1588482-1-vincent.donnefort@arm.com>
 <20220412134220.1588482-2-vincent.donnefort@arm.com>
 <Yl6uKPBfOoj8ABfI@geo.homenetwork>
From:   Vincent Donnefort <vincent.donnefort@arm.com>
In-Reply-To: <Yl6uKPBfOoj8ABfI@geo.homenetwork>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 7bit
Precedence: bulk

[...]

>> +#ifdef CONFIG_64BIT
>> +# define u64_u32_load_copy(var, copy)       var
>> +# define u64_u32_store_copy(var, copy, val) (var = val)
>> +#else
>> +# define u64_u32_load_copy(var, copy)					\
>> +({									\
>> +	u64 __val, __val_copy;						\
>> +	do {								\
>> +		__val_copy = copy;					\
>> +		/*							\
>> +		 * paired with u64_u32_store, ordering access		\
>> +		 * to var and copy.					\
>> +		 */							\
>> +		smp_rmb();						\
>> +		__val = var;						\
>> +	} while (__val != __val_copy);					\
>> +	__val;								\
>> +})
>> +# define u64_u32_store_copy(var, copy, val)				\
>> +do {									\
>> +	typeof(val) __val = (val);					\
>> +	var = __val;							\
>> +	/*								\
>> +	 * paired with u64_u32_load, ordering access to var and		\
>> +	 * copy.							\
>> +	 */								\
>> +	smp_wmb();							\
>> +	copy = __val;							\
> 
> `copy = __val;` should be `copy = var`.
> 
> If var equal to val we do not need to do store. Check this condition
> in the above macro to avoid a redundant store.
> 
>   if (var != __val)
>     var = __val;

Judging by the users of this macro, var = val is very much unlikely to
happen. Also, I don't think we want to waste a if here.

[...]