linux-kernel - Re: [PATCH v2 1/7] sched/fair: Provide u64 read for 32-bits arch helper

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Date:   Mon, 17 Jan 2022 19:42:04 +0000
From:   Vincent Donnefort <vincent.donnefort@....com>
To:     Tao Zhou <tao.zhou@...ux.dev>
Cc:     peterz@...radead.org, mingo@...hat.com, vincent.guittot@...aro.org,
        linux-kernel@...r.kernel.org, dietmar.eggemann@....com,
        valentin.schneider@....com, morten.rasmussen@....com,
        chris.redpath@....com, qperret@...gle.com, lukasz.luba@....com
Subject: Re: [PATCH v2 1/7] sched/fair: Provide u64 read for 32-bits arch
 helper

[...]

> > +/*
> > + * u64_u32_load/u64_u32_store
> > + *
> > + * Use a copy of a u64 value to protect against data race. This is only
> > + * applicable for 32-bits architectures.
> > + */
> > +#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;							\
> > +} while (0)
> 
> Code stay there some time from me. Just from my crude review;
> The above macro need a variable to load @var temporarily for
> later store; that means the @copy value is from @var not @val.
> 
>   # define u64_u32_store_copy(var, copy, val)				\
>   do {									\
>     typeof(val) __val = (val), __var = (var);					\
>     var = __val;							\
>     /*								\
>      * paired with u64_u32_load, ordering access to var and		\
>      * copy.							\
>      */								\
>     smp_wmb();							\
>     copy = __var;							\
>   } while (0)


Hi Tao,

__var would then contain the previous value of @var, wouldn't it? We need
both @var and @copy to be equal to @val.

> 
> 
> 
> Thanks,
> Tao