linux-kernel - Re: [PATCH] sched/fair: provide u64 read for 32-bits arch helper

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Date:   Tue, 28 Jul 2020 14:00:27 +0200
From:   peterz@...radead.org
To:     Vincent Donnefort <vincent.donnefort@....com>
Cc:     mingo@...hat.com, vincent.guittot@...aro.org,
        linux-kernel@...r.kernel.org, dietmar.eggemann@....com,
        lukasz.luba@....com, valentin.schneider@....com
Subject: Re: [PATCH] sched/fair: provide u64 read for 32-bits arch helper

On Tue, Jul 28, 2020 at 01:13:02PM +0200, peterz@...radead.org wrote:
> On Mon, Jul 27, 2020 at 04:23:03PM +0100, Vincent Donnefort wrote:
> 
> > For 32-bit architectures, both min_vruntime and last_update_time are using
> > similar access. This patch is simply an attempt to unify their usage by
> > introducing two macros to rely on when accessing those. At the same time, it
> > brings a comment regarding the barriers usage, as per the kernel policy. So
> > overall this is just a clean-up without any functional changes.
> 
> Ah, I though there was perhaps the idea to make use of armv7-lpae
> instructions.
> 
> Aside of that, I think we need to spend a little time bike-shedding the
> API/naming here:
> 
> > +# define u64_32read(val, copy) (val)
> > +# define u64_32read_set_copy(val, copy) do { } while (0)
> 
> How about something like:
> 
> #ifdef CONFIG_64BIT
> 
> #define DEFINE_U64_U32(name)	u64 name
> #define u64_u32_load(name)	name
> #define u64_u32_store(name, val)name = val
> 
> #else
> 
> #define DEFINE_U64_U32(name)			\
> 	struct {				\
> 		u64 name;			\
> 		u64 name##_copy;		\
> 	}
> 
> #define u64_u32_load(name)			\
> 	({					\
> 		u64 val, copy;			\
> 		do {				\
> 			val = name;		\
> 			smb_rmb();		\
> 			copy = name##_copy;	\
> 		} while (val != copy);		\

wrong order there; we should first read _copy and then the regular one
of course.

> 		val;
> 	})
> 
> #define u64_u32_store(name, val)		\
> 	do {					\
> 		typeof(val) __val = (val);	\
> 		name = __val;			\
> 		smp_wmb();			\
> 		name##_copy = __val;		\
> 	} while (0)
> 
> #endif

The other approach is making it a full type and inline functions I
suppose.