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SubjectRe: [PATCH for 4.16 v7 02/11] powerpc: membarrier: Skip memory barrier in switch_mm()
----- On Jun 18, 2021, at 1:13 PM, Christophe Leroy christophe.leroy@csgroup.eu wrote:
[...]
>
> I don't understand all that complexity to just replace a simple
> 'smp_mb__after_unlock_lock()'.
>
> #define smp_mb__after_unlock_lock() smp_mb()
> #define smp_mb() barrier()
> # define barrier() __asm__ __volatile__("": : :"memory")
>
>
> Am I missing some subtility ?

On powerpc CONFIG_SMP, smp_mb() is actually defined as:

#define smp_mb() __smp_mb()
#define __smp_mb() mb()
#define mb() __asm__ __volatile__ ("sync" : : : "memory")

So the original motivation here was to skip a "sync" instruction whenever
switching between threads which are part of the same process. But based on
recent discussions, I suspect my implementation may be inaccurately doing
so though.

Thanks,

Mathieu


>
> Thanks
> Christophe

--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com

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