[PATCH for 4.16 v7 02/11] powerpc: membarrier: Skip memory barrier in switch_mm()

[Christophe Leroy]

Le 18/06/2021 à 19:26, Mathieu Desnoyers a écrit :
> ----- On Jun 18, 2021, at 1:13 PM, Christophe Leroy christophe.leroy at 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.
> 

I see.

Then, if you think a 'sync' is a concern, shouldn't we try and remove the forest of 'sync' in the 
I/O accessors ?

I can't really understand why we need all those 'sync' and 'isync' and 'twi' around the accesses 
whereas I/O memory is usually mapped as 'Guarded' so memory access ordering is already garantied.

I'm sure we'll save a lot with that.

Christophe