Re: [RFC PATCH] sched: Add missing memory barrier in switch_mm_cid

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

On 2024-03-11 04:45, Yeo Reum Yun wrote:
> Hi. Mathieu. Sorry to late answer.
> 
>> diff --git a/arch/x86/include/asm/barrier.h b/arch/x86/include/asm/barrier.h
>> index 35389b2af88e..0d5e54201eb2 100644
>> --- a/arch/x86/include/asm/barrier.h
>> +++ b/arch/x86/include/asm/barrier.h
>> @@ -79,6 +79,9 @@ do {                                                                  \
>>   #define __smp_mb__before_atomic()      do { } while (0)
>>   #define __smp_mb__after_atomic()       do { } while (0)
> 
>> +/* Writing to CR3 provides a full memory barrier in switch_mm(). */
>> +#define smp_mb__after_switch_mm()      do { } while (0)
>> +
>> #include <asm-generic/barrier.h>
> 
> IIUC, ppc already does smp_mb() in switch_mm.
> 
> Would it better to add the same macro which do nothing to pcc?\\

Does it ?

Based on arch/powerpc/include/asm/membarrier.h, it appears that
powerpc does _not_ have a guaranteed barrier in switch_mm():

static inline void membarrier_arch_switch_mm(struct mm_struct *prev,
                                              struct mm_struct *next,
                                              struct task_struct *tsk)
{
         /*
          * Only need the full barrier when switching between processes.
          * Barrier when switching from kernel to userspace is not
          * required here, given that it is implied by mmdrop(). Barrier
          * when switching from userspace to kernel is not needed after
          * store to rq->curr.
          */
         if (IS_ENABLED(CONFIG_SMP) &&
             likely(!(atomic_read(&next->membarrier_state) &
                      (MEMBARRIER_STATE_PRIVATE_EXPEDITED |
                       MEMBARRIER_STATE_GLOBAL_EXPEDITED)) || !prev))
                 return;

         /*
          * The membarrier system call requires a full memory barrier
          * after storing to rq->curr, before going back to user-space.
          */
         smp_mb();
}

AFAIU the barrier provided in powerpc switch_mm_irqs_off() is only in the
"new_on_cpu" case. Am I missing something ?

Thanks,

Mathieu

> 
> Thanks!
> 
-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com