Re: [RFC PATCH 3/4] Cleanup: sched/membarrier: only sync_core before usermode for same mm

[Peter Zijlstra]

On Thu, Sep 05, 2019 at 11:12:59PM -0400, Mathieu Desnoyers wrote:
> When the prev and next task's mm change, switch_mm() provides the core
> serializing guarantees before returning to usermode. The only case
> where an explicit core serialization is needed is when the scheduler
> keeps the same mm for prev and next.
> 
> Suggested-by: Oleg Nesterov <oleg@xxxxxxxxxx>
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
> Cc: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Cc: Russell King - ARM Linux admin <linux@xxxxxxxxxxxxxxx>
> Cc: Chris Metcalf <cmetcalf@xxxxxxxxxx>
> Cc: Christoph Lameter <cl@xxxxxxxxx>
> Cc: Kirill Tkhai <tkhai@xxxxxxxxx>
> Cc: Mike Galbraith <efault@xxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> ---
>  include/linux/sched/mm.h | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
> index 4a7944078cc3..8557ec664213 100644
> --- a/include/linux/sched/mm.h
> +++ b/include/linux/sched/mm.h
> @@ -362,6 +362,8 @@ enum {
>  
>  static inline void membarrier_mm_sync_core_before_usermode(struct mm_struct *mm)
>  {
> +	if (current->mm != mm)
> +		return;
>  	if (likely(!(atomic_read(&mm->membarrier_state) &
>  		     MEMBARRIER_STATE_PRIVATE_EXPEDITED_SYNC_CORE)))
>  		return;

So SYNC_CORE is about I$ coherency and funny thing like that. Now it
seems 'natural' that if we flip the address space, that I$ also gets
wiped/updated, because the whole text mapping changes.

But did we just assume that, or did we verify the truth of this? (I'm
just being paranoid here)