linux-kernel - Re: [RFC PATCH 3/4] Cleanup: sched/membarrier: only sync

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Message-ID: <20190906074104.GT2349@hirez.programming.kicks-ass.net>
Date:   Fri, 6 Sep 2019 09:41:04 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     "Paul E. McKenney" <paulmck@...ux.ibm.com>,
        Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
        Oleg Nesterov <oleg@...hat.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Russell King - ARM Linux admin <linux@...linux.org.uk>,
        Chris Metcalf <cmetcalf@...hip.com>,
        Christoph Lameter <cl@...ux.com>,
        Kirill Tkhai <tkhai@...dex.ru>, Mike Galbraith <efault@....de>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [RFC PATCH 3/4] Cleanup: sched/membarrier: only sync_core before
 usermode for same mm

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@...hat.com>
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> Cc: "Paul E. McKenney" <paulmck@...ux.ibm.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Oleg Nesterov <oleg@...hat.com>
> Cc: "Eric W. Biederman" <ebiederm@...ssion.com>
> Cc: Linus Torvalds <torvalds@...ux-foundation.org>
> Cc: Russell King - ARM Linux admin <linux@...linux.org.uk>
> Cc: Chris Metcalf <cmetcalf@...hip.com>
> Cc: Christoph Lameter <cl@...ux.com>
> Cc: Kirill Tkhai <tkhai@...dex.ru>
> Cc: Mike Galbraith <efault@....de>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Ingo Molnar <mingo@...nel.org>
> ---
>  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)