linux-kernel - Re: [RFC PATCH 1/2] Fix: sched/membarrier: p->mm->membarrier

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Message-ID: <1457548021.1684.1567613478460.JavaMail.zimbra@efficios.com>
Date:   Wed, 4 Sep 2019 12:11:18 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     paulmck <paulmck@...ux.ibm.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        "Russell King, ARM Linux" <linux@...linux.org.uk>,
        Chris Metcalf <cmetcalf@...hip.com>,
        Chris Lameter <cl@...ux.com>, Kirill Tkhai <tkhai@...dex.ru>,
        Mike Galbraith <efault@....de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [RFC PATCH 1/2] Fix: sched/membarrier: p->mm->membarrier_state
 racy load

----- On Sep 4, 2019, at 7:11 AM, Oleg Nesterov oleg@...hat.com wrote:

> with or without these changes...
> 
> Why do membarrier_register_*_expedited() check get_nr_threads() == 1?
> This makes no sense to me, atomic_read(mm_users) == 1 should be enough.

Indeed, if every thread within a process hold a mm_users refcount, then
the get_nr_threads() == 1 check becomes redundant.

AFAIR, this check started out as "get_nr_threads() == 1", and then I changed
the code to also cover the multi-process CLONE_VM use-case by adding the
additional check.

> And I am not sure I understand membarrier_mm_sync_core_before_usermode().
> OK, membarrier_private_expedited() can race with user -> kernel -> user
> transition, but we do not care unless both user's above have the same mm?
> Shouldn't membarrier_mm_sync_core_before_usermode() do
> 
>	if (current->mm != mm)
>		return;
> 
> at the start to make it more clear and avoid sync_core_before_usermode()
> if possible?

Indeed, if we have taskA -> kernel -> taskB, it implies that we go through
switch_mm() when scheduling taskB, which provides the required core serializing
guarantees.

Moreover, if we look closely at the call to membarrier_mm_sync_core_before_usermode(),
the mm it receives as parameter is the rq->prev_mm. So using the prev_mm membarrier
state to decide whether we need to issue a sync_core before returning to a
different next mm is not really relevant unless the next mm == rq->prev_mm.

Nothing there seem to be actively buggy, but those are indeed nice cleanups.

Thanks,

Mathieu

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