From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To: Oleg Nesterov <oleg@redhat.com>
Cc: paulmck <paulmck@linux.ibm.com>,
Peter Zijlstra <peterz@infradead.org>,
Linus Torvalds <torvalds@linux-foundation.org>,
linux-kernel <linux-kernel@vger.kernel.org>,
"Eric W. Biederman" <ebiederm@xmission.com>,
"Russell King, ARM Linux" <linux@armlinux.org.uk>,
Chris Metcalf <cmetcalf@ezchip.com>, Chris Lameter <cl@linux.com>,
Kirill Tkhai <tkhai@yandex.ru>, Mike Galbraith <efault@gmx.de>,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@kernel.org>
Subject: Re: [RFC PATCH 1/2] Fix: sched/membarrier: p->mm->membarrier_state racy load
Date: Wed, 4 Sep 2019 12:11:18 -0400 (EDT) [thread overview]
Message-ID: <1457548021.1684.1567613478460.JavaMail.zimbra@efficios.com> (raw)
In-Reply-To: <20190904111126.GB24568@redhat.com>
----- On Sep 4, 2019, at 7:11 AM, Oleg Nesterov oleg@redhat.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
next prev parent reply other threads:[~2019-09-04 16:13 UTC|newest]
Thread overview: 24+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-09-03 20:11 [RFC PATCH 1/2] Fix: sched/membarrier: p->mm->membarrier_state racy load Mathieu Desnoyers
2019-09-03 20:11 ` [RFC PATCH 2/2] Fix: sched/membarrier: private expedited registration check Mathieu Desnoyers
2019-09-03 20:24 ` [RFC PATCH 1/2] Fix: sched/membarrier: p->mm->membarrier_state racy load Peter Zijlstra
2019-09-03 20:36 ` Linus Torvalds
2019-09-04 15:19 ` Mathieu Desnoyers
2019-09-04 16:09 ` Peter Zijlstra
2019-09-04 17:12 ` Mathieu Desnoyers
2019-09-04 18:26 ` Peter Zijlstra
2019-09-06 0:51 ` Mathieu Desnoyers
2019-09-03 20:41 ` Mathieu Desnoyers
2019-09-04 11:28 ` Peter Zijlstra
2019-09-04 11:49 ` Peter Zijlstra
2019-09-04 15:26 ` Mathieu Desnoyers
2019-09-04 12:03 ` Oleg Nesterov
2019-09-04 12:43 ` Peter Zijlstra
2019-09-04 13:17 ` Oleg Nesterov
2019-09-03 20:27 ` Linus Torvalds
2019-09-03 20:53 ` Mathieu Desnoyers
2019-09-04 10:53 ` Oleg Nesterov
2019-09-04 11:39 ` Peter Zijlstra
2019-09-04 15:24 ` Mathieu Desnoyers
2019-09-04 11:11 ` Oleg Nesterov
2019-09-04 16:11 ` Mathieu Desnoyers [this message]
2019-09-08 13:46 ` Mathieu Desnoyers
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