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Date:   Wed, 4 Sep 2019 13:11:27 +0200
From:   Oleg Nesterov <oleg@redhat.com>
To:     Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc:     "Paul E. McKenney" <paulmck@linux.ibm.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
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        "Eric W. Biederman" <ebiederm@xmission.com>,
        Russell King - ARM Linux admin <linux@armlinux.org.uk>,
        Chris Metcalf <cmetcalf@ezchip.com>,
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        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
Message-ID: <20190904111126.GB24568@redhat.com>
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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.


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?

Oleg.