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Date: Fri, 22 Sep 2017 16:56:17 +0800
From: Boqun Feng <boqun.feng@gmail.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        "Paul E . McKenney" <paulmck@linux.vnet.ibm.com>,
        linux-kernel@vger.kernel.org, Andrew Hunter <ahh@google.com>,
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        Avi Kivity <avi@scylladb.com>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
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        Michael Ellerman <mpe@ellerman.id.au>,
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        Alan Stern <stern@rowland.harvard.edu>,
        Will Deacon <will.deacon@arm.com>, Andy Lutomirski <luto@kernel.org>,
        linux-arch@vger.kernel.org
Subject: Re: [RFC PATCH v3 1/2] membarrier: Provide register expedited
 private command
Message-ID: <20170922085617.GB19090@tardis>
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On Fri, Sep 22, 2017 at 10:24:41AM +0200, Peter Zijlstra wrote:
> On Fri, Sep 22, 2017 at 11:22:06AM +0800, Boqun Feng wrote:
>=20
> > The idea is in membarrier_private_expedited(), we go through all ->curr
> > on each CPU and=20
> >=20
> > 1)	If it's a userspace task and its ->mm is matched, we send an ipi
> >=20
> > 2)	If it's a kernel task, we skip
> >=20
> > 	(Because there will be a smp_mb() implied by mmdrop(), when it
> > 	switchs to userspace task).
> >=20
> > 3)	If it's a userspace task and its ->mm is not matched, we take
> > 	the corresponding rq->lock and check rq->curr again, if its ->mm
> > 	matched, we send an ipi, otherwise we do nothing.
> >=20
> > 	(Because if we observe rq->curr is not matched with rq->lock
> > 	held, when a task having matched ->mm schedules in, the rq->lock
> > 	pairing along with the smp_mb__after_spinlock() will guarantee
> > 	it observes all memory ops before sys_membarrir()).
>=20
> 3) is an insta DoS.

OK, fair enough.

Regards,
Boqun

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