linux-kernel - Re: [RFC PATCH v3 1/2] membarrier: Provide register expedited private command

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Date:   Fri, 22 Sep 2017 16:56:17 +0800
From:   Boqun Feng <boqun.feng@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        "Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
        linux-kernel@...r.kernel.org, Andrew Hunter <ahh@...gle.com>,
        Maged Michael <maged.michael@...il.com>, gromer@...gle.com,
        Avi Kivity <avi@...lladb.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Dave Watson <davejwatson@...com>,
        Alan Stern <stern@...land.harvard.edu>,
        Will Deacon <will.deacon@....com>,
        Andy Lutomirski <luto@...nel.org>, linux-arch@...r.kernel.org
Subject: Re: [RFC PATCH v3 1/2] membarrier: Provide register expedited
 private command

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:
> 
> > The idea is in membarrier_private_expedited(), we go through all ->curr
> > on each CPU and 
> > 
> > 1)	If it's a userspace task and its ->mm is matched, we send an ipi
> > 
> > 2)	If it's a kernel task, we skip
> > 
> > 	(Because there will be a smp_mb() implied by mmdrop(), when it
> > 	switchs to userspace task).
> > 
> > 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.
> > 
> > 	(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()).
> 
> 3) is an insta DoS.

OK, fair enough.

Regards,
Boqun

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