From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S1751858AbdIVIZD (ORCPT <rfc822;w@1wt.eu>);
        Fri, 22 Sep 2017 04:25:03 -0400
Received: from bombadil.infradead.org ([65.50.211.133]:46743 "EHLO
        bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
        with ESMTP id S1751704AbdIVIZA (ORCPT
        <rfc822;linux-kernel@vger.kernel.org>);
        Fri, 22 Sep 2017 04:25:00 -0400
Date: Fri, 22 Sep 2017 10:24:41 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Boqun Feng <boqun.feng@gmail.com>
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>,
        Maged Michael <maged.michael@gmail.com>, gromer@google.com,
        Avi Kivity <avi@scylladb.com>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Dave Watson <davejwatson@fb.com>,
        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: <20170922082441.rxavx645ewoizuu2@hirez.programming.kicks-ass.net>
References: <20170919221342.29915-1-mathieu.desnoyers@efficios.com>
 <20170922032206.GE10893@tardis>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20170922032206.GE10893@tardis>
User-Agent: NeoMutt/20170609 (1.8.3)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

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.