From mboxrd@z Thu Jan  1 00:00:00 1970
From: Andrew Cooper <andrew.cooper3@citrix.com>
Subject: Re: [PATCH 2/7] xen: sched: fix locking for
 insert_vcpu() in credit1 and RTDS
Date: Thu, 8 Oct 2015 14:18:10 +0100
Message-ID: <56166D12.4040809@citrix.com>
References: <20151008124027.12522.42552.stgit@Solace.station>
	<20151008125243.12522.8999.stgit@Solace.station>
Mime-Version: 1.0
Content-Type: text/plain; charset="us-ascii"
Content-Transfer-Encoding: 7bit
Return-path: <xen-devel-bounces@lists.xen.org>
Received: from mail6.bemta5.messagelabs.com ([195.245.231.135])
	by lists.xen.org with esmtp (Exim 4.72)
	(envelope-from <prvs=71687dc37=Andrew.Cooper3@citrix.com>)
	id 1ZkB5O-0004bV-Da
	for xen-devel@lists.xenproject.org; Thu, 08 Oct 2015 13:18:18 +0000
In-Reply-To: <20151008125243.12522.8999.stgit@Solace.station>
List-Unsubscribe: <http://lists.xen.org/cgi-bin/mailman/options/xen-devel>,
	<mailto:xen-devel-request@lists.xen.org?subject=unsubscribe>
List-Post: <mailto:xen-devel@lists.xen.org>
List-Help: <mailto:xen-devel-request@lists.xen.org?subject=help>
List-Subscribe: <http://lists.xen.org/cgi-bin/mailman/listinfo/xen-devel>,
	<mailto:xen-devel-request@lists.xen.org?subject=subscribe>
Sender: xen-devel-bounces@lists.xen.org
Errors-To: xen-devel-bounces@lists.xen.org
To: Dario Faggioli <dario.faggioli@citrix.com>, xen-devel@lists.xenproject.org
Cc: George Dunlap <george.dunlap@eu.citrix.com>, Meng Xu <mengxu@cis.upenn.edu>, Jan Beulich <JBeulich@suse.com>
List-Id: xen-devel@lists.xenproject.org

On 08/10/15 13:52, Dario Faggioli wrote:
> The insert_vcpu scheduler hook is called with an inconsistent
> locking strategy. In fact, it is sometimes invoked while
> holding the runqueue lock and sometimes when that is not the
> case.
>
> For instance, in case of schedule_cpu_switch() the lock is
> acquired in generic code. On the other hand, in case of
> sched_move_domain(), locking is left as a responsibility
> of the schedulers implementing the hook.
>
> This results in Credit1 and RTDS schedulers ending up (in
> case of sched_move_domain()) doing runqueue manipulation
> without holding any runqueue lock, which is a bug. (Credit2
> was doing the locking by itself already.)
>
> The right thing is to defer locking to the specific schedulers,
> as it's them that know what, how and when it is best to lock
> (as in: runqueue locks, vs. private scheduler locks, vs. both,
> etc.).
>
> This patch, therefore:
>  - removes any locking around insert_vcpu() from generic
>    code;
>  - add proper locking in the hook implementations, for
>    both Credit1 and RTDS.
>
> Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>

Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>