From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751317AbdEHWQN (ORCPT ); Mon, 8 May 2017 18:16:13 -0400 Received: from mx1.redhat.com ([209.132.183.28]:35332 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750835AbdEHWQL (ORCPT ); Mon, 8 May 2017 18:16:11 -0400 DMARC-Filter: OpenDMARC Filter v1.3.2 mx1.redhat.com D64BE80C0A Authentication-Results: ext-mx02.extmail.prod.ext.phx2.redhat.com; dmarc=none (p=none dis=none) header.from=redhat.com Authentication-Results: ext-mx02.extmail.prod.ext.phx2.redhat.com; spf=pass smtp.mailfrom=jpoimboe@redhat.com DKIM-Filter: OpenDKIM Filter v2.11.0 mx1.redhat.com D64BE80C0A Date: Mon, 8 May 2017 17:16:09 -0500 From: Josh Poimboeuf To: "Paul E. McKenney" Cc: Steven Rostedt , Petr Mladek , Jessica Yu , Jiri Kosina , Miroslav Benes , live-patching@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 2/3] livepatch/rcu: Warn when system consistency is broken in RCU code Message-ID: <20170508221609.roaeaidj7mpfozcq@treble> References: <1493895316-19165-1-git-send-email-pmladek@suse.com> <1493895316-19165-3-git-send-email-pmladek@suse.com> <20170508165108.d3vd4h6ffa25bfui@treble> <20170508151322.76e8e9db@gandalf.local.home> <20170508194729.jjq7qrc7gkiq2s5v@treble> <20170508201558.GD3956@linux.vnet.ibm.com> <20170508204333.xc3isvr4riv26his@treble> <20170508210754.GE3956@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20170508210754.GE3956@linux.vnet.ibm.com> User-Agent: Mutt/1.6.0.1 (2016-04-01) X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.26]); Mon, 08 May 2017 22:16:11 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, May 08, 2017 at 02:07:54PM -0700, Paul E. McKenney wrote: > On Mon, May 08, 2017 at 03:43:33PM -0500, Josh Poimboeuf wrote: > > On Mon, May 08, 2017 at 01:15:58PM -0700, Paul E. McKenney wrote: > > > On Mon, May 08, 2017 at 02:47:29PM -0500, Josh Poimboeuf wrote: > > > > On Mon, May 08, 2017 at 03:13:22PM -0400, Steven Rostedt wrote: > > > > > > [ . . . ] > > > > > > > > If rcu is not watching, calling rcu_enter_irq() will have it watch > > > > > again. Even in NMI context I believe. > > > > > > > > What if you get an NMI while running in rcu_dynticks_eqs_enter() before > > > > it increments rdtp->dynticks? Will rcu_enter_irq() still work from the > > > rcu_irq_enter() > > > > NMI? > > > > > > The rcu_nmi_enter() function willl notice that RCU is not watching, and > > > will therefore atomically increment RCU's dynticks-idle counter, which > > > will be atomically incremented again upon return. Since the bottom bit > > > of this counter controls whether or not RCU is watching, RCU will be > > > watching during the NMI, will stop watching upon return from the NMI, > > > which restores state so as to allow rcu_irq_enter() to cause RCU to once > > > again watch. (NMI algorithm due to Andy Lutomirski.) > > > > > > > I'm just trying to understand what are the cases where rcu_enter_irq() > > > > *doesn't* work from an ftrace handler. > > > > > > It doesn't work from an NMI handler. Aside from possible architecture > > > specific special cases, it should work everywhere else. > > > > Ok, so just to clarify. Is there a bug in the ftrace stack tracer in > > the following situation? > > > > 1. RCU isn't watching > > 2. An NMI hits > > 3. ist_enter() calls into the ftrace stack tracer, before > > rcu_nmi_enter() is called, so RCU isn't watching yet > > 4. The ftrace stack tracer calls rcu_irq_enter(), which has no effect, > > so RCU still isn't watching > > 5. Hilarity ensues in the ftrace stack tracer > > This would be a problem if step 2's NMI hit rcu_irq_enter(), > rcu_irq_exit(), and friends in just the wrong place. > > I would suggest that ftrace() do something like this... > > if (in_nmi()) > rcu_nmi_enter(); > else > rcu_irq_enter(); > > Except that, as Steven will quickly point out, this won't work at the > very edges of the NMI, when NMI_MASK won't be set in preempt_count(). > > Other thoughts? Ok. So I think the livepatch ftrace handler would need the in_nmi() check, in case it's called early in the NMI. But on x86, rcu_nmi_enter() is also called in some non-NMI exception cases, from ist_enter(). So it appears that the in_nmi() check wouldn't be sufficient. We might instead need something like: if (in_nmi() || in_some_other_exception()) rcu_nmi_enter(); else rcu_irq_enter(); But unfortunately the in_some_other_exception() function doesn't currently exist. So, one more question. Would it work if we just always called rcu_nmi_enter()? -- Josh