From: Josh Poimboeuf <jpoimboe@redhat.com>
To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Steven Rostedt <rostedt@goodmis.org>,
Petr Mladek <pmladek@suse.com>, Jessica Yu <jeyu@redhat.com>,
Jiri Kosina <jikos@kernel.org>, Miroslav Benes <mbenes@suse.cz>,
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
Date: Mon, 8 May 2017 17:16:09 -0500 [thread overview]
Message-ID: <20170508221609.roaeaidj7mpfozcq@treble> (raw)
In-Reply-To: <20170508210754.GE3956@linux.vnet.ibm.com>
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
next prev parent reply other threads:[~2017-05-08 22:16 UTC|newest]
Thread overview: 31+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-05-04 10:55 [PATCH 0/3] livepatch/rcu: Handle some subtle issues between livepatching and RCU Petr Mladek
2017-05-04 10:55 ` [PATCH 1/3] livepatch/rcu: Guarantee consistency when patching idle kthreads Petr Mladek
2017-05-04 10:55 ` [PATCH 2/3] livepatch/rcu: Warn when system consistency is broken in RCU code Petr Mladek
2017-05-08 16:51 ` Josh Poimboeuf
2017-05-08 19:13 ` Steven Rostedt
2017-05-08 19:47 ` Josh Poimboeuf
2017-05-08 20:15 ` Paul E. McKenney
2017-05-08 20:43 ` Josh Poimboeuf
2017-05-08 20:51 ` Josh Poimboeuf
2017-05-08 21:08 ` Paul E. McKenney
2017-05-08 21:07 ` Paul E. McKenney
2017-05-08 21:18 ` Steven Rostedt
2017-05-08 21:30 ` Paul E. McKenney
2017-05-08 22:16 ` Josh Poimboeuf [this message]
2017-05-08 22:36 ` Paul E. McKenney
2017-05-09 16:18 ` Josh Poimboeuf
2017-05-09 16:36 ` Paul E. McKenney
2017-05-10 16:04 ` Petr Mladek
2017-05-10 16:45 ` Paul E. McKenney
2017-05-10 17:58 ` Josh Poimboeuf
2017-05-11 12:40 ` Miroslav Benes
2017-05-11 15:03 ` Josh Poimboeuf
2017-05-08 21:16 ` Steven Rostedt
2017-05-08 20:18 ` Steven Rostedt
2017-05-11 12:50 ` Miroslav Benes
2017-05-11 13:52 ` Petr Mladek
2017-05-11 14:50 ` Paul E. McKenney
2017-05-11 15:27 ` Josh Poimboeuf
2017-05-11 12:44 ` Petr Mladek
2017-05-04 10:55 ` [PATCH 3/3] livepatch/rcu: Disable livepatch removal when safety is not guaranteed Petr Mladek
2017-05-04 16:55 ` [PATCH 0/3] livepatch/rcu: Handle some subtle issues between livepatching and RCU Paul E. McKenney
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