From: "Paul E. McKenney" <paulmck@kernel.org>
To: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Steven Rostedt <rostedt@goodmis.org>,
Joel Fernandes <joel@joelfernandes.org>,
Peter Zijlstra <peterz@infradead.org>,
linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org,
mingo@kernel.org, gregkh@linuxfoundation.org,
gustavo@embeddedor.com, tglx@linutronix.de,
josh@joshtriplett.org, mathieu.desnoyers@efficios.com,
jiangshanlai@gmail.com
Subject: Re: [PATCH v2 3/9] rcu,tracing: Create trace_rcu_{enter,exit}()
Date: Mon, 17 Feb 2020 08:31:12 -0800 [thread overview]
Message-ID: <20200217163112.GM2935@paulmck-ThinkPad-P72> (raw)
In-Reply-To: <20200217175519.12a694a969c1a8fb2e49905e@kernel.org>
On Mon, Feb 17, 2020 at 05:55:19PM +0900, Masami Hiramatsu wrote:
> On Sat, 15 Feb 2020 06:59:34 -0800
> "Paul E. McKenney" <paulmck@kernel.org> wrote:
>
> > On Fri, Feb 14, 2020 at 03:19:06PM +0900, Masami Hiramatsu wrote:
> > > On Thu, 13 Feb 2020 14:39:18 -0800
> > > "Paul E. McKenney" <paulmck@kernel.org> wrote:
> > >
> > > > On Thu, Feb 13, 2020 at 05:04:51PM -0500, Steven Rostedt wrote:
> > > > > On Thu, 13 Feb 2020 13:50:04 -0800
> > > > > "Paul E. McKenney" <paulmck@kernel.org> wrote:
> > > > >
> > > > > > On Thu, Feb 13, 2020 at 04:38:25PM -0500, Steven Rostedt wrote:
> > > > > > > [ Added Masami ]
> > > > > > >
> > > > > > > On Thu, 13 Feb 2020 16:19:30 -0500
> > > > > > > Joel Fernandes <joel@joelfernandes.org> wrote:
> > > > > > >
> > > > > > > > On Thu, Feb 13, 2020 at 12:54:42PM -0800, Paul E. McKenney wrote:
> > > > > > > > > On Thu, Feb 13, 2020 at 03:44:44PM -0500, Joel Fernandes wrote:
> > > > > > > > > > On Thu, Feb 13, 2020 at 10:56:12AM -0800, Paul E. McKenney wrote:
> > > > > > > > > > [...]
> > > > > > > > > > > > > It might well be that I could make these functions be NMI-safe, but
> > > > > > > > > > > > > rcu_prepare_for_idle() in particular would be a bit ugly at best.
> > > > > > > > > > > > > So, before looking into that, I have a question. Given these proposed
> > > > > > > > > > > > > changes, will rcu_nmi_exit_common() and rcu_nmi_enter_common() be able
> > > > > > > > > > > > > to just use in_nmi()?
> > > > > > > > > > > >
> > > > > > > > > > > > That _should_ already be the case today. That is, if we end up in a
> > > > > > > > > > > > tracer and in_nmi() is unreliable we're already screwed anyway.
> > > > > > > > > > >
> > > > > > > > > > > So something like this, then? This is untested, probably doesn't even
> > > > > > > > > > > build, and could use some careful review from both Peter and Steve,
> > > > > > > > > > > at least. As in the below is the second version of the patch, the first
> > > > > > > > > > > having been missing a couple of important "!" characters.
> > > > > > > > > >
> > > > > > > > > > I removed the static from rcu_nmi_enter()/exit() as it is called from
> > > > > > > > > > outside, that makes it build now. Updated below is Paul's diff. I also added
> > > > > > > > > > NOKPROBE_SYMBOL() to rcu_nmi_exit() to match rcu_nmi_enter() since it seemed
> > > > > > > > > > asymmetric.
> > > > > > > > >
> > > > > > > > > My compiler complained about the static and the __always_inline, so I
> > > > > > > > > fixed those. But please help me out on adding the NOKPROBE_SYMBOL()
> > > > > > > > > to rcu_nmi_exit(). What bad thing happens if we leave this on only
> > > > > > > > > rcu_nmi_enter()?
> > > > > > > >
> > > > > > > > It seemed odd to me we were not allowing kprobe on the rcu_nmi_enter() but
> > > > > > > > allowing it on exit (from a code reading standpoint) so my reaction was to
> > > > > > > > add it to both, but we could probably keep that as a separate
> > > > > > > > patch/discussion since it is slightly unrelated to the patch.. Sorry to
> > > > > > > > confuse the topic.
> > > > > > > >
> > > > > > >
> > > > > > > rcu_nmi_enter() was marked NOKPROBE or other reasons. See commit
> > > > > > > c13324a505c77 ("x86/kprobes: Prohibit probing on functions before
> > > > > > > kprobe_int3_handler()")
> > > > > > >
> > > > > > > The issue was that we must not allow anything in do_int3() call kprobe
> > > > > > > code before kprobe_int3_handler() is called. Because ist_enter() (in
> > > > > > > do_int3()) calls rcu_nmi_enter() it had to be marked NOKPROBE. It had
> > > > > > > nothing to do with it being RCU nor NMI, but because it was simply
> > > > > > > called in do_int3().
> > > > > > >
> > > > > > > Thus, there's no reason to make rcu_nmi_exit() NOKPROBE. But a commont
> > > > > > > to why rcu_nmi_enter() would probably be useful, like below:
> > > > > >
> > > > > > Thank you, Steve! Could I please have your Signed-off-by for this?
> > > > >
> > > > > Sure, but it was untested ;-)
> > > >
> > > > No problem! I will fire up rcutorture on it. ;-)
> > > >
> > > > But experience indicates that you cannot even make a joke around here.
> > > > There is probably already someone out there somewhere building a
> > > > comment-checker based on deep semantic analysis and machine learning. :-/
> > > >
> > > > > Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
> > > > >
> > > > > I'd like a Reviewed-by from Masami though.
> > > >
> > > > Sounds good! Masami, would you be willing to review?
> > >
> > > Yes, the functions before calling kprobe_int3_handler() must not
> > > be kprobed. It can cause an infinite recursive int3 trapping.
> > >
> > > Reviewed-by: Masami Hiramatsu <mhiramat@kernel.org>
> >
> > Thank you both!
> >
> > Like this?
> >
> > Thanx, Paul
> >
>
> This is good to me.
Thank you for looking it over! (I already have your
> BTW, if you consider the x86 specific code is in the generic file,
> we can move NOKPROBE_SYMBOL() in arch/x86/kernel/traps.c.
> (Sorry, I've hit this idea right now)
Might this affect other architectures with NMIs and probe-like things?
If so, it might make sense to leave it where it is.
Thanx, Paul
> Thank you,
>
> > ------------------------------------------------------------------------
> >
> > commit 1817fdc8f4e4bd18c76305c9b937fb0dccbb1583
> > Author: Steven Rostedt <rostedt@goodmis.org>
> > Date: Sat Feb 15 06:54:50 2020 -0800
> >
> > rcu: Provide comment for NOKPROBE() on rcu_nmi_enter()
> >
> > The rcu_nmi_enter() function was marked NOKPROBE() by commit
> > c13324a505c77 ("x86/kprobes: Prohibit probing on functions before
> > kprobe_int3_handler()") because the do_int3() call kprobe code must
> > not be invoked before kprobe_int3_handler() is called. It turns out
> > that ist_enter() (in do_int3()) calls rcu_nmi_enter(), hence the
> > marking NOKPROBE() being added to rcu_nmi_enter().
> >
> > This commit therefore adds a comment documenting this line of reasoning.
> >
> > Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
> > Reviewed-by: Masami Hiramatsu <mhiramat@kernel.org>
> > Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> >
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index 132b53e..4a885af 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -835,6 +835,12 @@ void rcu_nmi_enter(void)
> > rdp->dynticks_nmi_nesting + incby);
> > barrier();
> > }
> > +/*
> > + * On x86, All functions in do_int3() must be marked NOKPROBE before
> > + * kprobe_int3_handler() is called. ist_enter() which is called in do_int3()
> > + * before kprobe_int3_handle() happens to call rcu_nmi_enter() which means
> > + * that rcu_nmi_enter() must be marked NOKRPOBE.
> > + */
> > NOKPROBE_SYMBOL(rcu_nmi_enter);
> >
> > /**
>
>
> --
> Masami Hiramatsu <mhiramat@kernel.org>
next prev parent reply other threads:[~2020-02-17 16:31 UTC|newest]
Thread overview: 63+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-02-12 21:01 [PATCH v2 0/9] tracing vs rcu vs nmi Peter Zijlstra
2020-02-12 21:01 ` [PATCH v2 1/9] rcu: Rename rcu_irq_{enter,exit}_irqson() Peter Zijlstra
2020-02-12 22:38 ` Joel Fernandes
2020-02-12 21:01 ` [PATCH v2 2/9] rcu: Mark rcu_dynticks_curr_cpu_in_eqs() inline Peter Zijlstra
2020-02-12 22:38 ` Joel Fernandes
2020-02-13 1:41 ` Steven Rostedt
2020-02-13 14:25 ` Joel Fernandes
2020-02-12 21:01 ` [PATCH v2 3/9] rcu,tracing: Create trace_rcu_{enter,exit}() Peter Zijlstra
2020-02-12 23:20 ` Joel Fernandes
2020-02-13 8:27 ` Peter Zijlstra
2020-02-13 13:31 ` Joel Fernandes
2020-02-13 13:51 ` Paul E. McKenney
2020-02-13 16:40 ` Peter Zijlstra
2020-02-13 18:56 ` Paul E. McKenney
2020-02-13 20:44 ` Joel Fernandes
2020-02-13 20:54 ` Paul E. McKenney
2020-02-13 21:19 ` Joel Fernandes
2020-02-13 21:38 ` Steven Rostedt
2020-02-13 21:50 ` Paul E. McKenney
2020-02-13 22:04 ` Steven Rostedt
2020-02-13 22:39 ` Paul E. McKenney
2020-02-14 6:19 ` Masami Hiramatsu
2020-02-15 14:59 ` Paul E. McKenney
2020-02-17 8:55 ` Masami Hiramatsu
2020-02-17 16:31 ` Paul E. McKenney [this message]
2020-02-18 4:33 ` Masami Hiramatsu
2020-02-18 16:12 ` Paul E. McKenney
2020-02-18 16:15 ` Mathieu Desnoyers
2020-02-18 16:35 ` Steven Rostedt
2020-02-18 17:46 ` Steven Rostedt
2020-02-18 20:18 ` Paul E. McKenney
2020-02-19 2:45 ` Masami Hiramatsu
2020-03-06 18:01 ` Masami Hiramatsu
2020-03-06 18:47 ` Joel Fernandes
2020-03-06 19:11 ` Joel Fernandes
2020-03-07 1:58 ` Masami Hiramatsu
2020-03-06 0:42 ` Thomas Gleixner
2020-02-13 21:48 ` Paul E. McKenney
2020-02-13 22:58 ` Joel Fernandes
2020-02-13 23:55 ` Steven Rostedt
2020-02-18 19:58 ` Peter Zijlstra
2020-02-18 20:17 ` Paul E. McKenney
2020-02-18 20:40 ` Peter Zijlstra
2020-02-18 21:39 ` Paul E. McKenney
2020-02-19 9:57 ` Peter Zijlstra
2020-02-19 12:46 ` Paul E. McKenney
2020-02-12 23:27 ` Joel Fernandes
2020-02-13 8:28 ` Peter Zijlstra
2020-02-13 18:39 ` Joel Fernandes
2020-02-12 21:01 ` [PATCH v2 4/9] sched,rcu,tracing: Avoid tracing before in_nmi() is correct Peter Zijlstra
2020-02-12 21:01 ` [PATCH v2 5/9] x86,tracing: Add comments to do_nmi() Peter Zijlstra
2020-02-12 21:01 ` [PATCH v2 6/9] perf,tracing: Prepare the perf-trace interface for RCU changes Peter Zijlstra
2020-02-12 23:28 ` Joel Fernandes
2020-02-13 8:29 ` Peter Zijlstra
2020-02-13 18:38 ` Joel Fernandes
2020-02-12 21:01 ` [PATCH v2 7/9] tracing: Employ trace_rcu_{enter,exit}() Peter Zijlstra
2020-02-12 21:01 ` [PATCH v2 8/9] tracing: Remove regular RCU context for _rcuidle tracepoints (again) Peter Zijlstra
2020-02-12 21:01 ` [PATCH v2 9/9] perf,tracing: Allow function tracing when !RCU Peter Zijlstra
2020-02-14 2:28 ` Sergey Senozhatsky
2020-02-14 2:42 ` Sergey Senozhatsky
2020-02-14 3:32 ` Steven Rostedt
2020-02-14 20:38 ` Kim Phillips
2020-02-14 22:48 ` Steven Rostedt
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