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Date:   Fri, 12 May 2017 11:13:57 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Masami Hiramatsu <mhiramat@...nel.org>
Subject: Re: [RFC][PATCH 0/5] perf/tracing/cpuhotplug: Fix locking order

On Fri, May 12, 2017 at 01:15:44PM -0400, Steven Rostedt wrote:
> NOTE: This was quickly written. The change logs and patches probably need
> some loving. This is for discussion. These may become legitimate patches,
> but for now, I'm seeing if this is an acceptable solution.
> 
> Also note, I checked out the last branch that I had Linus pull, and then
> merged with tip's commit b53e5129~1, which is the commit just before
> "trace/perf: Cure hotplug lock ordering issues" in tip/smp/hotplug
> that placed get_online_cpus() ahead of event_mutex. The issue is that
> event_mutex is a high level mutex, and trying to make sure that
> get_online_cpus() is held whenever it is held and is going to be a
> nightmare. So I started ahead of that commit, but I needed changes
> in my own repo to do full testing. I renamed this temp branch to
> "tip/cpuhotplug", and you can pull it as well. But this is not going
> to be a candidate for pushing to Linus.
> 
> The 5 patches here are:
> 
>  1) fix a lockdep splat when taking stack trace in irqsoff tracing
>     (one of my tests triggered this)
> 
>  2) Allow for get_online_cpus() to nest
> 
>  3) Have kprobes take get_online_cpus() before taking jump label lock.
> 
>  4) Have tracepoints always take get_online_cpus
> 
>  5) Have perf take event_mutex before taking get_online_cpus()
> 
> Thoughts?

First a thought on the use of get_online_cpus()...

Your mileage may vary, but from what I have seen, life is very good
if your piece of the kernel uses get_online_cpus(), but refrains from
using the CPU-hotplug notifiers.  Life is also very good if your piece
of the kernel uses CPU-hotplug notifiers, but refrains from using
get_online_cpus().  If your code uses both get_online_cpus() -and-
CPU-hotplug notifiers for the same data structures, life can be quite
complex and difficult.

For RCU, I cannot avoid using notifiers, so I have been removing uses
of get_online_cpus().  Removing each of these has made large numbers of
race conditions with CPU hotplug simply disappear -- not much reduction
in lines of code, but big reductions in complexity and in state space.
I have one more to go in _rcu_barrier().

Again, your mileage may vary.

							Thanx, Paul

> -- Steve
> 
> 
>   git://git.kernel.org/pub/scm/linux/kernel/git/rostedt/linux-trace.git
> tip/cpuhotplug
> 
> Head SHA1: ddb06e08fa10ed5030285267a9b1e25d40c337c8
> 
> 
> Steven Rostedt (VMware) (5):
>       tracing: Make sure RCU is watching before calling a stack trace
>       cpu-hotplug: Allow get_online_cpus() to nest
>       kprobes: Take get_online_cpus() before taking jump_label_lock()
>       tracepoints: Grab get_online_cpus() before taking tracepoints_mutex
>       perf: Grab event_mutex before taking get_online_cpus()
> 
> ----
>  include/linux/sched.h           | 1 +
>  include/linux/trace_events.h    | 2 ++
>  kernel/cpu.c                    | 9 +++++++++
>  kernel/events/core.c            | 9 +++++++++
>  kernel/fork.c                   | 1 +
>  kernel/kprobes.c                | 6 +++---
>  kernel/trace/trace.c            | 5 +++++
>  kernel/trace/trace.h            | 1 -
>  kernel/trace/trace_event_perf.c | 8 ++++----
>  kernel/tracepoint.c             | 5 +++++
>  10 files changed, 39 insertions(+), 8 deletions(-)
> 

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