linux-kernel - Re: [for-next][PATCH 4/4] ftrace: Add comment to why rcu_dereference

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Message-ID: <20200205104945.4a6f85de@oasis.local.home>
Date:   Wed, 5 Feb 2020 10:49:45 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     "Paul E. McKenney" <paulmck@...ux.ibm.com>,
        linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Amol Grover <frextrite@...il.com>
Subject: Re: [for-next][PATCH 4/4] ftrace: Add comment to why
 rcu_dereference_sched() is open coded

On Wed, 5 Feb 2020 10:42:12 -0500
Joel Fernandes <joel@...lfernandes.org> wrote:

> On Wed, Feb 05, 2020 at 09:28:47AM -0500, Steven Rostedt wrote:
> > On Wed, 5 Feb 2020 09:19:15 -0500
> > Joel Fernandes <joel@...lfernandes.org> wrote:
> >   
> > > Could you paste the stack here when RCU is not watching? In trace event code
> > > IIRC we call rcu_enter_irqs_on() to have RCU temporarily watch, since that
> > > code can be called from idle loop. Should we doing the same here as well?  
> > 
> > Unfortunately I lost the stack trace. And the last time we tried to use
> > rcu_enter_irqs_on() for ftrace, we couldn't find a way to do this
> > properly. Ftrace is much more invasive then going into idle. The
> > problem is that ftrace traces RCU itself, and calling
> > "rcu_enter_irqs_on()" in pretty much any place in the RCU code caused
> > lots of bugs ;-)
> > 
> > This is why we have the schedule_on_each_cpu(ftrace_sync) hack.  
> 
> The "schedule a task on each CPU" trick works on !PREEMPT though right?

It works on both, as I care more about the PREEMPT=y case then
the !PREEMPT, and the PREEMPT_RT which is even more preemptive than
PREEMPT!

> 
> Because it is possible in PREEMPT=y to get preempted in the middle of a
> read-side critical section, switch to the worker thread executing the
> ftrace_sync() and then switch back. But RCU still has to watch that CPU since
> the read-side critical section was not completed.
> 
> Or is there a subtlety here with ftrace that I missed?
> 

Hence Amol's patch:

> +       notrace_hash = rcu_dereference_protected(ftrace_graph_notrace_hash,
> +                                                !preemptible());

It checks to make sure preemption is off. There is no chance of being
preempted in the read side critical section.

-- Steve