linux-kernel - Re: [PATCH] rcu,ftrace: Fix ftrace recursion

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Date:   Tue, 29 Sep 2020 09:56:40 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>, kim.phillips@....com
Subject: Re: [PATCH] rcu,ftrace: Fix ftrace recursion

On Tue, Sep 29, 2020 at 10:54:16AM -0400, Steven Rostedt wrote:
> On Tue, 29 Sep 2020 07:41:06 -0700
> "Paul E. McKenney" <paulmck@...nel.org> wrote:
> 
> > On Tue, Sep 29, 2020 at 10:36:20AM -0400, Steven Rostedt wrote:
> > > On Tue, 29 Sep 2020 13:33:40 +0200
> > > Peter Zijlstra <peterz@...radead.org> wrote:
> > >   
> > > > Kim reported that perf-ftrace made his box unhappy. It turns out that
> > > > commit:
> > > > 
> > > >   ff5c4f5cad33 ("rcu/tree: Mark the idle relevant functions noinstr")
> > > > 
> > > > removed one too many notrace. Probably due to there not being a helpful
> > > > comment.
> > > > 
> > > > Reinstate the notrace and add a comment to avoid loosing it again.
> > > > 
> > > > Fixes: ff5c4f5cad33 ("rcu/tree: Mark the idle relevant functions noinstr")
> > > > Reported-by: Kim Phillips <kim.phillips@....com>
> > > > Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> > > > ---
> > > >  kernel/rcu/tree.c | 5 ++++-
> > > >  1 file changed, 4 insertions(+), 1 deletion(-)
> > > > 
> > > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > > > index ee5e595501e8..33020d84ec6b 100644
> > > > --- a/kernel/rcu/tree.c
> > > > +++ b/kernel/rcu/tree.c
> > > > @@ -1098,8 +1098,11 @@ noinstr bool __rcu_is_watching(void)
> > > >   * CPU can safely enter RCU read-side critical sections.  In other words,
> > > >   * if the current CPU is not in its idle loop or is in an interrupt or
> > > >   * NMI handler, return true.
> > > > + *
> > > > + * Must be notrace because __ftrace_ops_list_func() / ftrace_ops_assist_func()
> > > > + * will call this (for every function) outside of recursion protection.
> > > >   */
> > > > -bool rcu_is_watching(void)
> > > > +notrace bool rcu_is_watching(void)
> > > >  {
> > > >  	bool ret;
> > > >    
> > > 
> > > I think the patch I suggested is more suitable.  
> > 
> > OK, I will let you guys fight it out.  ;-)
> > 
> 
> Well, I think we should actually apply both, but the comment needs to be
> updated, as it will no longer be outside recursion. And the comment is
> wrong now as well, as its only outside recursion protection for the
> assist_func(). 
> 
> But it does prevent it from being always called for perf.
> 
>  * Make notrace because it can be called by the internal functions of
>  * ftrace, and making this notrace removes unnecessary recursion calls.

Fair enough.  ;-)

If I don't hear otherwise by late today (Tuesday), Pacific Time, I will
update the comment and pull it into -rcu.  If you guys have some other
route to mainline in mind, you have my Reviewed-by.  Either way, just
let me know.

							Thanx, Paul