linux-kernel - Re: [PATCH v2] tracing: Make sure RCU is watching before calling a stack trace

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Message-ID: <alpine.LSU.2.20.1705182016400.17317@pobox.suse.cz>
Date:   Thu, 18 May 2017 20:18:05 +0200 (CEST)
From:   Miroslav Benes <mbenes@...e.cz>
To:     Steven Rostedt <rostedt@...dmis.org>
cc:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v2] tracing: Make sure RCU is watching before calling a
 stack trace

On Thu, 18 May 2017, Steven Rostedt wrote:

> On Thu, 18 May 2017 15:48:55 +0200 (CEST)
> Miroslav Benes <mbenes@...e.cz> wrote:
> 
> > On Thu, 18 May 2017, Steven Rostedt wrote:
> > 
> > > 
> > > From: "Steven Rostedt (VMware)" <rostedt@...dmis.org>
> > > 
> > > As stack tracing now requires "rcu watching", force RCU to be watching when
> > > recording a stack trace.
> > > 
> > > Link: http://lkml.kernel.org/r/20170512172449.879684501@goodmis.org
> > > 
> > > Cc: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
> > > Signed-off-by: Steven Rostedt (VMware) <rostedt@...dmis.org>
> > > ---
> > > 
> > > Changes since v1:
> > > 
> > >    My testing discovered that the stack trace can be called with
> > >    interrupts enabled, which is a no no to have when calling
> > >    rcu_irq_enter(). When interrupts are enabled, as with being in an
> > >    NMI, RCU will also be watching.  
> > 
> > Would rcu_irq_enter_irqson() help then? This is what Petr used in a live 
> > patching handler.
> > 
> 
> Yes, that could work too, but I wanted to avoid disabling interrupts if
> we didn't have to.

Ok, that makes sense.

> > Your solution works too, of course. Just asking if I am not missing 
> > something.
> >
> 
> Nope, I was just trying to keep the overhead down. As this can be
> called by every event enabled, as well as functions being traced. I
> figured that local_save_irqs() is faster than a pair of
> local_irq_save()/ local_irq_restore() calls.

(noticed Paul's reply)... yeah, it'd great. Damn, this is mindblowing.

Thanks,
Miroslav