linux-kernel - Re: [PATCH] clocksource/drivers/ti-32k: Prevent ftrace recursion

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Message-ID: <20160922224514.696ae61b@gandalf.local.home>
Date:   Thu, 22 Sep 2016 22:45:14 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Jisheng Zhang <jszhang@...vell.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>, <daniel.lezcano@...aro.org>,
        LKML <linux-kernel@...r.kernel.org>,
        LAK <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] clocksource/drivers/ti-32k: Prevent ftrace recursion

On Fri, 23 Sep 2016 10:04:31 +0800
Jisheng Zhang <jszhang@...vell.com> wrote:

> Hi Thomas,
> 
> On Thu, 22 Sep 2016 15:58:03 +0200 Thomas Gleixner wrote:
> 
> > On Thu, 22 Sep 2016, Jisheng Zhang wrote:
> >   
> > > Currently ti-32k can be used as a scheduler clock. We properly marked
> > > omap_32k_read_sched_clock() as notrace but we then call another
> > > function ti_32k_read_cycles() that _wasn't_ notrace.
> > > 
> > > Having a traceable function in the sched_clock() path leads to a
> > > recursion within ftrace and a kernel crash.    
> > 
> > Kernel crash? Doesn't ftrace core prevent recursion?  
> 
> a recent similar issue:
> 
> http://www.spinics.net/lists/arm-kernel/msg533480.html

Right. But Thomas brought up recursion detection. And I said that would
be the fix, but now thinking about it, I've updated the recursion
protection so that timer issues should not cause a crash.

I'd like to know more, as this appears to be mostly arm related.

-- Steve