linux-kernel - Re: [patch 06/41] x86/traps: Simplify pagefault tracing logic

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Date:   Fri, 25 Aug 2017 17:11:57 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Steven Rostedt <rostedt@...dmis.org>
cc:     LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>, Peter Anvin <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...en8.de>
Subject: Re: [patch 06/41] x86/traps: Simplify pagefault tracing logic

On Fri, 25 Aug 2017, Steven Rostedt wrote:

> On Fri, 25 Aug 2017 12:31:09 +0200
> Thomas Gleixner <tglx@...utronix.de> wrote:
> 
> >  dotraplinkage void notrace
> > -trace_do_page_fault(struct pt_regs *regs, unsigned long error_code)
> > +do_page_fault(struct pt_regs *regs, unsigned long error_code)
> >  {
> > +	unsigned long address = read_cr2(); /* Get the faulting address */
> > +	enum ctx_state prev_state;
> > +
> >  	/*
> > -	 * The exception_enter and tracepoint processing could
> > -	 * trigger another page faults (user space callchain
> > -	 * reading) and destroy the original cr2 value, so read
> > -	 * the faulting address now.
> > +	 * We must have this function tagged with __kprobes, notrace and call
> > +	 * read_cr2() before calling anything else. To avoid calling any kind
> > +	 * of tracing machinery before we've observed the CR2 value.
> > +	 *
> > +	 * exception_{enter,exit}() contain all sorts of tracepoints.
> >  	 */
> > -	unsigned long address = read_cr2();
> > -	enum ctx_state prev_state;
> > +	if (trace_irqvectors_enabled())
> > +		trace_page_fault_entries(address, regs, error_code);
> >  
> >  	prev_state = exception_enter();
> > -	trace_page_fault_entries(address, regs, error_code);
> 
> I believe you need to keep the tracing after the "exception_enter()", as
> for NO_HZ_FULL, that enables RCU again, and the tracepoint needs RCU
> enabled.

Indeed. Thanks for spotting it.