linux-kernel - Re: frequent lockups in 3.18rc4

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Message-ID: <20141121121136.18df4ecb@gandalf.local.home>
Date:	Fri, 21 Nov 2014 12:11:36 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Tejun Heo <tj@...nel.org>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Dave Jones <davej@...hat.com>, Don Zickus <dzickus@...hat.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	the arch/x86 maintainers <x86@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Andy Lutomirski <luto@...capital.net>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>
Subject: Re: frequent lockups in 3.18rc4

On Fri, 21 Nov 2014 12:01:51 -0500
Steven Rostedt <rostedt@...dmis.org> wrote:
 
> Looking at kernel/trace/trace_functions.c: function_trace_call() which is
> registered with RECURSION_SAFE, I see that the recursion check is done
> before the per_cpu_ptr() call to the dynamically allocated per_cpu data.
> 
> It looks OK, but...
> 
> Oh! but if we trace the page fault handler, and we fault here too
> we just nuked the cr2 register. Not good.

Ah! Looking at the code, I see that do_page_fault (called from
assembly) is marked notrace. And the first thing it does is:

	unsigned long address = read_cr2();

And uses that. Thus if the function tracer were to fault on
exception_enter() or __do_page_fautt(), the address wont be
clobbered.

-- Steve
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