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Date: Fri, 21 Nov 2014 12:11:36 -0500
From: Steven Rostedt <rostedt@goodmis.org>
To: Tejun Heo <tj@kernel.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Dave Jones <davej@redhat.com>, Don Zickus <dzickus@redhat.com>,
        Linux Kernel <linux-kernel@vger.kernel.org>,
        the arch/x86 maintainers <x86@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Andy Lutomirski <luto@amacapital.net>,
        Arnaldo Carvalho de Melo <acme@ghostprotocols.net>
Subject: Re: frequent lockups in 3.18rc4
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In-Reply-To: <20141121170151.GC30603@home.goodmis.org>
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On Fri, 21 Nov 2014 12:01:51 -0500
Steven Rostedt <rostedt@goodmis.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