linux-kernel - Re: BUG - function tracing with breakpoints (was: Re: tracing ring_buffer

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Date:	Fri, 25 May 2012 11:29:06 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	"H. Peter Anvin" <hpa@...ux.intel.com>
Cc:	Dave Jones <davej@...hat.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...hat.com>, Andi Kleen <ak@...ux.intel.com>
Subject: Re: BUG - function tracing with breakpoints (was: Re: tracing
 ring_buffer_resize oops.)

On Fri, 2012-05-25 at 10:31 -0400, Steven Rostedt wrote:

> Looks like we set RSP to code. Again pointing to a corrupted iretq.
> Maybe we are having nested debug stack usage, where we are hitting a
> breakpoint before setting the idt to not change the stack?

Another clue. If I do not trace the following functions:

func_ptr_is_kernel_text
kprobe_exceptions_notify
hw_breakpoint_exceptions_notify
notifier_call_chain*

it works fine.

# echo func_ptr_is_kernel_text kprobe_exceptions_notify \
 hw_breakpoint_exceptions_notify notifier_call_chain* > set_ftrace_notrace
# echo function > current_tracer

works!

These notifiers are being called by the breakpoint. So perhaps the
breakpoint is still being called by int3 when it shouldn't be. It
shouldn't because we have:


dotraplinkage void __kprobes notrace do_int3(struct pt_regs *regs, long error_code)
{
#ifdef CONFIG_DYNAMIC_FTRACE
	/* ftrace must be first, everything else may cause a recursive crash */
	if (unlikely(modifying_ftrace_code) && ftrace_int3_handler(regs))
		return;
#endif

The fix I added (but hasn't fixed it completely) was:

 void arch_ftrace_update_code(int command)
 {
        modifying_ftrace_code++;
+       /*
+        * Make sure that all CPUs see this before we start
+        * adding breakpoints.
+        */
+       smp_mb();
 
        ftrace_modify_all_code(command);
 
+       /* Finish all breakpoints before clearing */
+       smp_mb();
+
        modifying_ftrace_code--;
 }


This would make sense for this bug, as if modifying_ftrace_code was not
seen by other CPUs, it wouldn't go into the ftrace_int3_handler() path.
That would cause this issue. But the bug remains after the smp_mb()'s
were put in place. Although it behaves a little differently not. Maybe
there's something else I missed?

-- Steve


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