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Re: [PATCH] mips: function tracer: Fix broken function tracing

To: Al Cooper <>
Subject: Re: [PATCH] mips: function tracer: Fix broken function tracing
From: David Daney <>
Date: Fri, 11 Jan 2013 09:01:01 -0800
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On 01/11/2013 06:33 AM, Al Cooper wrote:
Function tracing is currently broken for all 32 bit MIPS platforms.
When tracing is enabled, the kernel immediately hangs on boot.
This is a result of commit b732d439cb43336cd6d7e804ecb2c81193ef63b0
that changes the kernel/trace/Kconfig file so that is no longer
forces FRAME_POINTER when FUNCTION_TRACING is enabled.

MIPS frame pointers are generally considered to be useless because
they cannot be used to unwind the stack. Unfortunately the MIPS
function tracing code has bugs that are masked by the use of frame
pointers. This commit fixes the bugs so that MIPS frame pointers do
not need to be enabled.

The bugs are a result of the odd calling sequence used to call the trace
routine. This calling sequence is inserted into every tracable function
when the tracing CONFIG option is enabled. This sequence is generated
for 32bit MIPS platforms by the compiler via the "-pg" flag.
Part of the sequence is "addiu sp,sp,-8" in the delay slot after every
call to the trace routine "_mcount" (some legacy thing where 2 arguments
used to be pushed on the stack). The _mcount routine is expected to
adjust the sp by +8 before returning.

One of the bugs is that when tracing is disabled for a function, the
"jalr _mcount" instruction is replaced with a nop, but the
"addiu sp,sp,-8" is still executed and the stack pointer is left
trashed. When frame pointers are enabled the problem is masked
because any access to the stack is done through the frame
pointer and the stack pointer is restored from the frame pointer when
the function returns. This patch uses a branch likely instruction
"bltzl zero, f1" instead of "nop" to disable the call because this
instruction will not execute the "addiu sp,sp,-8" instruction in
the delay slot. The other possible solution would be to nop out both
the jalr and the "addiu sp,sp,-8", but this would need to be interrupt
and SMP safe and would be much more intrusive.

I thought all CPUs were in stop_machine() when the modifications were done, so that there is no issue with multi-word instruction patching.

Am I wrong about this?

So really I think you can do two NOP just as easily.

The only reason I bring this up is that I am not sure all 32-bit CPUs implement the 'Likely' branch variants. Also there may be an affect on the branch predictor.

A third possibility would be to replace the JALR with 'ADDIU SP,SP,8' That way the following adjustment would be canceled out. This would work well on single-issue CPUs, but the instructions may not be able to dual-issue on a multi issue machine due to data dependencies.

David Daney

A few other bugs were fixed where the _mcount routine itself did not
always fix the sp on return.

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