Re: [PATCH] mips/ftrace: Fix function tracing return address to match

[Corey Minyard <cminyard@mvista.com>]

On 07/18/2013 03:11 PM, Steven Rostedt wrote:
> On Mon, 2013-07-15 at 15:17 -0700, David Daney wrote:
>> From: Corey Minyard <cminyard@mvista.com>
>>
>> Dynamic function tracing was not working on MIPS.  When doing dynamic
>> tracing, the tracer attempts to match up the passed in address with
>> the one the compiler creates in the mcount tables.  The MIPS code was
>> passing in the return address from the tracing function call, but the
>> compiler tables were the address of the function call.  So they
>> wouldn't match.
>>
>> Just subtracting 8 from the return address will give the address of
>> the function call.  Easy enough.
>>
>> Signed-off-by: Corey Minyard <cminyard@mvista.com>
>> [david.daney@cavium.com: Adjusted code comment and patch Subject.]
>> Signed-off-by: David Daney <david.daney@cavium.com>
>> ---
>>   arch/mips/kernel/mcount.S | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/arch/mips/kernel/mcount.S b/arch/mips/kernel/mcount.S
>> index a03e93c..539b629 100644
>> --- a/arch/mips/kernel/mcount.S
>> +++ b/arch/mips/kernel/mcount.S
>> @@ -83,7 +83,7 @@ _mcount:
>>   	PTR_S	MCOUNT_RA_ADDRESS_REG, PT_R12(sp)
>>   #endif
>>   
>> -	move	a0, ra		/* arg1: self return address */
>> +	PTR_SUBU a0, ra, 8	/* arg1: self address */
>>   	.globl ftrace_call
>>   ftrace_call:
>>   	nop	/* a placeholder for the call to a real tracing function */
> I applied this patch to my Yeeloong Lemote laptop and it causes the
> system to crash. Not sure why. I'll try to investigate.
>
> -- Steve
>
>
That is bizarre.  That value should just be used for comparisons, it 
isn't dereferenced or anything like that.  What tracing do you have enabled?

-corey