linux-kernel - Re: [PATCH V2] mips: function tracer: Fix broken function tracing

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Date:	Thu, 17 Jan 2013 10:35:45 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Alan Cooper <alcooperx@...il.com>
Cc:	Geert Uytterhoeven <geert@...ux-m68k.org>, ddaney.cavm@...il.com,
	ralf@...ux-mips.org, linux-mips@...ux-mips.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH V2] mips: function tracer: Fix broken function tracing

On Thu, 2013-01-17 at 09:58 -0500, Alan Cooper wrote:
> When the kernel first boots we have to be able to handle the gcc
> generated jalr, addui sequence until ftrace_init gets a chance to run
> and change the sequence. At this point mcount just adjusts the stack
> and returns. When ftrace_init runs, we convert the jalr/addui to nops.
> Then whenever tracing is enabled we convert the first nop to a "jalr
> mcount+8". The mcount+8 entry point skips the stack adjust.
> 

I was confused by that too.

> 
> On Thu, Jan 17, 2013 at 1:27 AM, Geert Uytterhoeven

> >
> >> @@ -69,7 +68,7 @@ NESTED(ftrace_caller, PT_SIZE, ra)
> >>         .globl _mcount
> >>  _mcount:
> >>         b       ftrace_stub
> >> -        nop
> >> +       addiu sp,sp,8

Can you add a comment here:

	/* When tracing is activated, it calls ftrace_caller+8 (aka here) */

> >>         lw      t1, function_trace_stop
> >>         bnez    t1, ftrace_stub
> >>         nop
> >

-- Steve


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