linux-kernel - Re: [PATCH 1/2] MIPS: ftrace.h: Fix the MCOUNT_INSN

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Message-ID: <54214F76.6070808@imgtec.com>
Date:	Tue, 23 Sep 2014 11:46:14 +0100
From:	Markos Chandras <Markos.Chandras@...tec.com>
To:	Steven Rostedt <rostedt@...dmis.org>,
	David Daney <ddaney.cavm@...il.com>
CC:	<linux-mips@...ux-mips.org>, Ingo Molnar <mingo@...hat.com>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] MIPS: ftrace.h: Fix the MCOUNT_INSN_SIZE definition

On 09/22/2014 07:25 PM, Steven Rostedt wrote:
> On Mon, 22 Sep 2014 09:55:09 -0700
> David Daney <ddaney.cavm@...il.com> wrote:
> 
>> On 09/22/2014 06:32 AM, Markos Chandras wrote:
>>> The MCOUNT_INSN_SIZE is meant to be used to denote the overall
>>> size of the mcount() call. Since a jal instruction is used to
>>> call mcount() the delay slot should be taken into consideration
>>> as well.
>>> This also replaces the MCOUNT_INSN_SIZE usage with the real size
>>> of a single MIPS instruction since, as described above, the
>>> MCOUNT_INSN_SIZE is used to denote the total overhead of the
>>> mcount() call.
>>
>> Are you seeing errors with the existing code?  If so please state what 
>> they are.
>>
>> By changing this, we can no longer atomically replace the instruction. 
>> So I think shouldn't be changing this stuff unless there is a real bug 
>> we are fixing.
> 
> Actually, it looks like the code still works the same, as it uses the
> old size of 4 (FTRACE_MIPS_INSN_SIZE) to do the update.

Indeed I haven't seen any functional change when it comes to replacing
the instruction.

> [...]
> 
> It may also fix the stack tracer, as it searches for the ip saved in
> the return address to find where the true stack is (skipping the stack
> part that calls the strack tracer itself). If the link register holds
> the location after the delay slot, then this would require
> MCOUNT_INSN_SIZE to include the delay slot as well.

Yes, this is the only case I spotted as well. Perhaps I should put that
in the changelog.

Or I could add
> another macro called MCOUNT_DELAY_SLOT_SIZE that can be defined by an
> arch (and keep it zero for all other archs). That wouldn't be too much
> of an issue to implement.

If you want to fix that in the generic code then I am fine with it.

-- 
markos
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