linux-kernel - Re: [PATCH] MIPS: lib: Mark intrinsics notrace

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Date:	Tue, 31 May 2016 13:18:28 +0100
From:	Harvey Hunt <harvey.hunt@...tec.com>
To:	Ralf Baechle <ralf@...ux-mips.org>
CC:	<linux-mips@...ux-mips.org>, <linux-kernel@...r.kernel.org>,
	"# 4 . 2 . x-" <stable@...r.kernel.org>
Subject: Re: [PATCH] MIPS: lib: Mark intrinsics notrace

Hi Ralf,

On 29/05/16 22:03, Ralf Baechle wrote:
> On Wed, May 25, 2016 at 11:06:35AM +0100, Harvey Hunt wrote:
>
>> On certain MIPS32 devices, the ftrace tracer "function_graph" uses
>> __lshrdi3() during the capturing of trace data. ftrace then attempts to
>> trace __lshrdi3() which leads to infinite recursion and a stack overflow.
>> Fix this by marking __lshrdi3() as notrace. Mark the other compiler
>> intrinsics as notrace in case the compiler decides to use them in the
>> ftrace path.
>
> Makes perfect sense - but I'm wondering how you triggered it.  Was this
> a build with the GCC option -Os that is CONFIG_CC_OPTIMIZE_FOR_SIZE?
> Usually people build with CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE that is -O2
> which results in intrinsics being inlined.

This is triggered by building with CONFIG_CC_OPTIMIZE_FOR_SIZE. This 
explains why I only saw it on certain MIPS32 devices - Malta's 
defconfigs don't have CONFIG_CC_OPTIMIZE_FOR_SIZE enabled, but the 
pistachio and Ci20 defconfigs do.

>
>   Ralf
>

Thanks,

Harvey