linux-kernel - Re: [PATCH] ftrace: use a real variable for ftrace

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Message-ID: <alpine.DEB.1.10.0810240916350.3882@gandalf.stny.rr.com>
Date:	Fri, 24 Oct 2008 09:17:37 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Ingo Molnar <mingo@...e.hu>
cc:	Rakib Mullick <rakib.mullick@...il.com>,
	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] ftrace: use a real variable for ftrace_nop in x86


On Fri, 24 Oct 2008, Steven Rostedt wrote:

> 
> The dynamic ftrace determines which nop is safe to use at start up.
> When it finds a safe nop for patching, it sets a pointer called ftrace_nop
> to point to the code. All call sites are then patched to this nop.
> 
> Later, when tracing is turned on, this ftrace_nop variable is again used
> to compare the location to make sure it is a nop before we update it to
> a mcount call. If this fails just once, a warning is outputed and ftrace
> is disabled.
> 
> Rakib Mullick noted that the code that sets up the nop is a .init section
> where as the nop itself is in the .text section. This is needed because
> the nop is used later on after boot up. The problem is that the test of the
> nop jumps back to the setup code and causes a "section mismatch" warning.
> 
> Rabik first recommended to convert the nop to .init.text, but as stated
> above, this would fail since that text is used later.
> 
> The real solution is to extend Rabik's patch, and to make the ftrace_nop
> into an array, and just save the code from the assembly to this array.

Rereading what I sent, I see I misspelled Rakib's name twice.

Ingo, could you fix the changelog here before commiting it.

Thanks,

-- Steve

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