linux-kernel - Re: [PATCH -mm] tracing: Fix section mismatch in trace_hw

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Date:	Sun, 22 Feb 2009 17:22:35 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Rakib Mullick <rakib.mullick@...il.com>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>, markus.t.metzger@...il.com
Subject: Re: [PATCH -mm] tracing: Fix section mismatch in
	trace_hw_branches.c

On Sun, Feb 22, 2009 at 07:33:08PM +0600, Rakib Mullick wrote:
> On 2/21/09, Frederic Weisbecker <fweisbec@...il.com> wrote:
> 
> > Hi,
> >
> >  When I saw this patch, I searched the real purpose of __cpuinit and its
> >  real impact.
> >  But I didn't find any comments about it inside the kernel.
> >
> >  But today, by looking at the discussion around latest git pull for x86
> >  to mainline, I discover that __cpuinit becomes __init on UP.
> >
> >  So, unless I missed something, this patch seems to me very dangerous.
> >  The init and reset callbacks of a tracer can be called at any time, not only
> >  on initcalls time (__init functions are freed from memory after the middle stage
> >  of the boot).
> >  With this patch, on UP we will dereference freed memory while activating this tracer.
> If the init and reset callbacks of a tracer can be called regardless
> of cpu hotpluging then it is. If the tracer's init or reset doesn't
> rely on cpuhotplug then it shouldn't use it.
> There's a another way to fix the warning is by remove __cpuinitdata
> from bts_hotcpu_notifier.


Yes, they can be called on UP, on SMP with or without cpu hotplug,
and everytime (boot, runtime).

init() is called when you switch to a tracer:

echo tracer_name > /debug/tracing/current_tracer

and reset() is called when you switch to another one.

But removing __cpuinitdata will mean a kind of waste of memory (though it's only
a little struct).

 
> Thanks,

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