linux-kernel - Re: [PATCH] Linux Kernel Markers

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Date:	Tue, 19 Sep 2006 13:54:05 -0400
From:	Mathieu Desnoyers <compudj@...stal.dyndns.org>
To:	Martin Bligh <mbligh@...gle.com>
Cc:	prasanna@...ibm.com, Andrew Morton <akpm@...l.org>,
	"Frank Ch. Eigler" <fche@...hat.com>, Ingo Molnar <mingo@...e.hu>,
	Paul Mundt <lethal@...ux-sh.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Jes Sorensen <jes@....com>, Tom Zanussi <zanussi@...ibm.com>,
	Richard J Moore <richardj_moore@...ibm.com>,
	Michel Dagenais <michel.dagenais@...ymtl.ca>,
	Christoph Hellwig <hch@...radead.org>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	William Cohen <wcohen@...hat.com>, ltt-dev@...fik.org,
	systemtap@...rces.redhat.com, Alan Cox <alan@...rguk.ukuu.org.uk>
Subject: Re: [PATCH] Linux Kernel Markers

* Martin Bligh (mbligh@...gle.com) wrote:
> How about we combine all three ideas together ...
> 
> 1. Load modified copy of the function in question.
> 2. overwrite the first instruction of the routine with an int3 that
> does what you say (atomically)
> 3. Then overwrite the second instruction with a jump that's faster
> 4. Now atomically overwrite the int3 with a nop, and let the jump
> take over.
> 

Very good idea.. However, overwriting the second instruction with a jump could
be dangerous on preemptible and SMP kernels, because we never know if a thread
has an IP in any of its contexts that would return exactly at the middle of the
jump. I think it would be doable to overwrite a 5+ bytes instruction with a NOP
non-atomically in all cases, but as the instructions nin the prologue seems to
be smaller :

prologue on x86
   0:   55                      push   %ebp
   1:   89 e5                   mov    %esp,%ebp
epilogue on x86
   3:   5d                      pop    %ebp
   4:   c3                      ret

Then is can be a problem. Ideas are welcome.

Mathieu


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