This is the mail archive of the
systemtap@sourceware.org
mailing list for the systemtap project.
Re: [PATCH] Linux Kernel Markers
- From: Mathieu Desnoyers <compudj at krystal dot dyndns dot org>
- To: Martin Bligh <mbligh at google dot com>
- Cc: prasanna at in dot ibm dot com, Andrew Morton <akpm at osdl dot org>, "Frank Ch. Eigler" <fche at redhat dot com>, Ingo Molnar <mingo at elte dot hu>, Paul Mundt <lethal at linux-sh dot org>, linux-kernel <linux-kernel at vger dot kernel dot org>, Jes Sorensen <jes at sgi dot com>, Tom Zanussi <zanussi at us dot ibm dot com>, Richard J Moore <richardj_moore at uk dot ibm dot com>, Michel Dagenais <michel dot dagenais at polymtl dot ca>, Christoph Hellwig <hch at infradead dot org>, Greg Kroah-Hartman <gregkh at suse dot de>, Thomas Gleixner <tglx at linutronix dot de>, William Cohen <wcohen at redhat dot com>, ltt-dev at shafik dot org, systemtap at sources dot redhat dot com, Alan Cox <alan at lxorguk dot ukuu dot org dot uk>
- Date: Tue, 19 Sep 2006 13:54:05 -0400
- Subject: Re: [PATCH] Linux Kernel Markers
- References: <20060918234502.GA197@Krystal> <20060919081124.GA30394@elte.hu> <451008AC.6030006@google.com> <20060919154612.GU3951@redhat.com> <4510151B.5070304@google.com> <20060919093935.4ddcefc3.akpm@osdl.org> <45101DBA.7000901@google.com> <20060919063821.GB23836@in.ibm.com> <45102641.7000101@google.com>
* Martin Bligh (mbligh@google.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
OpenPGP public key: http://krystal.dyndns.org:8080/key/compudj.gpg
Key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68