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Re: [PATCH 1/1] x86: fix text_poke
- From: Jeremy Fitzhardinge <jeremy at goop dot org>
- To: Masami Hiramatsu <mhiramat at redhat dot com>
- Cc: Mathieu Desnoyers <mathieu dot desnoyers at polymtl dot ca>, Linus Torvalds <torvalds at linux-foundation dot org>, "H. Peter Anvin" <hpa at zytor dot com>, Andi Kleen <andi at firstfloor dot org>, Ingo Molnar <mingo at elte dot hu>, Jiri Slaby <jirislaby at gmail dot com>, David Miller <davem at davemloft dot net>, zdenek dot kabelac at gmail dot com, rjw at sisk dot pl, paulmck at linux dot vnet dot ibm dot com, akpm at linux-foundation dot org, linux-ext4 at vger dot kernel dot org, herbert at gondor dot apana dot org dot au, penberg at cs dot helsinki dot fi, clameter at sgi dot com, linux-kernel at vger dot kernel dot org, pageexec at freemail dot hu, "Frank Ch. Eigler" <fche at redhat dot com>, systemtap at sources dot redhat dot com
- Date: Fri, 25 Apr 2008 23:21:54 -0700
- Subject: Re: [PATCH 1/1] x86: fix text_poke
- References: <20080425163035.GE9503@Krystal> <481209F2.4050908@zytor.com> <20080425170929.GA16180@Krystal> <20080425183748.GB16180@Krystal> <48123C9B.9020306@zytor.com> <20080425203717.GB25950@Krystal> <481241DC.3070601@zytor.com> <alpine.LFD.1.10.0804251349510.2779@woody.linux-foundation.org> <20080425211205.GC25950@Krystal> <alpine.LFD.1.10.0804251456410.2779@woody.linux-foundation.org> <20080425230028.GC31226@Krystal> <481265B7.9040505@goop.org> <48126A80.4000203@redhat.com>
Masami Hiramatsu wrote:
Jeremy Fitzhardinge wrote:
Mathieu Desnoyers wrote:
This idea has been considered a few years ago at OLS in the tracing BOF
if I remember well. The results were this : First, there is no way to
guarantee that no code path, nor any return address from any function,
interrupt, sleeping thread, will return to the "old" version of the
function. Nor is it possible to determine when a quiescent state is
reached. Therefore, we couldn't see how we can do the teardown.
Does that matter? The new function is semantically identical to the old
one, and the old code will remain in place. If there's still users in
the old function it may take a while for them to get flushed out (and
won't be traced in the meantime), but you have to expect some missed
events if you're shoving any kind of dynamic marker into the code. The
main problem is if there's something still depending on the first 5
bytes of the function (most likely if there's a loop head somewhere near
the top of the function).
I think we have to ensure no threads sleeping or being interrupted on
the function when removing new function. How would you check it?
Not sure I follow you. You'd never remove any code. But you'd only
start tracing new callers of the function. If the function loops
indefinitely, you could potentially have some users which never end up
getting traced. Also, if those users depend on instructions in the
first 5 bytes of the function, they would crash because of the jump to
the new function patched on top of them.
Overall, it doesn't seem like a very satisfactory mechanism...
J