Re: [PATCH] Linux Kernel Markers 0.13 for 2.6.17

From: Frank Ch. Eigler
Date: Tue Sep 26 2006 - 15:51:52 EST

Mathieu Desnoyers <compudj@xxxxxxxxxxxxxxxxxx> writes:

> [...]
> > Yep, that looks reasonable. Though you could just directly test a
> > per-marker enable flag, rather than using "condition"...
> [...]
> I am not sure I understand your suggestion correctly.. do you mean having
> a per-marker flag that would be loaded and tested at every marker site ?

I gather that one reason for working so hard with the inline assembly
is a race condition problem with the plain STAP_MARK style of marker

if (pointer) (*pointer)(args ...);

Granted, but this problem could almost certainly be dealt with simpler
than that. How about a compxchg or other atomic-fetch of the static
pointer with a local variable? That should solve the worry of an
(*NULL) call.

If we then become concerned with a valid pointer become obsolete (the
probe handler function wanting to unload), we might be able to use
some RCU-type deferral mechanism and/or preempt controls to ensure
that this does not happen.

- FChE
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