Re: [PATCH] Linux Kernel Markers 0.13 for 2.6.17

[Mathieu Desnoyers <compudj@krystal.dyndns.org>]

* Frank Ch. Eigler (fche@redhat.com) wrote:
> Mathieu Desnoyers <compudj@krystal.dyndns.org> 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
> disconnection:
> 
>         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.
> 

I don't really see how cmpxchg might be needed here.

Atomic fetch of a static variable is how I will do it in my next version for the
non optimized case :

volatile static var = 0;
if(var) {
  preempt disable
  call
  preempt_enable_no_resched
}

But, still, in the optimized case, the if(var) will depend on an immediate
value, therefore saving the memory read.


> 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.
> 

This is exactly why the preemption is disabled around the call. However, RCU
must always _see_ a coherent version of the structure in memory.

Calling an empty function, disabling preemption around the call and calling
synchronize_sched() before deleting the removed function looks very much like
a RCU-style protection (actually, that's what it is).

Mathieu


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