Messages in this thread | | | Date | Tue, 26 Sep 2006 16:05:14 -0400 | From | Mathieu Desnoyers <> | Subject | Re: [PATCH] Linux Kernel Markers 0.13 for 2.6.17 |
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* 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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