`On Thu, 30 Oct 2008, Ingo Molnar wrote:> > hm, all those magic constants look a bit like voodoo and make the > patch ugly, and people who read this will be confused about the > purpose for sure.Point taken.> > But the checks are still worth having in practice. So could you please > improve the comments, to come up with some tangible calculation that > leads to these constants?> > For example the '1000' constant, how did you come to that? Could you > estimate what type of interrupt storm is needed to trigger it falsely? > So instead of this comment:My original number was 100,000, but I thought that a bit high ;-)Since it is OK for an interrupt to preempt this code and perform a trace, which would make the condition fail by the one being preempted. The likelyhood of an interrupt coming in at that location 1000 times in a row seems to be awefully low. It's not enough that a 1000 interrupts come in, the task being preempted must loop 1000 times and have a trace interrupt cause the condition to fail each time. I'll explain it this way in the comments.I picked a big number because I can see a traced interrupt that is very active causing several interruptions in this code.> > > +	 * If we loop here 1,000 times, that means we are either> > +	 * in an interrupt storm, or we have something buggy.> > +	 * Bail!> > something like this might look more acceptable:> > > +	 * If we loop here 1,000 times, that means we are either> > +	 * in an interrupt storm that preempted the same trace-entry> > +	 * attempt 1000 times in a row, or we have a bug in the tracer.> > +	 * Bail!> > i.e. please exaplain every single magic number there so that it can be > followed how you got to that number, and what precise effects that > number has.> > In the cases where you just guessed a number based on experiments, > please think it through and insert an analysis about the effects of > that number.> > Would this be doable?Again, there are small "allowable" races that causes the code to loop a few times.  I'll try to explain them a bit better in the comments.There's small races between the reader and writer that can hit just right to cause a "loop again". But these chances are much smaller than the interrupt tracing situation.I'll look deeper at the reasons for the races and explain them a bit better.Thanks,-- Steve`