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Comment by mmm_grayons | Hacker News Reader

> Using threads -especially low-level threads- can lead to a real education in debugging.

Oh yeah it can. I'm still scarred by an error that took me a month to solve: I was self-teaching pthreads and kept getting an access violation with a stack trace starting at 0x0.

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The only bug that ever beat me in my two decades coding was in a custom lock free data structure using an early version of c++11. I spent a month on it, including writing custom visualization and logging tools, but I never found the issue that caused a crash once every few hundred billion operations.

Later I saw some suspicious compiler bug fixes around compare and swap, and I still wonder if that was maybe it.

1 in every hundred billion... is that low enough odds to be cosmic rays?

A hundred billion operations is spinning on a loop for a couple minutes, so not really.

It took about ten minutes at full system utilization on all cores, running the "torture test" in a loop in each thread.

For a cosmic ray bit-flip, or to perform a hundred billion operations?

One hundred billion operations. The machine didn't have ECC memory, so a bit flip is a possible explanation. I should have checked if it was reproducible with ECC.

I still think a logic bug or compiler bug was at fault though.

Obligatory sometimes, just sometimes, it is the compiler.

I read that some lock free libraries needed to be updated when Apple released it's first dual core ARM processors. Something about ARM's read or write ordering guarantees are looser than Intels.

Me I'd rather roll my own cryto than try and write my own lock free data structure.

Fun fact: Apple's recent processors have a mode where they emulate stricter ordering guarantees (for Rosetta, of course).

That reminds me I think I saw some blurb in an MCU data sheet that mentioned the processor can do out of order memory accesses which if you're an embedded guy tends to freak you out. However the chip designers disabled that for peripheral memory accesses.

Which is very nice of them.

Yes, this is something where different CPU architectures do it differently, even between ARMv7 and ARMv8 etc. There's a great presentation on the differences here:

https://channel9.msdn.com/Shows/Going+Deep/Cpp-and-Beyond-20... https://channel9.msdn.com/Shows/Going+Deep/Cpp-and-Beyond-20...

He goes into the details of ARM vs Intel etc in the middle of part 2, but really part 1 is well worth a watch.

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