The curious case of #367: Infinite loops can hang Concurrent Haskell

Moritz Angermann moritz.angermann at
Mon Aug 17 08:40:06 UTC 2020

Hi there!

While working on a NCG, I eventually came across #367[0], which make GHC produce
code that looks similar to this:

  brach-instruction label

so essentially an uninterruptible loop. The solution for GHC to
produce code that
can be interrupted is to pass -fno-omit-yields.

So far so good. Out of curiosity, I did add a small piece of code to
detect this to my NCG
to complain if code like the above was generated[1].

Three weeks ago, I kind of maneuvered myself into a memory blow up
corner, and then
life happened, but this weekend I managed to find some time to revert
some memory
blow up and continue working on the NCG.  Turns out I can build a
stage2 "quick" flavour
of the NCG without dynamic support just fine.  I never saw the dead
lock detection code fire.

Now I did leave the test suite running yesterday night, and when
looking through the
test suite results, there were quite a few failure. Curiously a lot of
them were due to
ghc missing dynamic support (doh!).  But also quite a few that failed
due to the deadlock

T12485, hs_try_putmvar003, ds-wildcard, ds001, read029, T2817, tc011,
tc021, T4524

So, my question then is this: are we fine with ghc generating this
code? Or, if we are not, do we want to figure out if we can eliminate
it? The issue 367 goes into quite a bit of detail why this is tricky
to handle generally.

Or should we add -fno-omit-yields to the test-cases? The ultimate
option is to just turn of the
detection, and I'm fine with doing so. However I'd rather ask if
anyone sees value in detecting
this or not.



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