I got this on Linux in libraries/unix/tests Wrong exit code for forkprocess01(threaded1_ls)(expected 0 , actual 134 ) Stderr ( forkprocess01 ): forkprocess01: internal error: multiple ACQUIRE_LOCK: rts/Task.c 226 (GHC version 8.3.20170428 for x86_64_unknown_linux) Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug Aborted when validaiting. It worked fine when I said "make TEST=forkprocess01" (see below). I have no idea if this matters. Simon make TEST=forkprocess01 PYTHON="python3" "python3" ../../../testsuite/driver/runtests.py -e ghc_compiler_always_flags="'-dcore-lint -dcmm-lint -no-user-package-db -rtsopts -fno-warn-missed-specialisations -fshow-warning-groups -fdiagnostics-color=never -fno-diagnostics-show-caret -dno-debug-output'" -e config.compiler_debugged=False -e ghc_with_native_codegen=1 -e config.have_vanilla=True -e config.have_dynamic=True -e config.have_profiling=False -e ghc_with_threaded_rts=1 -e ghc_with_dynamic_rts=1 -e config.have_interp=True -e config.unregisterised=False -e config.ghc_dynamic_by_default=False -e config.ghc_dynamic=True -e ghc_with_smp=1 -e ghc_with_llvm=0 -e windows=False -e darwin=False -e config.in_tree_compiler=True -e config.cleanup=True -e config.local=True --rootdir=. --configfile=../../../testsuite/config/ghc -e 'config.confdir="../../../testsuite/config"' -e 'config.platform="x86_64-unknown-linux"' -e 'config.os="linux"' -e 'config.arch="x86_64"' -e 'config.wordsize="64"' -e 'config.timeout=int() or config.timeout' -e 'config.exeext=""' -e 'config.top="/5playpen/simonpj/HEAD-6/testsuite"' --config 'compiler="/5playpen/simonpj/HEAD-6/inplace/test spaces/ghc-stage2"' --config 'ghc_pkg="/5playpen/simonpj/HEAD-6/inplace/test spaces/ghc-pkg"' --config 'haddock=' --config 'hp2ps="/5playpen/simonpj/HEAD-6/inplace/test spaces/hp2ps"' --config 'hpc="/5playpen/simonpj/HEAD-6/inplace/test spaces/hpc"' --config 'gs="gs"' --config 'timeout_prog="../../../testsuite/timeout/install-inplace/bin/timeout"' -e "config.stage=2" \ --only=forkprocess01 \ \ \ \ \ \ Timeout is 300 Found 2 .T files... Beginning test run at Tue May 2 10:11:39 2017 BST ====> Scanning ./all.T ====> Scanning ./libposix/all.T =====> forkprocess01(normal) 1 of 1 [0, 0, 0] cd "./forkprocess01.run" && "/5playpen/simonpj/HEAD-6/inplace/test spaces/ghc-stage2" -o forkprocess01 forkprocess01.hs -dcore-lint -dcmm-lint -no-user-package-db -rtsopts -fno-warn-missed-specialisations -fshow-warning-groups -fdiagnostics-color=never -fno-diagnostics-show-caret -dno-debug-output -package unix cd "./forkprocess01.run" && ./forkprocess01 =====> forkprocess01(threaded1_ls) 1 of 1 [0, 0, 0] cd "./forkprocess01.run" && "/5playpen/simonpj/HEAD-6/inplace/test spaces/ghc-stage2" -o forkprocess01 forkprocess01.hs -dcore-lint -dcmm-lint -no-user-package-db -rtsopts -fno-warn-missed-specialisations -fshow-warning-groups -fdiagnostics-color=never -fno-diagnostics-show-caret -dno-debug-output -threaded -debug -package unix cd "./forkprocess01.run" && ./forkprocess01 +RTS -ls -RTS SUMMARY for test run started at Tue May 2 10:11:39 2017 BST 0:00:03 spent to go through 1 total tests, which gave rise to 8 test cases, of which 6 were skipped 0 had missing libraries 2 expected passes 0 expected failures 0 caused framework failures 0 caused framework warnings 0 unexpected passes 0 unexpected failures 0 unexpected stat failures simonpj@cam-05-unx:~/5builds/HEAD-6/libraries/unix/tests$