Please record the repro steps exactly, including the git hashes of any repositories that you use.  If it is a GC bug, the last thing we want is for it to disappear, because then we lose the opportunity to find it.

If you could put the build steps into a script that we can run to reproduce the error, that will help too.  And as Edward says, if there's any way you can find to reduce the test case so that it still fails, that's really useful.

Cheers,
Simon

On 28/05/2014 11:04, Ömer Sinan Ağacan wrote:

Hi all,

I'm suffering from a RTS bug(probably GC related) that makes making
progress in my GSoC project impossible. I have very limited knowledge
of GHC internals and I currently have no idea how to produce a minimal
program that demonstrates the bug. I wrote how to reproduce it and gdb
backtrace when segfault happens in a short blog post:
http://osa1.net/posts/2014-05-27-worst-bug.html . As also written in
the blog post, changing generation count of generational GC will makes
the bug disappear in some cases, but it's not a solution.

I also pasted backtrace output below for those who don't want to click links.

GHC version used is 7.8.2.

If anyone give me some pointers to understand what's going wrong or
how can I produce a simple program that demonstrates the bug, I'd like
to work on that. I'm basically stuck and I can't make any progress
with this bug.

Thanks,
Ömer

[  5 of 202] Compiling GHC.Unicode[boot] ( GHC/Unicode.hs-boot,
dist/build/GHC/Unicode.js_p_o-boot )
Detaching after fork from child process 3382.
[  6 of 202] Compiling GHC.IO[boot]     ( GHC/IO.hs-boot,
dist/build/GHC/IO.js_p_o-boot )
Detaching after fork from child process 3383.
[  7 of 202] Compiling GHC.Exception[boot] ( GHC/Exception.lhs-boot,
dist/build/GHC/Exception.js_p_o-boot )
Detaching after fork from child process 3384.
[ 51 of 202] Compiling GHC.Fingerprint[boot] (
GHC/Fingerprint.hs-boot, dist/build/GHC/Fingerprint.js_p_o-boot )
Detaching after fork from child process 3385.
[ 55 of 202] Compiling GHC.IO.Exception[boot] (
GHC/IO/Exception.hs-boot, dist/build/GHC/IO/Exception.js_p_o-boot )
Detaching after fork from child process 3386.
[ 75 of 202] Compiling Foreign.C.Types  ( Foreign/C/Types.hs,
dist/build/Foreign/C/Types.js_p_o )

Program received signal SIGSEGV, Segmentation fault.
0x000000000425d5c4 in LOOKS_LIKE_CLOSURE_PTR (p=0x0) at
includes/rts/storage/ClosureMacros.h:258
258     includes/rts/storage/ClosureMacros.h: No such file or directory.
(gdb) bt
#0  0x000000000425d5c4 in LOOKS_LIKE_CLOSURE_PTR (p=0x0) at
includes/rts/storage/ClosureMacros.h:258
#1  0x000000000425f776 in scavenge_mutable_list1 (bd=0x7fffe5c02a00,
gen=0x4d1fd48) at rts/sm/Scav.c:1400
#2  0x000000000425fa13 in scavenge_capability_mut_Lists1
(cap=0x4cfe5c0 <MainCapability>) at rts/sm/Scav.c:1493
#3  0x0000000004256b66 in GarbageCollect (collect_gen=0,
do_heap_census=rtsFalse, gc_type=2,
     cap=0x4cfe5c0 <MainCapability>) at rts/sm/GC.c:342
#4  0x00000000042454a3 in scheduleDoGC (pcap=0x7fffffffc198,
task=0x4d32b60, force_major=rtsFalse)
     at rts/Schedule.c:1650
#5  0x0000000004243de4 in schedule (initialCapability=0x4cfe5c0
<MainCapability>, task=0x4d32b60)
     at rts/Schedule.c:553
#6  0x0000000004246436 in scheduleWaitThread (tso=0x7ffff6708d60,
ret=0x0, pcap=0x7fffffffc2c0) at rts/Schedule.c:2346
#7  0x000000000423e9b4 in rts_evalLazyIO (cap=0x7fffffffc2c0,
p=0x477f850, ret=0x0) at rts/RtsAPI.c:500
#8  0x0000000004241666 in real_main () at rts/RtsMain.c:63
#9  0x0000000004241759 in hs_main (argc=237, argv=0x7fffffffc448,
main_closure=0x477f850, rts_config=...)
     at rts/RtsMain.c:114
#10 0x0000000000408ea7 in main ()
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