Re: [Haskell-cafe] How do I debug this RTS segfault?

On 21:25 Sun 24 Jul , Anatoly Yakovenko wrote:

It's probably out of file descriptors. It's possible that it tries to open another one during the error handling. On Sun, Jul 24, 2016 at 10:50 AM Lana Black wrote:

...

Hello,

I have run into this RTS bug recently. In short, when executing multiple consequtive forks, after 500-600 or so the process is terminated by SIGSEGV. I know this kind of thing is totally artificial, but still.

The problem I have is that I can't get any meaningful backtrace in gdb. For example, for threaded RTS I get this

(gdb) bt #0 0x0000000000560d63 in base_GHCziEventziThread_ensureIOManagerIsRunning1_info () Backtrace stopped: Cannot access memory at address 0x7fffff7fcea0

For non-threaded RTS I get this

(gdb) bt #0 0x00000000007138c9 in stg_makeStablePtrzh () Backtrace stopped: Cannot access memory at address 0x7fffff7fc720

Build command: ghc --make -O2 -g -fforce-recomp fork.hs Add threaded if needed.

I was able to reproduce this bug with both GHC 7.10.3 and todays HEAD with the code below.

...

import System.Exit (exitSuccess) import System.Posix.Process (forkProcess)

fork_ n | n > 0 = processPid =<< forkProcess (fork_ $! n - 1) | otherwise = putStrLn "I'm done!"

processPid pid | pid > 0 = exitSuccess | pid < 0 = putStrLn "OOOPS, forkProcess failed!" | otherwise = pure ()

main = fork_ 1000

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Seems like this is not the case. I actually overlooked GHCs -debug option, with it I'm now able to get a stacktrace. Furthermore, the number of used file descriptors is well within the limit, and changing the latter with `ulimit -n` does not affect the outcome. Curiously, the stacks are rather different for threaded and non-threaded RTS. Non-threaded: (gdb) bt #0 INFO_PTR_TO_STRUCT (info=) at includes/rts/storage/ClosureMacros.h:60 #1 0x000000000070e956 in get_itbl (c=0x20006e7f8) at includes/rts/storage/ClosureMacros.h:87 #2 0x000000000070ec3c in closure_sizeW (p=0x20006e7f8) at includes/rts/storage/ClosureMacros.h:439 #3 0x000000000070ecf7 in overwritingClosure (p=0x20006e7f8) at includes/rts/storage/ClosureMacros.h:555 #4 0x0000000000725dd7 in stg_upd_frame_info () #5 0x0000000000000000 in ?? () Threaded: (gdb) bt #0 0x00007ffff6ce49ce in _IO_vfprintf_internal (s=s@entry=0x7fffff7ff430, format=format@entry=0x7ffff75c3550 "/proc/self/task/%u/comm", ap=ap@entry=0x7fffff7ff558) at vfprintf.c:1266 #1 0x00007ffff6d0954b in __IO_vsprintf (string=0x7fffff7ff630 "`\366\177\377\377\177", format=0x7ffff75c3550 "/proc/self/task/%u/comm", args=args@entry=0x7fffff7ff558) at iovsprintf.c:42 #2 0x00007ffff6cecd47 in __sprintf (s=s@entry=0x7fffff7ff630 "`\366\177\377\377\177", format=format@entry=0x7ffff75c3550 "/proc/self/task/%u/comm") at sprintf.c:32 #3 0x00007ffff75c1f2b in pthread_setname_np (th=140737317025536, name=0x78ba04 "ghc_ticker") at ../sysdeps/unix/sysv/linux/pthread_setname.c:49 #4 0x000000000072ce4e in initTicker (interval=10000000, handle_tick=0x71a23d ) at rts/posix/itimer/Pthread.c:173 #5 0x000000000071a32f in initTimer () at rts/Timer.c:111 #6 0x0000000000703c26 in forkProcess (entry=0x207) at rts/Schedule.c:2072 #7 0x0000000000405bf7 in s7dF_info () #8 0x0000000000000000 in ?? ()