0005141: IO mutex and threads

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0005141

OCaml

OCaml general

public

2010-09-02 14:43

2012-07-08 10:59

Reporter

ygrek

Assigned To

Priority

normal

Severity

major

Reproducibility

always

Status

resolved

Resolution

suspended

Platform

OS Version

Product Version

3.11.2

Target Version

Fixed in Version

Summary

0005141: IO mutex and threads

Description

Consider :

let () =
  let ok = match Sys.argv with [|_;"ok"|] -> true | _ -> false in
  Sys.set_signal Sys.sigint (Sys.Signal_handle (fun _ ->
    (if ok then prerr_endline else print_endline) "signal received, sleep 1 second";
    Thread.delay 1.));
  while true do print_endline "ha ha ha ha ha ha ha" done

Compile with : ocamlopt -thread unix.cmxa threads.cmxa io.ml -o io

Running `./io hang` and pressing Ctrl-C will deadlock :
(gdb) bt
#0 __lll_lock_wait ()
    at ../nptl/sysdeps/unix/sysv/linux/x86_64/lowlevellock.S:136
#1 0x00007f0fda04e0e9 in _L_lock_953 () from /lib/libpthread.so.0
#2 0x00007f0fda04df0b in __pthread_mutex_lock (mutex=0x1f30360)
    at pthread_mutex_lock.c:61
0000003 0x000000000041a1ab in caml_io_mutex_lock ()
0000004 0x00000000004266fb in caml_ml_output ()
0000005 0x000000000042e65c in caml_c_call ()
0000006 0x0000000000000001 in ?? ()
0000007 0x0000000000410e9a in camlPervasives__print_endline_298 ()
0000008 0x0000000000000000 in ?? ()

Running `./io ok` will behave as expected (pause 1 sec then continue).

Additional Information

Reproduces :
  debian x86_64 with ocaml 3.11.2 and 3.12.0
  rhel4 with ocaml 3.11.2
  opensolaris x86_64 with ocaml 3.10.2

Reproduces but prints "signal received" before hanging :
  openbsd x86_64 with ocaml 3.11.1
  FreeBSD 6.3-STABLE i386 with ocaml 3.11.2

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Notes
(0007636) xleroy (administrator) 2012-07-04 20:00 edited on: 2012-07-04 20:05	Well, taking a lock from a signal handler can cause deadlock, indeed. I don't think there is anything we can do about it, except advise you not to use locks (directly or indirectly through standard I/O) in signal handlers. Even better: never use signals in a multithreaded program, because threads and signals just don't mix, in OCaml as in C and in any other language running under Unix. I "suspend" this PR in the faint hope that someone will come up with a really clever solution, but I'm not holding my breath.

(0007656) ygrek (reporter) 2012-07-08 10:59 edited on: 2012-07-08 11:00	I agree. In my (multithreaded) programs I now follow the rule to either use ExtUnix.signalfd for non-trivial signal handling or only set boolean flag in signal handler when using Unix module. Maybe this problem should be noted in documentation?

Issue History
Date Modified	Username	Field	Change
2010-09-02 14:43	ygrek	New Issue
2011-05-20 17:54	doligez	Status	new => acknowledged
2012-07-04 17:15	doligez	Target Version	=> 4.00.0+dev
2012-07-04 20:00	xleroy	Note Added: 0007636
2012-07-04 20:00	xleroy	Status	acknowledged => resolved
2012-07-04 20:00	xleroy	Resolution	open => suspended
2012-07-04 20:00	xleroy	Target Version	4.00.0+dev =>
2012-07-04 20:05	xleroy	Note Edited: 0007636	View Revisions
2012-07-08 10:59	ygrek	Note Added: 0007656
2012-07-08 11:00	ygrek	Note Edited: 0007656	View Revisions