0003236: Queue+Threads+Cygwin native causes crash

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0003236

OCaml

OCaml general

public

2004-10-25 22:06

2012-09-11 09:35

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administrator

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Priority

normal

Severity

minor

Reproducibility

always

Status

acknowledged

Resolution

open

Platform

OS Version

Product Version

Target Version

4.00.2+dev

Fixed in Version

Summary

0003236: Queue+Threads+Cygwin native causes crash

Description

Full_Name: Matthew W. Boyd
Version: 3.08.1
OS: Windows/Cygwin
Submission from: outbound.mlog.com (216.127.133.66)

The following application will cause a core dump when compiled in the following
way (Under Cygwin):

ocamlopt -thread -o qthr unix.cmxa threads.cmxa qthr.ml

(* ---------------------------------------- *)
open Unix
open Printf
open Pervasives

let q = Queue.create ()

let try_take () =
  try Queue.take q
  with Queue.Empty -> 0

let rec randomaction n =
  let () =
    match Random.int 2 with
    | 0 -> printf "Taking: %d\n" (try_take ())
    | _ -> printf "Adding\n"; Queue.add n q
  in
  flush stdout;
  randomaction (n+1)

let v =
  let a = Array.create 10 (Thread.self()) in
  for i = 0 to 9 do
    a.(i) <- Thread.create randomaction i
  done;
  a
;;

let hent = Unix.gethostbyname "localhost" in
for i = 0 to 9 do
  Thread.join v.(i)
done
;;
(* ---------------------------------------- *)

I noticed this while I was attempting to use a shared queue with multiple
threads.

In this sample application, the program will not crash unless I call
Unix.gethostbyname.

I also noticed that if I call Unix.gethostbyname after unblocking Sys.sigint,
the signal will be blocked again after the call to Unix.gethostbyname; however,
if I
unblock the signal after calling gethostbyname, it will remain unblocked.

I also noticed that I can prevent the segmentation fault by wrapping any Queue
function calls with Mutex locks.

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Notes
(0003092) administrator (administrator) 2005-02-02 16:56	Unlocked concurrent accesses to Queue are inherently unsafe. Still, can't reproduce crash under Linux. Can reproduce it under Cygwin with and without the gethostbyname. gdb gives no useful context. No can do.

(0008054) doligez (manager) 2012-09-11 09:35	Just looking at the code of Queue.take, it's obvious that concurrent access to a queue can cause a crash. This should be documented. We also need to investigate the interference between Unix.gethostbyname and signals under Cygwin.

Issue History
Date Modified	Username	Field	Change
2005-11-18 10:14	administrator	New Issue
2012-07-11 17:32	doligez	Target Version	=> 4.01.0+dev
2012-07-11 17:32	doligez	Description Updated	View Revisions
2012-07-31 13:37	doligez	Target Version	4.01.0+dev => 4.00.1+dev
2012-09-11 09:35	doligez	Note Added: 0008054
2012-09-11 09:35	doligez	Target Version	4.00.1+dev => 4.00.2+dev
2012-09-11 09:35	doligez	Resolution	unable to reproduce => open