0005013: Condition.broadcast may not wake up all threads in native Windows build

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0005013

OCaml

OCaml general

public

2010-04-01 00:20

2011-05-29 12:19

Reporter

omion

Assigned To

xleroy

Priority

normal

Severity

major

Reproducibility

random

Status

closed

Resolution

fixed

Platform

OS Version

Product Version

3.11.1

Target Version

Fixed in Version

3.12.0+dev

Summary

0005013: Condition.broadcast may not wake up all threads in native Windows build

Description

If two threads are waiting on a condition, Condition.broadcast should wake both of them up. However, if the first thread to be awoken waits on the same condition again it may be woken up again instead of the other thread.

I marked reproducibility as random since it depends on threads, but it happens about 50% of the time.

Linux and bytecode-Windows seem to be immune from this problem.

Additional Information

The attached code can display the problem. Each line of the output begins with the thread which is printing it. two threads "t1" and "t2" wait on condition "c". The main thread then broadcasts the condition, which SHOULD wake up both threads eventually. On Linux this program always outputs:
1: waiting first
2: waiting first
0: BROADCAST
2: going from broadcast; waiting second
1: going from broadcast; waiting second

indicating that both threads woke up. On Windows the program may return:
1: waiting first
2: waiting first
0: BROADCAST
1: going from broadcast; waiting second
2: going from broadcast; waiting second

which is also correct (the threads were woken up in the opposite order as on Linux, but that doesn't matter). However Windows may also return the following:
1: waiting first
2: waiting first
0: BROADCAST
2: going from broadcast; waiting second
2: going from broadcast a second time

basically the broadcast woke up thread #2 TWICE, without waking up thread #1 at all. If this were a real program which was waiting on thread #1 to continue it would deadlock.

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Attached Files

threadconditions.ml [^] (838 bytes) 2010-04-01 00:20

Relationships

Notes
(0005283) omion (reporter) 2010-04-01 03:06	Actually, after further testing, it looks like it only occurs in 64-bit Windows compiles. My 32-bit compilation of the code seems to work properly...

(0005394) xleroy (administrator) 2010-04-27 10:04	The Win32 implementation of condition variables was indeed incorrect w.r.t. Condition.broadcast. Reimplemented condition variables under Win32 in a way that should conform to POSIX semantics. Will go in 3.12.0.

Issue History
Date Modified	Username	Field	Change
2010-04-01 00:20	omion	New Issue
2010-04-01 00:20	omion	File Added: threadconditions.ml
2010-04-01 03:06	omion	Note Added: 0005283
2010-04-01 11:13	xleroy	Assigned To	=> xleroy
2010-04-01 11:13	xleroy	Status	new => acknowledged
2010-04-27 10:04	xleroy	Note Added: 0005394
2010-04-27 10:04	xleroy	Status	acknowledged => resolved
2010-04-27 10:04	xleroy	Resolution	open => fixed
2010-04-27 10:04	xleroy	Fixed in Version	=> 3.12.0+dev
2011-05-29 12:19	xleroy	Status	resolved => closed