From: Bill Kelly <billk@...>
Date: 2004-08-27T05:18:27+09:00
Subject: Re: [BUG?] erroneous deadlock?  (was: apparent hang...)

Hi yet again,  :)

I think my script is simple enough to post now... 
After running it for a few minutes, I get:

w{-}{}{}S+w{-}{}{}S+w{-}{}{}S+w{-}{
deadlock 0x2a74888: sleep:-  - c:/dev/ruby-dbg/lib/ruby/1.8/thread.rb:195
deadlock 0x2a85628: sleep:- (main) - c:/dev/ruby-dbg/lib/ruby/1.8/thread.rb:195
c:/dev/ruby-dbg/lib/ruby/1.8/thread.rb:195:in `stop': Thread(0x2a85628): deadlock (fatal)
        from c:/dev/ruby-dbg/lib/ruby/1.8/thread.rb:195:in `wait'
        from c:/dev/ruby-dbg/lib/ruby/1.8/thread.rb:193:in `exclusive_unlock'
        from c:/dev/ruby-dbg/lib/ruby/1.8/thread.rb:142:in `exclusive'
        from c:/dev/ruby-dbg/lib/ruby/1.8/thread.rb:142:in `exclusive_unlock'
        from c:/dev/ruby-dbg/lib/ruby/1.8/thread.rb:193:in `wait'
        from buffered-io-test4d.rb:46:in `global_wait'
        from buffered-io-test4d.rb:44:in `synchronize'
        from buffered-io-test4d.rb:44:in `global_wait'
        from buffered-io-test4d.rb:66
        from buffered-io-test4d.rb:60:in `loop'
        from buffered-io-test4d.rb:60


I believe this script is simple enough that based on
inspection we should be able to say there is no way
it could deadlock.

I'll intersperse some comments to try to support this
assertion:

01:
02: require 'socket'
03: require 'thread'
04: require 'timeout'
05:
06: $frame = 0
07:
08: $global_mutex = Mutex.new
09: $global_signal = ConditionVariable.new
10:
11: $background_mutex = Mutex.new
12: $background_signal = ConditionVariable.new
13:
14: $sock = TCPSocket.new("curmudgeon", 27999)

# "curmudgeon:27999" is a server that will merely
# send back a prompt then wait for us to respond.
# We are not going to respond - so after we've read
# the initial prompt, we expect line 18 to wait
# forever in the select.  This is by intention, as
# it seems to be important to have a thread doing
# a time-unlimited select to get these problems to
# occur.
#
# (Note that we can replace line 14 with:
#    $server = TCPServer.new(12345)
#    $sock = TCPSocket.new("localhost", 12345)
# and get similar erroneous behavior.  But for 
# whatever reason the localhost version exhibits
# the "hanging" behavior, while the "curmudgeon:27999"
# version prefers to deadlock.)

# So - once background_read has read the initial
# data it receives from the server, we expect it
# to just wait forever on line 18.

15:
16: def background_read
17:   loop do
18:     if select([$sock], nil, nil, nil)
19:       print "r"
20:       dat = $bg_rd_sock.recv(65536)
21:       $global_signal.signal
22:     end
23:   end
24: end
25:

# background_write is the only code owning the
# $background_mutex... so there should never be
# any deadlock contention there.  So all we do
# is repeatedly wait for the main thread to
# signal us.  Then print chars to stdout and go
# back to sleep.

26: def background_write
27:   loop do
28:     $background_mutex.synchronize {
29:       print "-"
30:       $background_signal.wait($background_mutex)
31:       print "+"
32:     }
33:
34:     print "w"
35:   end
36: end
37:

# global_wait creates a secondary alarm thread to
# awaken the primary thread doing the wait after
# the timeout elapses.  Line 44 is the only code
# owning the $global_mutex, so there should not be
# any deadlock contention there.
# Line 46 is where the deadlocks and hangs always
# occur.

38: def global_wait(timeout_secs)
39:   begin
40:     alarm_th = Thread.start {
41:       sleep timeout_secs
42:       $global_signal.broadcast  # %%WAS: .signal
43:     }
44:     $global_mutex.synchronize {
45:       print "{"
46:       $global_signal.wait($global_mutex)
47:       print "}"
48:     }
49:   ensure
50:     alarm_th.kill if alarm_th and alarm_th.alive?
51:   end
52: end
53:
54:
55: bg_read_th = Thread.new { background_read }
56: bg_write_th = Thread.new { background_write }
57:
58: $stdout.sync = true
59:

# Our main loop does nothing but periodically signal
# the background_write thread, and sleep in global_wait.

60: loop do
61:   $frame += 1
62:   if ($frame % 3) == 0
63:     print "S"
64:     $background_signal.signal
65:   end
66:   global_wait(0.003)
67: end
68:



This script will either deadlock, or hang, on both
Windows (1.8.2preview2) and Linux (1.8.0).

I believe it is simple enough to claim that by inspection
it should never deadlock or hang.  (Unless I am just 
being dense.)

I will begin delving into rb_thread_shedule to try to
determine what is going awry.  Any tips from the
experts would be most welcome.  :)


Regards,

Bill