I tried submitting this at
Ruby Issue Tracking System
but it said “« Mailing list » can’t be blank” but wouldn’t let me fill
in anything in that field.
If a third thread is involved, the code in eval.c in function
rb_thread_join0 to detect a mutual join dead lock is faulty.
This effects ruby 1.8.6 p368 and ruby 1.8.7 p 160 but not ruby 1.9.1 p
129
What seems to be happening is a sequence like this…
main thread grabs mutex
thread two blocks waiting for mutex
thread one blocks waiting for mutex and th->join is set to the main
thread.
the main thread unlocks, which wakes thread two. Thread two now has
mutex, thread one is still blocked.
the main thread attempts to join thread one, rb_thread_join0 notes
that thread one is in a WAIT_JOIN waitfor state AND th->join still
points at the main thread and (erroneously) reports deadlock.
ruby-1.8.7-p160/eval.c================================================
rb_thread_join0(th, limit)
rb_thread_t th;
double limit;
{
enum rb_thread_status last_status = THREAD_RUNNABLE;
if (rb_thread_critical) rb_thread_deadlock();
if (!rb_thread_dead(th)) {
if (th == curr_thread)
rb_raise(rb_eThreadError, “thread 0x%lx tried to join itself”,
th->thread);
if ((th->wait_for & WAIT_JOIN) && th->join == curr_thread)
rb_raise(rb_eThreadError, “Thread#join: deadlock 0x%lx - mutual
join(0x%lx)”,
curr_thread->thread, th->thread);
Affected versions.
/opt/ruby/ruby-1.8.6-p368/bin/ruby -v
ruby 1.8.6 (2009-03-31 patchlevel 368) [i686-linux]
/opt/ruby/ruby-1.8.7-p160/bin/ruby -v
ruby 1.8.7 (2009-04-08 patchlevel 160) [i686-linux]
Here is a test script to trigger it…
=mutual_join_bug.rb============================================================
require ‘thread’
m = Mutex.new
m.lock
wt2 = Thread.new do
m.lock
sleep 2
m.unlock
end
Ensure wt2 is waiting on m
sleep 0.1
wt1 = Thread.new do
m.lock
m.unlock
end
Ensure wt1 is waiting on m
sleep 0.1
Give it to wt2
m.unlock
wt1.join
Output…
/opt/ruby/ruby-1.8.7-p160/bin/ruby -w mutual_join_bug.rb
mutual_join_bug.rb:24:in `join’: Thread#join: deadlock 0xb7daa1bc -
mutual join(0xb7d9bc48) (ThreadError)
from mutual_join_bug.rb:24
John C. Phone : (64)(3) 358 6639
Tait Electronics Fax : (64)(3) 359 4632
PO Box 1645 Christchurch Email : [email protected]
New Zealand