Okay, I've run it multiple times. It hangs for me too, but randomly - about 1 out of 25 runs - more often if the machine is under heavy load. I moved the warn statements after the lock in both subs and moved the "warn m-processing" above the signal to reduce the time the variable was unlocked. That seemed to help, but how do we quantify it?

The $done race was not the only one. Looking at the TRACE output, the warn statements don't get executed uniformly. I guess hat's to be expected, since the threads run asynchronously.

However, when it hangs, I see one of two things: a missed signal or a signal being raised when the other thread isn't waiting. 

m-locking
m-waiting
t-Locking
t-Waiting
t-Setting
t-Signalling  # look ma, nobody received my signal!
t-Locking
t-Waiting     # and now we're both blocked, waiting for the other to c
+all

or (I reconstructed this second scenario from memory)

m-processing
t-Locking
t-Waiting
t-Signalling  # if a tree signals in a forest, and noone's listening
t-Locking
t-Waiting
m-locking
m-waiting     # and again, we both wait...

[download]

Now threads::shared says the following about the second condition: 

If there are no threads blocked in a "cond_wait" on the variable,
the signal is discarded. By always locking before signaling, you can
(with care), avoid signaling before another thread has entered
cond_wait().

[download]
Uh... what does it mean "with care"?

Two more random notes -

  1. random lines (from the file to copy) get skipped when TRACE=1, again from missed signals.
  2. FWIW, 5.8.4 on Debian runs this correctly consistently.
My conclusion is that there's too much happening between lock, wait and signal.

In reply to Re^5: Printing to STDERR causes deadlocks. by bmann
in thread Printing to STDERR causes deadlocks. by BrowserUk

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