Hi everyone,
While I was testing and debugging some of the SRFI-18 code that Neil
and I were working on, I noticed a deadlock that happens in
scm_join_thread_timed. I'm pretty sure it affects the 1.8 codebase as
well, although it's probably more common when doing timed joins.
Thread joining in Guile (1.9 or 1.8) works as follows:
1. If the target thread has exited, return.
2. Block on the target thread's join queue.
3. When woken (because of a pthread_cond_signal, a spurious pthreads
wakeup, or, in 1.9, a timeout expiration), check the target thread's
exit status -- if it has exited, return.
4. Otherwise, SCM_TICK.
5. Go to step 2.
The deadlock can happen if the thread exits during the tick, because
there's no check of the exit status before block_self is called again.
I'm pretty sure that moving step 1 into the beginning of the loop
would fix this -- I can submit a patch against 1.8, 1.9, or both.
Let me know what you guys would like.
Hi Julian,
Based on the synopsis above, I agree that moving step 1 inside the loop should fix this. In addition, though, I think it would be very good if we could add a minimal test that currently reproduces the deadlock, and so will serve to guard against future regressions here. Do you have such a test?
No need for a patch against both 1.8 and 1.9; just one will do, and git cherry-pick will handle the other for us (unless the fix is significantly different in the two branches).
Regards,
Neil
Regards,
Julian