ruby-core

Issue #13794 has been updated by catphish (Charlie Smurthwaite).


I am now testing the following patch:

~~~
diff --git a/thread_pthread.c b/thread_pthread.c
index 4aa2d620a2..fe99524a54 100644
--- a/thread_pthread.c
+++ b/thread_pthread.c
@@ -1685,6 +1685,7 @@ native_stop_timer_thread(void)
 static void
 native_reset_timer_thread(void)
 {
+    timer_thread_pipe.writing = 0;
     if (TT_DEBUG)  fprintf(stderr, "reset timer thread\n");
 }
~~~

I don't know if this is the correct way to solve the problem, but I will update this thread when I know if it is effective or not. Even if this is the wrong way to solve the issue it will serve to provide more information about it.

----------------------------------------
Bug #13794: Infinite loop of sched_yield
https://bugs.ruby-lang.org/issues/13794#change-66214

* Author: catphish (Charlie Smurthwaite)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
* ruby -v: ruby 2.3.4p301 (2017-03-30 revision 58214) [x86_64-linux]
* Backport: 2.2: UNKNOWN, 2.3: UNKNOWN, 2.4: UNKNOWN
----------------------------------------
I have been encountering an issue with processes hanging in an infinite loop of calling sched_yield(). The looping code can be found at https://github.com/ruby/ruby/blob/v2_3_4/thread_pthread.c#L1663

while (ATOMIC_CAS(timer_thread_pipe.writing, (rb_atomic_t)0, 0)) {
  native_thread_yield();
}

It is my belief that by some mechanism I have not been able to identify, timer_thread_pipe.writing is incremented but it never decremented, causing this loop to run infinitely.

I am not able to create a reproducible test case, however this issue occurs regularly in my production application. I have attached backtraces and thread lists from 2 processes exhibiting this behaviour. gdb confirms that timer_thread_pipe.writing = 1 in these processes.

I believe one possibility of the cause is that rb_thread_wakeup_timer_thread() or rb_thread_wakeup_timer_thread_low() is called, and before it returns, another thread calls fork(), leaving the value of timer_thread_pipe.writing incremented, but leaving behind the thread that would normally decrement it.

If this is correct, one solution would be to reset timer_thread_pipe.writing to 0 in native_reset_timer_thread() immediately after a fork.

Other examples of similar bugs being reported:
https://github.com/resque/resque/issues/578
https://github.com/zk-ruby/zk/issues/50

---Files--------------------------------
backtrace_1.txt (14 KB)
backtrace_2.txt (10.9 KB)


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