Bug #10076: 2nd thread can't get mutex even though 1st thread released it (race)

• Status: Closed
• Assignee: -
• Target version: -
• ruby -v: ruby 2.1.2p95 (2014-05-08 revision 45877) [i686-linux]
• Backport: 2.0.0: UNKNOWN, 2.1: UNKNOWN

[ledestin (Dmitry Maksyoma)]

require 'thread'

m = Mutex.new

Thread.abort_on_exception = true
Thread.new {
  loop {
    m.synchronize {
      puts 'got mutex in thread'
      sleep 0.1
    }
  }
}

loop {
  m.synchronize {
    puts 'got mutex in loop'
    sleep 1
  }
  # Without sleep, the thread above has no chance of getting the mutex.
  #sleep 0.1
}

[ledestin (Dmitry Maksyoma)]

• File monitor-test monitor-test added

Attaching source code with reproduced problem.

[nobu (Nobuyoshi Nakada)]

• Description updated (diff)

Thread fairness issue?
Checking interrupts seems to help it, but not sure.

diff --git a/thread.c b/thread.c
index 682e05f..6f67df6 100644
--- a/thread.c
+++ b/thread.c
@@ -4528,11 +4528,15 @@ rb_mutex_synchronize(VALUE mutex, VALUE (*func)(VALUE arg), VALUE arg)
 static VALUE
 rb_mutex_synchronize_m(VALUE self, VALUE args)
 {
+    VALUE ret;
+
     if (!rb_block_given_p()) {
 	rb_raise(rb_eThreadError, "must be called with a block");
     }
 
-    return rb_mutex_synchronize(self, rb_yield, Qundef);
+    ret = rb_mutex_synchronize(self, rb_yield, Qundef);
+    RUBY_VM_CHECK_INTS(GET_THREAD());
+    return ret;
 }
 
 void rb_mutex_allow_trap(VALUE self, int val)

[naruse (Yui NARUSE)]

• Status changed from Open to Feedback

[kosaki (Motohiro KOSAKI)]

Your code is very artifactual and the patch of comment #2 reduce a performance of normal case. So, until someone provide a real usecase, I don't take it.

[jeremyevans0 (Jeremy Evans)]

• Status changed from Feedback to Closed