Merge pull request #919 from turnon/can-get-write-lock-after-read-locked-more-than-once-only-by-myself

can get write lock after read locked more than once only by myself

Author: chrisseaton

lib/concurrent-ruby/concurrent/atomic/reentrant_read_write_lock.rb

@@ -267,12 +267,10 @@ def acquire_write_lock
         #   running right now, AND no writers who came before us still waiting to
         #   acquire the lock
         # Additionally, if any read locks have been taken, we must hold all of them
-        if c == held
-          # If we successfully swap the RUNNING_WRITER bit on, then we can go ahead
-          if @Counter.compare_and_set(c, c+RUNNING_WRITER)
-            @HeldCount.value = held + WRITE_LOCK_HELD
-            return true
-          end
+        if held > 0 && @Counter.compare_and_set(1, c+RUNNING_WRITER)
+          # If we are the only one reader and successfully swap the RUNNING_WRITER bit on, then we can go ahead
+          @HeldCount.value = held + WRITE_LOCK_HELD
+          return true
         elsif @Counter.compare_and_set(c, c+WAITING_WRITER)
           while true
             # Now we have successfully incremented, so no more readers will be able to increment

spec/concurrent/atomic/reentrant_read_write_lock_spec.rb

@@ -149,6 +149,23 @@ def wait_up_to(secs, &condition)
           end
         end
 
+        it "can be upgraded to a write lock when read lock acquired more than once" do
+          Timeout.timeout(3) do
+            expect(lock.acquire_read_lock).to be true
+            expect(lock.acquire_read_lock).to be true
+            expect(Thread.current).to hold(lock).for_read
+
+            # now we want to upgrade...
+            expect(lock.acquire_write_lock).to be true
+            expect(lock.release_read_lock).to be true
+            expect(lock.release_read_lock).to be true
+            expect(Thread.current).to hold(lock).for_write
+
+            expect(lock.release_write_lock).to be true
+            expect(lock).to be_free
+          end
+        end
+
         it "cannot be released when not held" do
           expect { lock.release_read_lock }.to raise_error(IllegalOperationError)
         end