[ruby-core:103995] [Ruby master Bug#17827] Monitor is not fiber safe

Issue #17827 has been updated by ioquatix (Samuel Williams).


@nagachika thank you so much for your effort.

----------------------------------------
Bug #17827: Monitor is not fiber safe
https://bugs.ruby-lang.org/issues/17827#change-92132

* Author: ioquatix (Samuel Williams)
* Status: Closed
* Priority: Normal
* Assignee: Eregon (Benoit Daloze)
* Backport: 2.6: DONTNEED, 2.7: DONTNEED, 3.0: DONE
----------------------------------------
According to our discussion here https://github.com/rspec/rspec-support/issues/501 it seems like typical implementation of per-thread reentrant mutex is no longer valid and can lead to some deadlock situation.

```
#!/usr/bin/env ruby

require 'monitor'

def monitor_failure
	m = Monitor.new
	
	f1 = Fiber.new do
		m.synchronize do
			puts "f1 A"
			Fiber.yield
			puts "f1 B"
		end
	end
	
	f2 = Fiber.new do
		m.synchronize do
			puts "f2 A"
			# Fiber.yield
			f1.resume
			puts "f2 B"
		end
	end
	
	f1.resume
	f2.resume
end

monitor_failure
```



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