From: "byroot (Jean Boussier)" <noreply@...>
Date: 2022-11-04T22:53:56+00:00
Subject: [ruby-core:110611] [Ruby master Bug#17827] Monitor is not fiber safe

Issue #17827 has been updated by byroot (Jean Boussier).


> It would be nice to raise a ThreadError when detecting in Mutex#lock that the Mutex is owned by the same Thread but a different Fiber, if there is no Fiber scheduler.

I just ran into this issue, that would indeed be good to error out rather that deadlock forever.

----------------------------------------
Bug #17827: Monitor is not fiber safe
https://bugs.ruby-lang.org/issues/17827#change-99943

* 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https://github.com/rspec/rspec-support/issues/501 it seems like typical implementation of per-thread reentrant mutex is no longer valid and can lead to some deadlock situation.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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