From: "luke-gru (Luke Gruber) via ruby-core" <ruby-core@...>
Date: 2026-01-22T19:30:47+00:00
Subject: [ruby-core:124611] [Ruby Bug#21836] RUBY_MN_THREADS deadlock and sleep issues

Issue #21836 has been updated by luke-gru (Luke Gruber).


I'm confused about what should happen. Shouldn't it return roughly after 1 second instead of 10 seconds? I'll look into the sleep issue with `RUBY_MN_THREADS=1`, but I can't reproduce the deadlock or segfault with a more recent commit (ad6b85450d). 

----------------------------------------
Bug #21836: RUBY_MN_THREADS deadlock and sleep issues
https://bugs.ruby-lang.org/issues/21836#change-116198

* Author: jpl-coconut (Jacob Lacouture)
* Status: Open
* ruby -v: ruby 3.4.7 (2025-11-08) +PRISM [aarch64-linux]
* Backport: 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN, 4.0: UNKNOWN
----------------------------------------
I created a benchmark for the purpose of testing [a fix](https://github.com/ruby/ruby/pull/15840) for [Issue 21685](https://bugs.ruby-lang.org/issues/21685)

The benchmark is inline below and saved as 100usec.rb. It should always take just a bit over 10 seconds, regardless of how many cores or threads are assigned. It should look like this:
```
> time taskset --cpu-list 1 ./ruby 100usec.rb 1
real	0m10.130s
user	0m0.094s
sys	0m0.218s
```

It works fine with MN_THREADS disabled. However, with RUBY_MN_THREADS=1, I see two separate issues.

### Issue #1 is that when the number of cores is set to more than 1, the benchmark completes too fast.
These results are consistent across the ruby versions I tried. I did a little debugging and found that the call to `sleep(0.001)` (1ms) is returning after only 0.000005 seconds (5us).

```
> time RUBY_MN_THREADS=1 taskset --cpu-list 1,2 ./ruby 100usec.rb 1
real	0m0.359s
user	0m0.075s
sys	0m0.134s
```

### Issue #2 shows up when the number of cores is limited to 1.

- On ruby 3.4.7 it runs very slow, about 6x slower than expected.
```
> time RUBY_MN_THREADS=1 taskset --cpu-list 1 ./ruby 100usec.rb 1
real	1m2.277s
user	0m0.249s
sys	0m0.408s
```

- On ruby 4.0.0-preview2 it usually deadlocks, but sometimes it segfaults.

```
> time RUBY_MN_THREADS=1 taskset --cpu-list 1 ./ruby ruby/100usec.rb 1
[BUG] unreachable
ruby 4.0.0preview2 (2025-11-17 master 4fa6e9938c) +MN +PRISM [aarch64-linux]

-- Control frame information -----------------------------------------------


-- Threading information ---------------------------------------------------
Total ractor count: 1
Ruby thread count for this ractor: 1

-- C level backtrace information -------------------------------------------
[BUG] Illegal instruction at 0x0000ffff93777250
ruby 4.0.0preview2 (2025-11-17 master 4fa6e9938c) +MN +PRISM [aarch64-linux]

Crashed while printing bug report
Illegal instruction

```

or

```
> time RUBY_MN_THREADS=1 taskset --cpu-list 1 ./ruby ruby/100usec.rb 1
<<DEADLOCK>>
```


### The benchmark code (100usec.rb)



``` ruby
ITRCOUNT = 10000

def inner_test
	r, w = IO.pipe
	reader = Thread.new do
		ITRCOUNT.times.map {|i|
			r.getbyte
		}
	end

	ITRCOUNT.times.map {|i|
		sleep 0.0001
		w.write('0')
	}

	reader.join
end

def outer_test(count)
	count.times.map{|j|
		Thread.new do
			inner_test
		end
	}.each{|t| t.join}
end

outer_test(ARGV[0].to_i)
```






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