[ruby-core:93054] [Ruby trunk Bug#15645] It is possible to escape `Mutex#synchronize` without releasing the mutex

[merch-redmine@jeremyevans.net]

Issue #15645 has been updated by jeremyevans0 (Jeremy Evans).


Eregon (Benoit Daloze) wrote:
> @jeremyevans0 The patch looks wrong, `rb_mutex_lock` should be changed, not `rb_mutex_unlock`.

Correct, apologies for that.  It turns out there is a pull request for this that should may the issue (https://github.com/ruby/ruby/pull/2131).

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Bug #15645: It is possible to escape `Mutex#synchronize` without releasing the mutex
https://bugs.ruby-lang.org/issues/15645#change-78454

* Author: jneen (Jeanine Adkisson)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
* ruby -v: ruby 2.6.1p33 (2019-01-30 revision 66950) [x86_64-linux]
* Backport: 2.4: UNKNOWN, 2.5: UNKNOWN, 2.6: UNKNOWN
----------------------------------------
Hello, I hope this finds you well.

I have a persistent deadlocking issue in a project that relies both on `Mutex#synchronize` and `Thread#raise`, and I believe I have reduced the problem to the following example, in which it is possible to exit a `synchronize` block without releasing the mutex.

``` ruby
mutex = Mutex.new
class E < StandardError; end

t1 = Thread.new do
  10000.times do
    begin
      mutex.synchronize do
        puts 'acquired'
        # sleep 0.01
        raise E if rand < 0.5
        puts 'releasing'
      end
    rescue E
      puts "interrupted"
    end

    puts "UNRELEASED MUTEX" if mutex.owned?
  end
end

t2 = Thread.new do
  1000.times do
    mutex.synchronize { sleep 0.01 }
    sleep 0.01
    t1.raise(E)
  end
end

t3 = Thread.new do
  1000.times do
    mutex.synchronize { sleep 0.01 }
    sleep 0.01
    t1.raise(E)
  end
end

t2.join
t3.join
```

I would expect `mutex.owned?` to always return `false` outside of the `synchronize { ... }` block, but when I run the above script, I see the following output:

```
; ruby tmp/testy.rb
acquired
interrupted
interrupted
UNRELEASED MUTEX
#<Thread:0x00005577aaa07860@tmp/testy.rb:4 run> terminated with exception (report_on_
exception is true):
Traceback (most recent call last):
        3: from tmp/testy.rb:5:in `block in <main>'
        2: from tmp/testy.rb:5:in `times'
        1: from tmp/testy.rb:7:in `block (2 levels) in <main>'
tmp/testy.rb:7:in `synchronize': deadlock; recursive locking (ThreadError)
```

I do not fully understand why this is possible, and it is possible there is a simpler example that would reproduce the issue. But it seems at least that it is necessary for two different threads to be running `Thread#raise` simultaneously.

Occasionally, especially if the timing of the `sleep` calls are tuned, the thread `t1` will display an stack trace for an error `E` - which I believe is the expected behavior in the case that the error is raised during its rescue block.

Thank you for your time!




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