Issue #16792 has been updated by ioquatix (Samuel Williams).


> Any example? Please don't claim such things if there is no example.

I'll have to defer to @Charles for examples, since he was the one that mentioned it. I can only assume he is telling the truth when he said some application code was broken.

The only reason to look at it is to check if it's a valid use case or not. If we can't find any valid use case, it confirms your assessment, which is a good thing.

> It doesn't.

I did play around with this a bit, and I also wondered if we should add it to Ruby spec before making changes.

https://github.com/ruby/ruby/pull/3032/files#diff-1a99f5621b805d95b67228708b652ff9R25

That is the current semantic and if we introduce non-blocking fiber, it can cause deadlock, as shown in the test, if you disable the mutex -> blocking relationship.



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Feature #16792: Make Mutex held per Fiber instead of per Thread
https://bugs.ruby-lang.org/issues/16792#change-85138

* Author: Eregon (Benoit Daloze)
* Status: Open
* Priority: Normal
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Currently, Mutex in CRuby is held per Thread.
In JRuby and TruffleRuby, Mutex is held per Fiber (because it's simply easier implementation-wise).

While a user could theoretically notice the difference, it seems extremely uncommon in practice (probably incorrect synchronization).

The usage pattern for a Mutex is using #synchronize or lock+unlock.
Such a pattern protects/surrounds a region of code using some resource, and such a region of code is always on the same Fiber since it's on a given Ruby "stack".

With #16786 it becomes more relevant to have Mutex held per Fiber, otherwise Mutex#lock will hurt scalability of that proposal significantly.
This means, if a Fiber does Mutex#lock and it's already held by another Fiber of the same Thread, and the Thread#scheduler is enabled, instead of just raising an error (which made sense before, because it would be a deadlock, but no longer the case with scheduler),
or disabling fiber scheduling entirely until #unlock (current state in #16786, makes Mutex#lock special and hurts scalability),
we would just go to the scheduler and schedule another Fiber (for instance, the one holding that Mutex, or any other ready to be run Fiber).

This is not a new idea and in fact Crystal already does this with its non-blocking Fibers, which is very similar with #16786:
https://github.com/crystal-lang/crystal/blob/612825a53c831ce7d17368c8211342b199ca02ff/src/mutex.cr#L72

Mutex#lock is just like other blocking operations, so let's make it so building on #16786.
I believe it's the natural and intuitive thing to do for Fiber concurrency with a scheduler.

Queue#pop and SizedQueue#push could be other candidates to handle in a similar way.

Here is an early commit to make Mutex held per Fiber, it's quite trivial as you can see:
https://github.com/ruby/ruby/compare/master...eregon:mutex-per-fiber
It passes test-all and test-spec.



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