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


I have played around with this and I see the most basic operation is a way to tell the scheduler that a fiber can make progress. We can use a generic approach - in a loop, you may call `Fiber.yield`. When that fiber is ready to proceed (e.g. mutex is unlocked), you need to notify scheduler, e.g. either of the following interfaces:

```
class Scheduler
  # Inform the scheduler that the fiber can be resumed. If urgent, the scheduler will wake up as soon as possible.
  # @parameter fiber [Object] Must respond to `#resume`.
  def ready(fiber, urgent = false)
  end
  
  # Execute the block in the run loop. If urgent, the scheduler will wake up as soon as possible.
  def invoke(urgent = false, &block)
  end
end
```

Also, maybe this is crazy idea, but in order to unify the interface, maybe we should introduce `Fiber#call` as an alias for `Fiber#resume` (or `Fiber#transfer`). That way, we could make it valid to substitute a proc for a fiber.

----------------------------------------
Feature #16792: Make Mutex held per Fiber instead of per Thread
https://bugs.ruby-lang.org/issues/16792#change-87084

* Author: Eregon (Benoit Daloze)
* Status: Open
* Priority: Normal
----------------------------------------
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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