From: "jhawthorn (John Hawthorn) via ruby-core" <ruby-core@...>
Date: 2026-10-06T23:00:06+00:00
Subject: [ruby-core:126944] [Ruby Bug#19172] `ruby_thread_has_gvl_p` is innacurate sometimes -- document or change?

Issue #19172 has been updated by jhawthorn (John Hawthorn).


Eregon (Benoit Daloze) wrote in #note-4:
> So now we have a public C API with known incorrect semantics when used in a signal handler, what should we do then?

This public API is not documented as async-signal-safe, so there is no issue here. Calling `ruby_thread_has_gvl_p() ` or almost any other function in this context is invalid. The list of stdlib functions which are valid to call in this context is tiny, and the same is true of Ruby: only rb_postponed_job_*, rb_internal_thread_specific_{get,set}, and rb_close_before_exec are async signal safe (I use rb_profile_frames in vernier/stackprof as if it's async-signal-safe, but it isn't officially and there's definitely been issues).

I don't think this is a bug and should be closed.

----------------------------------------
Bug #19172: `ruby_thread_has_gvl_p` is innacurate sometimes -- document or change?
https://bugs.ruby-lang.org/issues/19172#change-119336

* Author: ivoanjo (Ivo Anjo)
* Status: Assigned
* Assignee: ko1 (Koichi Sasada)
* ruby -v: (All Ruby versions)
* Backport: 2.7: UNKNOWN, 3.0: UNKNOWN, 3.1: UNKNOWN
----------------------------------------
Howdy ����! I work for Datadog [on the ddtrace gem](https://github.com/DataDog/dd-trace-rb) and I found a... sharp edge on the internal `ruby_thread_has_gvl_p` API.

I am aware that `ruby_thread_has_gvl_p` is documented an experimental API that is exported as a symbol but not present on the VM include files.

### Background

In the ddtrace profiling component, we setup a signal handler and then periodically send SIGPROF signals to try to interrupt the running Ruby thread (e.g. the thread that is holding the global VM lock or equivalent).

In the signal handler, we need to perform some API calls which are not safe to do without the GVL. So we need to check if the signal handler got called in the thread that has the GVL.

### The issue

```c
int
ruby_thread_has_gvl_p(void)
{
    rb_thread_t *th = ruby_thread_from_native();

    if (th && th->blocking_region_buffer == 0) {
        return 1;
    }
    else {
        return 0;
    }
}
```

In its current implementation, `ruby_thread_has_gvl_p` only checks if the thread has a `blocking_region_buffer` or not. Unfortunately, this means that when called from a thread that lost the GVL but not due to blocking (e.g. via `rb_thread_schedule()`), it can still claim that a thread is holding the GVL when that is not the case.

I ran into this issue in https://github.com/DataDog/dd-trace-rb/pull/2415, and needed to find a workaround.

### Next steps

Since this is an internal VM API, I'm not sure you'd want to change the current behavior, so I was thinking of perhaps two options:

* Is it worth changing `ruby_thread_has_gvl_p` to be accurate in the case I've listed?

* If not, would you accept a PR to document its current limitations, so that others don't run into the same issue I did?



-- 
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