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Narrow deadlock check for sibling fibers locking mutex #9226

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Merged
headius merged 2 commits into from
Feb 10, 2026
Merged

Narrow deadlock check for sibling fibers locking mutex #9226

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0cb6ddb
Narrow deadlock check for fibers and fix message
headius Feb 10, 2026
a418dd9
Add specs for sibling fibers deadlock on mutex lock
headius Feb 10, 2026
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7 changes: 4 additions & 3 deletions core/src/main/java/org/jruby/ext/thread/Mutex.java
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Original file line number Diff line number Diff line change
Expand Up @@ -106,7 +106,8 @@ public IRubyObject lock(ThreadContext context) {

checkRelocking(context);

if (this.lockingThread == parentThread) {
RubyThread lockingThread = this.lockingThread;
if (lockingThread == parentThread && context.getFiber() != lockingThread.getContext().getFiber()) {
throw context.runtime.newThreadError("deadlock; lock already owned by another fiber belonging to the same thread");
}

Expand All @@ -123,7 +124,7 @@ public IRubyObject lock(ThreadContext context) {
}
}

this.lockingThread = thread;
this.lockingThread = parentThread;

// always check for thread interrupts after acquiring lock
try {
Expand Down Expand Up @@ -204,7 +205,7 @@ public IRubyObject owned_p(ThreadContext context) {

private void checkRelocking(ThreadContext context) {
if (lock.isHeldByCurrentThread()) {
throw context.runtime.newThreadError("Mutex relocking by same thread");
throw context.runtime.newThreadError("deadlock; recursive locking");
}
}

Expand Down
25 changes: 23 additions & 2 deletions spec/ruby/core/mutex/lock_spec.rb
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Original file line number Diff line number Diff line change
Expand Up @@ -20,11 +20,32 @@

# Unable to find a specific ticket but behavior change may be
# related to this ML thread.
it "raises a ThreadError when used recursively" do
it "raises a deadlock ThreadError when used recursively" do
m = Mutex.new
m.lock
-> {
m.lock
}.should raise_error(ThreadError)
}.should raise_error(ThreadError, /deadlock/)
end

it "raises a deadlock ThreadError when multiple fibers from the same thread try to lock" do
m = Mutex.new

m.lock
f0 = Fiber.new do
m.lock
end
-> { f0.resume }.should raise_error(ThreadError, /deadlock/)

m.unlock
f1 = Fiber.new do
m.lock
Fiber.yield
end
f2 = Fiber.new do
m.lock
end
f1.resume
-> { f2.resume }.should raise_error(ThreadError, /deadlock/)
end
end
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