[ruby-core:127182] [Ruby Bug#22453] GET_EC() may break when the native thread switches under M:N threads

20 Oct 2026

      Issue #22453 has been updated by luke-gru (Luke Gruber).

Something like this should definitely work too, and it's arguably safer as it doesn't rely the compiler's optimizations.

```patch
+    // Under M:N threads the native thread can change in the middle of a C
+    // function. The compiler assumes it doesn't, so it may reuse a TLS address
+    // (e.g. the result of __tls_get_addr() with -fPIC) after a switch, which
+    // then points into another thread's TLS. Reading the variable entirely in
+    // asm volatile hides the TLS address from the compiler, so each read uses
+    // the current thread's %fs.
+    # if defined(__x86_64__) && defined(__ELF__)
+    static inline struct rb_execution_context_struct *
+    rb_current_ec_tls(void)
+    {
+        struct rb_execution_context_struct *ec;
+        __asm__ volatile("movq ruby_current_ec@gottpoff(%%rip), %0\n\t"
+                         "movq %%fs:(%0), %0"
+                         : "=r"(ec));
+        return ec;
+    }
+    # else
+    #  define rb_current_ec_tls() ruby_current_ec
+    # endif
+

-    rb_execution_context_t * volatile ec = ruby_current_ec;
+    rb_execution_context_t * volatile ec = rb_current_ec_tls();
```

----------------------------------------
Bug #22453: GET_EC() may break when the native thread switches under M:N threads
https://bugs.ruby-lang.org/issues/22453#change-119597

* Author: mame (Yusuke Endoh)
* Status: Open
* Assignee: ko1 (Koichi Sasada)
* Backport: 3.3: UNKNOWN, 3.4: UNKNOWN, 4.0: UNKNOWN
----------------------------------------
There are many bugs caused by the bad interaction between the M:N thread model and `GET_EC()`.

A simple example is code that switches the native thread inside a block of `String#split`.

```ruby
8.times.map do
  Ractor.new do
    50.times do
      ",,".split(/(,)/) { sleep 0.001 }
    end
    :ok
  end
end.map(&:value)
```

When Ruby is built with Clang 21, this causes a segmentation fault:

```
$ ./build-clang21/miniruby 31_string_split.rb
31_string_split.rb:2: warning: Ractor API is experimental and may change in future versions of Ruby.
31_string_split.rb:4: [BUG] Segmentation fault at 0x0000000000000030
ruby 4.1.0dev (2026-10-14T18:01:17Z master 90a3551c8a) +PRISM [x86_64-linux]
```

I had Claude Code analyze the problem, and it found 32 similar bugs (listed at the end). I believe there are many more.

## Cause

The cause is a bad interaction between C compiler optimizations and the M:N thread model.

A C compiler may optimize code assuming that the native thread does not change during the execution of a function.
For an access to a variable stored in Thread-local Storage (TLS), it may cache the pointer to the variable in the function and generate code that dereferences the cached pointer in later accesses.

On the other hand, under the M:N thread model, the native thread may change in the middle of the execution of a C function.
So, if the native thread switches after the pointer is cached, the pointer still points to the old TLS, and dereferencing it may cause a SEGV or other failures.

In the `String#split` example above, `str_alloc_embed` is inlined into `rb_str_split_m`, and the TLS address used by `GET_EC()` in it is cached.
Then, when `sleep 0.001` in the block of `split` switches the native thread, the invalid address is dereferenced.

This problem has already been reported in #20243, #21046, #21534, #22440 and others, but it has been addressed by fixing individual places or by workarounds for specific platforms.
However, with newer compilers such as GCC 16 and Clang 21, I have confirmed that it happens in many places on x86-64.

## How to fix

There are two options.

1. Avoid `GET_EC()` as much as possible and pass `ec` explicitly.
2. Make `rb_current_ec()` NOINLINE so that the compiler cannot cache the TLS address.

I think option 1 is not realistic because it requires rewriting a large amount of existing code.

Option 2 is already applied on arm64 and ppc64le (https://github.com/ruby/ruby/blob/61d31c4d2b956e1bf7fc09cced93ad20d9c5c81c/t...). I propose to apply it unconditionally.

PR: https://github.com/ruby/ruby/pull/19421

With this change, none of the 32 reproducers reproduce anymore.

## Performance

The concern with NOINLINE is performance, so I measured it.
Some cases get slower and others get faster. Overall, I think the impact is small.

Change in execution time from before to after the patch (median of 20 runs; negative means faster):

| Benchmark | GCC 16 | Clang 21 |
|---|---|---|
| alloc (`Object.new`) | -0.4% | +0.0% |
| method (method call) | -0.9% | +0.4% |
| block (`yield`) | -1.1% | -1.4% |
| cfunc (`Array#size` etc.) | -0.8% | +1.5% |

Environment: AMD Ryzen 9 6900HX, Ubuntu, source at 90a3551c8a, built with --enable-shared.

## Reproducers

### 1. `rb_f_raise`

```ruby
class E < StandardError
  def exception(*) = (sleep 0.001; self)
end
8.times.map { Ractor.new { 50.times { raise E.new rescue nil } } }.each(&:join)
#=> [BUG] Segmentation fault at 0x0000000000000010 (GCC 16)
```

### 2. `fiber_initialize`

```ruby
class H < Hash
  def initialize_copy(*) = (sleep 0.001; super)
end
8.times.map { Ractor.new { 50.times { Fiber.new(storage: H.new) {}.resume } } }.each(&:join)
#=> [BUG] Segmentation fault at 0x0000000000000030 (GCC 16)
```

### 3. `nurat_convert`

```ruby
class N
  def to_int = (sleep 0.001; 3)
end
8.times.map { Ractor.new { 25.times { Rational(N.new, N.new) } } }.each(&:join)
#=> [BUG] Segmentation fault at 0x0000000000000030 (GCC 16)
```

### 4. `f_complex_polar_real`

```ruby
class N < Numeric
  def real? = true
  def coerce(o) = [self, self]
  def to_f = 1.0
  def *(o) = (sleep 0.001; 2)
  def +(o) = 3
end
8.times.map { Ractor.new { 25.times { Complex.polar(N.new, 2) } } }.each(&:join)
#=> [BUG] Segmentation fault at 0x0000000000000030 (GCC 16)
```

### 5. `nurat_rationalize`

```ruby
class Eps < Numeric
  def real? = true
  def abs = self
  def zero? = false
  def coerce(o) = (sleep 0.001; [o, 1r / 1_000_000_000])
end
8.times.map { Ractor.new { 25.times { (3r / 7).rationalize(Eps.new) } } }.each(&:join)
#=> [BUG] Segmentation fault at 0x0000000000000030 (GCC 16)
```

### 6. `rb_complex_uminus`

```ruby
class A < Numeric
  def real? = true
  def -@ = self
  def -(o) = (sleep 0.001; 5)
end
class B < Numeric
  def real? = true
  def -@ = Complex(1, 2)
end
8.times.map { Ractor.new { 50.times { -Complex.rect(A.new, B.new) } } }.each(&:join)
#=> [BUG] Segmentation fault at 0x0000000000000030 (GCC 16)
```

### 7. `rb_complex_conjugate`

```ruby
class A < Numeric
  def real? = true
  def -@ = self
  def -(o) = (sleep 0.001; 5)
end
class B < Numeric
  def real? = true
  def -@ = Complex(1, 2)
end
8.times.map { Ractor.new { 50.times { Complex.rect(A.new, B.new).conjugate } } }.each(&:join)
#=> [BUG] Segmentation fault at 0x0000000000000030 (GCC 16)
```

### 8. `rb_complex_pow`

```ruby
class Z < Numeric
  def ==(o) = (sleep 0.001; true)
end
8.times.map { Ractor.new { 50.times { Complex(1, 2) ** Z.new } } }.each(&:join)
#=> [BUG] Segmentation fault at 0x0000000000000030 (GCC 16)
```

### 9. `rb_complex_pow (complex_pow_for_special_angle)`

```ruby
class N < Numeric
  def real? = true
  def <(o) = (sleep 0.001; true)
  def -@ = 3
end
8.times.map { Ractor.new { 50.times { Complex.rect(N.new, 0) ** 2 } } }.each(&:join)
#=> [BUG] Segmentation fault at 0x0000000000000030 (GCC 16)
```

### 10. `f_muldiv`

```ruby
4.times.map do |i|
  Ractor.new(i) do |i|
    q = Rational(7**3000 + i, 3**1500 + 1)
    t = Time.now
    2.times.map { Thread.new { q.round(5) while Time.now - t < 0.5 } }.each(&:join)
  end
end.each(&:join)
#=> [BUG] Segmentation fault at 0x0000000000000030 (GCC 16)
```

### 11. `exec_recursive`

```ruby
class Slow
  def inspect = (sleep 0.001; raise "boom")
end
8.times.map { Ractor.new { 50.times { [Slow.new].inspect rescue nil } } }.each(&:join)
#=> [BUG] Segmentation fault at 0x0000000000000010 (GCC 16, Clang 21)
```

### 12. `exec_recursive (outermost)`

```ruby
class SlowKey
  def hash = (sleep 0.001; raise "boom")
end
8.times.map { Ractor.new { h = {}; 50.times { (h[SlowKey.new] = 1) rescue nil } } }.each(&:join)
#=> [BUG] Segmentation fault at 0x0000000000000010 (GCC 16, Clang 21)
```

### 13. `rb_thread_sleep_deadly`

```ruby
class Ping < StandardError; end
8.times.map do
  Ractor.new do
    finished = false
    sleeper = Thread.new do
      until finished
        begin
          Thread.handle_interrupt(Object => :never) { Thread.stop }
        rescue Ping
        end
      end
    end
    Thread.new do
      t_end = Time.now + 0.05
      while Time.now < t_end
        5.times { sleeper.raise(Ping) rescue nil; sleep 0.0005 }
        sleeper.wakeup rescue nil
        sleep 0.002
      end
      finished = true
    end.join
    sleeper.wakeup rescue nil until sleeper.join(0.001)
  end
end.each(&:join)
#=> [BUG] Segmentation fault at 0x0000000000000030 (GCC 16)
```

### 14. `mutex_lock_uninterruptible`

```ruby
class Ping < StandardError; end
8.times.map do
  Ractor.new do
    m = Mutex.new
    cv = ConditionVariable.new
    finished = false
    sleeper = Thread.new do
      until finished
        begin
          Thread.handle_interrupt(Object => :never) { m.synchronize { cv.wait(m) } }
        rescue Ping
        end
      end
    end
    Thread.new do
      t_end = Time.now + 0.05
      while Time.now < t_end
        m.synchronize do
          cv.signal
          5.times { sleeper.raise(Ping) rescue nil; sleep 0.0005 }
        end
        sleep 0.002
      end
      finished = true
    end.join
    m.synchronize { cv.broadcast } until sleeper.join(0.001)
  end
end.each(&:join)
#=> [BUG] Segmentation fault at 0x0000000000000030 (GCC 16)
```

### 15. `gc_finalize_deferred`

```ruby
8.times.map do
  Ractor.new do
    10.times do
      4.times { ObjectSpace.define_finalizer(Object.new, proc { sleep 0.0005 }) }
      GC.start
      sleep 0.0001
    end
  end
end.each(&:join)
#=> [BUG] Segmentation fault at 0x0000000000000024 (GCC 16, Clang 21)
```

### 16. `rb_str_times`

```ruby
class N
  def to_int = (sleep 0.001; 3)
end
8.times.map { Ractor.new { 50.times { "ab" * N.new } } }.each(&:join)
#=> [BUG] Segmentation fault at 0x0000000000000010 (GCC 16, Clang 21)
```

### 17. `rb_const_search_from`

```ruby
50.times do |i|
  path = File.expand_path("nt_switch_autoload_#{i}.rb")
  File.write(path, "sleep 0.001\n") # does not define the constant
  autoload :"C#{i}", path
end
8.times.map { Ractor.new { 50.times { |i| Object.const_get(:"C#{i}") rescue nil } } }.each(&:join)
#=> [BUG] Segmentation fault at 0x0000000000000098 (GCC 16)
```

### 18. `rb_to_int`

```ruby
class N
  def to_int = (sleep 0.001; 3)
end
8.times.map { Ractor.new { 50.times { [1, 2, 3, 4].first(N.new) } } }.each(&:join)
#=> [BUG] Segmentation fault at 0x0000000000000010 (GCC 16, Clang 21)
```

### 19. `rb_check_integer_type`

```ruby
class N
  def to_int = (sleep 0.001; 3)
end
8.times.map { Ractor.new { 50.times { "abc".byteslice(N.new) } } }.each(&:join)
#=> [BUG] Segmentation fault at 0x0000000000000010 (GCC 16, Clang 21)
```

### 20. `rb_f_integer1`

```ruby
class N
  def to_i = (sleep 0.001; 3)
end
8.times.map { Ractor.new { 50.times { Integer(N.new) } } }.each(&:join)
#=> [BUG] Segmentation fault at 0x0000000000000010 (GCC 16, Clang 21)
```

### 21. `nucomp_s_new`

```ruby
class N < Numeric
  def real? = (sleep 0.001; true)
end
8.times.map { Ractor.new { 50.times { Complex.rect(N.new) rescue nil } } }.each(&:join)
#=> [BUG] Segmentation fault at 0x0000000000000030 (GCC 16)
```

### 22. `nucomp_s_polar`

```ruby
class N < Numeric
  def real? = (sleep 0.001; true)
end
8.times.map { Ractor.new { 50.times { Complex.polar(N.new) rescue nil } } }.each(&:join)
#=> [BUG] Segmentation fault at 0x0000000000000030 (GCC 16)
```

### 23. `detach_process_watcher`

```ruby
8.times.map do
  Ractor.new do
    10.times do
      ths = 2.times.map { Process.detach(Process.spawn("sleep", "0.001")) }
      ths.each { raise "bad #{it.value.inspect}" unless Process::Status === it.value }
    end
  end
end.each(&:join)
#=> [BUG] Segmentation fault at 0x0000000000000030 (Clang 21)
```

### 24. `proc_waitall`

```ruby
8.times.map do
  Ractor.new do
    12.times do
      4.times { Process.spawn("sleep", "0.001") }
      2.times.map { Thread.new { Process.waitall } }.each(&:join)
    end
  end
end.each(&:join)
#=> [BUG] Segmentation fault at 0x0000000000000030 (Clang 21)
```

### 25. `rb_thread_io_close_wait`

```ruby
12.times.map do
  Ractor.new do
    100.times do
      r, w = IO.pipe
      ths = 8.times.map { Thread.new { r.read rescue nil } }
      Thread.pass until ths.all? { it.status == "sleep" }
      r.close
      ths.each(&:join)
      w.close
    end
  end
end.each(&:join)
#=> [BUG] Segmentation fault at 0x0000000000000030 (Clang 21)
```

### 26. `rb_error_frozen_object`

```ruby
class F
  def self.to_s = (sleep 0.001; "F")
  def self.inspect = (sleep 0.001; "F")
end
8.times.map do
  Ractor.new do
    50.times do
      F.new.freeze.instance_variable_set(:@a, 1)
    rescue FrozenError
    end
  end
end.each(&:join)
#=> [BUG] Segmentation fault at 0x0000000000000010 (Clang 21)
```

### 27. `rb_ary_permutation`

```ruby
8.times.map { Ractor.new { 12.times { n = 0; [*0...200].permutation(200) { sleep 0.001; break if (n += 1) > 5 } } } }.each(&:join)
#=> [BUG] Segmentation fault at 0x0000000000000030 (Clang 21)
```

### 28. `rb_ary_repeated_combination`

```ruby
8.times.map { Ractor.new { 12.times { n = 0; [1, 2].repeated_combination(200) { sleep 0.001; break if (n += 1) > 5 } } } }.each(&:join)
#=> [BUG] Segmentation fault at 0x0000000000000030 (Clang 21)
```

### 29. `rb_ary_repeated_permutation`

```ruby
8.times.map { Ractor.new { 12.times { n = 0; [1, 2].repeated_permutation(200) { sleep 0.001; break if (n += 1) > 5 } } } }.each(&:join)
#=> [BUG] Segmentation fault at 0x0000000000000030 (Clang 21)
```

### 30. `rb_ary_transpose`

```ruby
class A
  def initialize(slow) = @slow = slow
  def to_ary = (sleep 0.001 if @slow; [1, 2, 3])
end
8.times.map { Ractor.new { 2.times { Array.new(200) { A.new(it == 0) }.transpose } } }.each(&:join)
#=> [BUG] Segmentation fault at 0x0000000000000030 (Clang 21)
```

### 31. `rb_str_split_m`

```ruby
8.times.map { Ractor.new { 12.times { ",,".split(/(,)/) { sleep 0.001 } } } }.each(&:join)
#=> [BUG] Segmentation fault at 0x0000000000000030 (Clang 21)
```

### 32. `ruby_float_step`

```ruby
8.times.map { Ractor.new { 12.times { 1e300.step(2e300, 2e299) { sleep 0.001 } } } }.each(&:join)
#=> [BUG] Segmentation fault at 0x0000000000000030 (Clang 21)
```

-- 
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