From: thedarkone2@...
Date: 2015-07-31T04:59:11+00:00
Subject: [ruby-core:70197] [Ruby trunk - Bug #10892] Deadlock in autoload

Issue #10892 has been updated by Vit Z.

File 0001-load.c-unlock-the-new-shield.patch added

That broken rubyspec was written by me. The problem lies with repeatedly `autoload`ing the same `.rb` file, since this should be impossible, the spec manually deletes the loaded path from `$LOADED_FEATURES` and then re-declares the `autoload`, this is currently broken on MRI.

Here's a much smaller repro script:

~~~
def with_autoload_file(const_name, file_name = 'foo.rb')
  mangled_file_name = file_name.sub(/\.rb\Z/, '____temp____autoload.rb') # avoid accidentally overwriting any files
  File.write(mangled_file_name, "sleep 1; module #{const_name}; end")
  autoload const_name, File.expand_path(mangled_file_name.sub(/\.rb\Z/, ''))
  $LOADED_FEATURES.delete(File.expand_path(mangled_file_name)) if $LOADED_FEATURES.include?(File.expand_path(mangled_file_name))
  yield
ensure
  File.delete(mangled_file_name)
end

foo_ready = bar_waiting = bar_ready = false
t = Thread.new do
  Thread.pass until foo_ready
  Foo
  bar_waiting = true
  Thread.pass until bar_ready
  Bar
end

with_autoload_file('Foo') do
  foo_ready = true
  Foo
end

Thread.pass until bar_waiting

with_autoload_file('Bar') do
  bar_ready = true
  Bar
end

t.join
~~~

Running this results in an "`uninitialized constant Bar`" exception from the non-main thread.

If the last block is rearranged like this:

~~~
with_autoload_file('Bar') do
  Bar
  bar_ready = true
end
~~~

the script deadlocks (main thread deadlocks, while secondary thread `t` busy spins in `Thread.pass until bar_ready`).

If the last `autoload` block uses a different `.rb` file, everything works fine:

~~~
with_autoload_file('Bar', 'bar.rb') do
  Bar
  bar_ready = true
end
~~~

I think I've tracked the issue to an incorrectly locked `load_lock`'s `thread_shield`: when `rb_thread_shield_wait()` returns `Qfalse` the failed thread creates a new `thread_shield` via `rb_thread_shield_new()`, however because `rb_thread_shield_new()` automatically locks the newly created shield and the branch does not return a successful `ftptr`, the newly installed shield is then never unlocked.

The attached patch seems to fix the issue for me.

----------------------------------------
Bug #10892: Deadlock in autoload
https://bugs.ruby-lang.org/issues/10892#change-53621

* Author: Benoit Daloze
* Status: Open
* Priority: Normal
* Assignee: 
* ruby -v: ruby 2.3.0dev (2015-02-23 trunk 49693) [x86_64-linux]
* Backport: 2.0.0: UNKNOWN, 2.1: UNKNOWN, 2.2: UNKNOWN
----------------------------------------
Updating to recent RubySpec seems to show a bug under concurrent autoload.
I attach the extracted logic to reproduce.

At me, the script ends with either, in most cases,

    autoload_bug.rb:105:in `value': No live threads left. Deadlock? (fatal)
    	from autoload_bug.rb:105:in `map'
    	from autoload_bug.rb:105:in `<main>'

Or:

    autoload_bug.rb:95:in `const_get': uninitialized constant Mod1 (NameError)
    	from autoload_bug.rb:95:in `block (3 levels) in <main>'
    	from autoload_bug.rb:86:in `each'
    	from autoload_bug.rb:86:in `block (2 levels) in <main>'

Which both seem incorrect behavior.
All versions from 2.0 seem affected, and 1.9.3 behavior seems wrong but differently.

Could someone confirm this is a bug?
Is it likely to be fixed?

---Files--------------------------------
autoload_bug.rb (2.18 KB)
0001-load.c-unlock-the-new-shield.patch (1005 Bytes)


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