From: Hiroshi Nakamura <nahi@...>
Date: 2011-11-19T08:24:00+09:00
Subject: [ruby-core:41144] Re: [Backport93 - Backport #5621] Please backport thread-safe autoloading patch

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(2011/11/19 6:31), Charles Oliver Nutter wrote:
>> Sure.  This deadlock from cross-require is intentional at this
>> moment I believe, and that's the reason for the warning 'loading
>> in progress, circular require considered harmful'.  Related
>> discussion is at #920.
>>=20
>> I don't know how Java's classloader avoids this problem.
>=20
> Each classloader has a single lock, and if it needs to call a
> parent classloader the locks are still always acquired in the same
> order (meaning deadlock is impossible).

1) No parallel loading in single classloader
   (single lock, disallowing concurrent loading)

2) Classloaders know class loading order at compile time to avoid
   deadlock

Am I right?

> I don't see how it can be fixed if concurrent requires are
> allowed, but individual files have their own locks. If there's ever
> any potential for two different locks to be acquired by two threads
> in opposing order, it can deadlock.

Indeed.  Warning would be the best we can do.

> You're right, though...it's important to point out Yehuda's problem
> is not an autoload problem. Autoload thread-safety itself is
> achievable. Require thread-safety doesn't exist even today. That's
> the real problem.

Sure.  Moreover, I think that require thread-safety could not be
solved easily in Ruby, so I want to find a different way to solve
Yehuda's real problem.


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