Subject: Re: bug#31878: Module autoloading is not thread safe

From: [email-masked] (Ludovic Courtès)
Date: Fri, 24 Aug 2018 10:45:57 +0200

Hi Mark,

Mark H Weaver <[email-masked]> skribis:

> [email-masked] (Ludovic Courtès) writes:
>
>> Mark H Weaver <[email-masked]> skribis:
>>
>>> Since Guile (unfortunately) allows cyclic module dependencies, we would
>>> need a mechanism to avoid deadlocks in case modules A and B both import
>>> each other, and two threads concurrently attempt to load those modules.
>>>
>>> The first idea that comes to mind is to also have a global structure
>>> storing a partial order on the modules currently being loaded.  If,
>>> while module A is being loaded, there's an attempt to auto-load module
>>> B, then an entry (A < B) would added to the partial order.  The partial
>>> order would not allow cycles to be introduced, reporting an error in
>>> that case.  In case a cycle would be introduced when adding (A < B),
>>> then the thread would simply be given access to the partially-loaded
>>> module B, by adding B to its local list of modules-being-loaded.
>>
>> Would it enough to (1) use recursive mutexes, and (2) have
>> ‘resolve-module’ lookup modules first in the global name space, and
>> second in the local list of modules being loaded?
>
> Item (2) above is something that I had already envisioned in my
> proposal, although I neglected to mention it.
>
> However, I don't see how recursive mutexes would help here, or how they
> could obviate the need for the other mechanisms I described above.
>
> Suppose module A and module B are mutually dependent on each other.  If
> thread 1 is loading module A concurrently with thread 2 loading module
> B, then thread 1 will be the only thread with access to module A (via
> thread 1's local list) and will hold the lock on it, and similarly for
> thread 2 and module B.
>
> Now, when thread 1 tries to load module B (while it's in the process of
> loading module A), it should normally be blocked until module B is
> finished loading.  If those modules were _not_ mutually dependent on
> each other, we should insist on thread 1 waiting for module B to finish
> loading before gaining access to it.  Only if there is a cyclic
> dependency should it be granted access to the partially-loaded module.
>
> If we simply use recursive mutexes, I think deadlock would occur in this
> case.  Thread 1 would try to grab the lock on module B, which is already
> held by thread 2, and vice versa.  Since it's not self-held, I fail to
> see the relevance of the recursive mutex.

Oh, got it; you’re right.  So yes, the solution you outlined above is
probably what’s needed.

Thanks for explaining!

Ludo’.