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

From: [email-masked] (Ludovic Courtès)
Date: Thu, 23 Aug 2018 15:54:00 +0200

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

> I forgot to mention an important aspect of the proposed auto-load
> locking here.  It would not be a global lock, but rather a lock specific
> to the module being loaded.  So, I guess we would need a global table of
> locks for modules-being-loaded.

Right (we can’t add a mutex to the module record without breaking the
ABI.)

> 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?

Thanks,
Ludo’.