Dear thread lovers,
The code below spawns a bunch of threads that look up imported bindings
in a module. It usually enters an infinite loop and possibly eats all
your memory (on Guile 2.2.3):
--8<---------------cut here---------------start------------->8---
(use-modules (ice-9 threads)
(ice-9 match)
(srfi srfi-1))
(define bindings
;; Bindings exported by (guile).
'())
(module-for-each (lambda (name var)
(set! bindings (cons name bindings)))
(resolve-module '(guile)))
(define thread-count 8)
(define bindings-per-thread
(floor (/ (length bindings) thread-count)))
(define iface
(resolve-module '(ice-9 q)))
(define threads
(unfold (lambda (x) (>= x thread-count))
(lambda (n)
(call-with-new-thread
(lambda ()
(let loop ((bindings (take (drop bindings
(* n bindings-per-thread))
bindings-per-thread)))
(match bindings
(() #t)
((head . tail)
(module-variable iface head)
(loop tail)))))))
1+
0))
(for-each join-thread threads)
(pk 'done thread-count)
--8<---------------cut here---------------end--------------->8---
The issue lies in the “import obarray”, which is used as a cache and is
accessed in a non-thread-safe manner:
--8<---------------cut here---------------start------------->8---
static inline SCM
module_imported_variable (SCM module, SCM sym)
{
#define SCM_BOUND_THING_P scm_is_true
register SCM var, imports;
/* Search cached imported bindings. */
imports = SCM_MODULE_IMPORT_OBARRAY (module);
var = scm_hashq_ref (imports, sym, SCM_UNDEFINED);
if (SCM_BOUND_THING_P (var))
return var;
[...]
if (SCM_BOUND_THING_P (found_var))
{
/* Save the lookup result for future reference. */
(void) scm_hashq_set_x (imports, sym, found_var);
return found_var;
}
--8<---------------cut here---------------end--------------->8---
Possible solutions:
1. Add a field in the module record containing a mutex. Downside is
that this breaks the ABI, though I’m not sure how much of a problem
it is.
2. Make the import obarray a weak hash table since they are
thread-safe in 2.2. This should be semantically equivalent to
using a hash table provided interned symbols are not GC’d.
Thoughts?
Ludo’.
