[Guile-commits] 33/58: Module import obarrays are accessed in a critical section.

[Andy Wingo]

wingo pushed a commit to branch lightning
in repository guile.

commit edc80bd595a442d059098fd6b605bbe1aa7e81ca
Author: Ludovic Courtès <address@hidden>
Date:   Mon Jun 18 17:39:35 2018 +0200

    Module import obarrays are accessed in a critical section.
    
    Fixes <https://bugs.gnu.org/31879>.
    
    * libguile/modules.c (import_obarray_mutex): New variable.
    (resolve_duplicate_binding, module_imported_variable): Acquire it before
    accessing an obarray.
---
 libguile/modules.c | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/libguile/modules.c b/libguile/modules.c
index 751d907..b0327fc 100644
--- a/libguile/modules.c
+++ b/libguile/modules.c
@@ -275,6 +275,13 @@ default_duplicate_binding_handlers (void)
   return (scm_call_0 (get_handlers));
 }
 
+/* Each module has an "import obarray" that may be accessed concurrently
+   by several threads.  This mutex protects access to any obarray.  This
+   is coarse-grain but (1) pthread mutexes are quite cheap, and (2)
+   Scheme "programs" have a cache for free variables anyway.  */
+static scm_i_pthread_mutex_t import_obarray_mutex =
+  SCM_I_PTHREAD_MUTEX_INITIALIZER;
+
 /* Resolve the import of SYM in MODULE, where SYM is currently provided by
    both IFACE1 as VAR1 and IFACE2 as VAR2.  Return the variable chosen by the
    duplicate binding handlers or `#f'.  */
@@ -300,7 +307,11 @@ resolve_duplicate_binding (SCM module, SCM sym,
   args[5] = SCM_VARIABLE_REF (var2);
   if (SCM_UNBNDP (args[5]))
     args[5] = SCM_BOOL_F;
+
+  scm_i_pthread_mutex_lock (&import_obarray_mutex);
   args[6] = scm_hashq_ref (SCM_MODULE_IMPORT_OBARRAY (module), sym, 
SCM_BOOL_F);
+  scm_i_pthread_mutex_unlock (&import_obarray_mutex);
+
   args[7] = SCM_BOOL_F;
       
   handlers = SCM_MODULE_DUPLICATE_HANDLERS (module);
@@ -338,7 +349,11 @@ module_imported_variable (SCM module, SCM sym)
 
   /* Search cached imported bindings.  */
   imports = SCM_MODULE_IMPORT_OBARRAY (module);
+
+  scm_i_pthread_mutex_lock (&import_obarray_mutex);
   var = scm_hashq_ref (imports, sym, SCM_UNDEFINED);
+  scm_i_pthread_mutex_unlock (&import_obarray_mutex);
+
   if (SCM_BOUND_THING_P (var))
     return var;
 
@@ -386,7 +401,9 @@ module_imported_variable (SCM module, SCM sym)
     if (SCM_BOUND_THING_P (found_var))
       {
        /* Save the lookup result for future reference.  */
+        scm_i_pthread_mutex_lock (&import_obarray_mutex);
        (void) scm_hashq_set_x (imports, sym, found_var);
+        scm_i_pthread_mutex_unlock (&import_obarray_mutex);
        return found_var;
       }
   }