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-rw-r--r--kernel/module.c35
1 files changed, 27 insertions, 8 deletions
diff --git a/kernel/module.c b/kernel/module.c
index 9f8d23d8b3a8..1016b75b026a 100644
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -521,11 +521,13 @@ static void module_unload_init(struct module *mod)
int cpu;
INIT_LIST_HEAD(&mod->modules_which_use_me);
- for_each_possible_cpu(cpu)
- per_cpu_ptr(mod->refptr, cpu)->count = 0;
+ for_each_possible_cpu(cpu) {
+ per_cpu_ptr(mod->refptr, cpu)->incs = 0;
+ per_cpu_ptr(mod->refptr, cpu)->decs = 0;
+ }
/* Hold reference count during initialization. */
- __this_cpu_write(mod->refptr->count, 1);
+ __this_cpu_write(mod->refptr->incs, 1);
/* Backwards compatibility macros put refcount during init. */
mod->waiter = current;
}
@@ -664,12 +666,28 @@ static int try_stop_module(struct module *mod, int flags, int *forced)
unsigned int module_refcount(struct module *mod)
{
- unsigned int total = 0;
+ unsigned int incs = 0, decs = 0;
int cpu;
for_each_possible_cpu(cpu)
- total += per_cpu_ptr(mod->refptr, cpu)->count;
- return total;
+ decs += per_cpu_ptr(mod->refptr, cpu)->decs;
+ /*
+ * ensure the incs are added up after the decs.
+ * module_put ensures incs are visible before decs with smp_wmb.
+ *
+ * This 2-count scheme avoids the situation where the refcount
+ * for CPU0 is read, then CPU0 increments the module refcount,
+ * then CPU1 drops that refcount, then the refcount for CPU1 is
+ * read. We would record a decrement but not its corresponding
+ * increment so we would see a low count (disaster).
+ *
+ * Rare situation? But module_refcount can be preempted, and we
+ * might be tallying up 4096+ CPUs. So it is not impossible.
+ */
+ smp_rmb();
+ for_each_possible_cpu(cpu)
+ incs += per_cpu_ptr(mod->refptr, cpu)->incs;
+ return incs - decs;
}
EXPORT_SYMBOL(module_refcount);
@@ -846,10 +864,11 @@ void module_put(struct module *module)
{
if (module) {
preempt_disable();
- __this_cpu_dec(module->refptr->count);
+ smp_wmb(); /* see comment in module_refcount */
+ __this_cpu_inc(module->refptr->decs);
trace_module_put(module, _RET_IP_,
- __this_cpu_read(module->refptr->count));
+ __this_cpu_read(module->refptr->decs));
/* Maybe they're waiting for us to drop reference? */
if (unlikely(!module_is_live(module)))
wake_up_process(module->waiter);