[PATCH v2 5/5] module: Remove stop_machine from module unloading

Masami Hiramatsu
2014-10-23T11:30:59Z

Remove stop_machine from module unloading by adding new reference
counting algorithm.

This atomic refcounter works like a semaphore, it can get (be
incremented) only when the counter is not 0. When loading a module,
kmodule subsystem sets the counter MODULE_REF_BASE (= 1). And when
unloading the module, it subtracts MODULE_REF_BASE from the counter.
If no one refers the module, the refcounter becomes 0 and we can
remove the module safely. If someone referes it, we try to recover
the counter by adding MODULE_REF_BASE unless the counter becomes 0,
because the referrer can put the module right before recovering.
If the recovering is failed, we can get the 0 refcount and it
never be incremented again, it can be removed safely too.

Note that __module_get() forcibly gets the module refcounter,
users should use try_module_get() instead of that.

Signed-off-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
---
kernel/module.c | 67 ++++++++++++++++++++++++++++++++-----------------------
1 file changed, 39 insertions(+), 28 deletions(-)

diff --git a/kernel/module.c b/kernel/module.c
index b1d485d..e772595 100644
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -42,7 +42,6 @@
#include <linux/vermagic.h>
#include <linux/notifier.h>
#include <linux/sched.h>
-#include <linux/stop_machine.h>
#include <linux/device.h>
#include <linux/string.h>
#include <linux/mutex.h>
@@ -98,7 +97,7 @@
* 1) List of modules (also safely readable with preempt_disable),
* 2) module_use links,
* 3) module_addr_min/module_addr_max.
- * (delete uses stop_machine/add uses RCU list operations). */
+ * (delete and add uses RCU list operations). */
DEFINE_MUTEX(module_mutex);
EXPORT_SYMBOL_GPL(module_mutex);
static LIST_HEAD(modules);
@@ -628,14 +627,23 @@ static char last_unloaded_module[MODULE_NAME_LEN+1];

EXPORT_TRACEPOINT_SYMBOL(module_get);

+/* MODULE_REF_BASE is the base reference count by kmodule loader. */
+#define MODULE_REF_BASE 1
+
/* Init the unload section of the module. */
static int module_unload_init(struct module *mod)
{
+ /*
+ * Initialize reference counter to MODULE_REF_BASE.
+ * refcnt == 0 means module is going.
+ */
+ atomic_set(&mod->refcnt, MODULE_REF_BASE);
+
INIT_LIST_HEAD(&mod->source_list);
INIT_LIST_HEAD(&mod->target_list);

/* Hold reference count during initialization. */
- atomic_set(&mod->refcnt, 1);
+ atomic_inc(&mod->refcnt);

return 0;
}
@@ -734,39 +742,39 @@ static inline int try_force_unload(unsigned int flags)
}
#endif /* CONFIG_MODULE_FORCE_UNLOAD */

-struct stopref
+/* Try to release refcount of module, 0 means success. */
+static int try_release_module_ref(struct module *mod)
{
- struct module *mod;
- int flags;
- int *forced;
-};
+ int ret;

-/* Whole machine is stopped with interrupts off when this runs. */
-static int __try_stop_module(void *_sref)
-{
- struct stopref *sref = _sref;
+ /* Try to decrement refcnt which we set at loading */
+ ret = atomic_sub_return(MODULE_REF_BASE, &mod->refcnt);
+ BUG_ON(ret < 0);
+ if (ret)
+ /* Someone can put this right now, recover with checking */
+ ret = atomic_add_unless(&mod->refcnt, MODULE_REF_BASE, 0);
+
+ return ret;
+}

+static int try_stop_module(struct module *mod, int flags, int *forced)
+{
/* If it's not unused, quit unless we're forcing. */
- if (module_refcount(sref->mod) != 0) {
- if (!(*sref->forced = try_force_unload(sref->flags)))
+ if (try_release_module_ref(mod) != 0) {
+ *forced = try_force_unload(flags);
+ if (!(*forced))
return -EWOULDBLOCK;
}

/* Mark it as dying. */
- sref->mod->state = MODULE_STATE_GOING;
- return 0;
-}
-
-static int try_stop_module(struct module *mod, int flags, int *forced)
-{
- struct stopref sref = { mod, flags, forced };
+ mod->state = MODULE_STATE_GOING;

- return stop_machine(__try_stop_module, &sref, NULL);
+ return 0;
}

unsigned long module_refcount(struct module *mod)
{
- return (unsigned long)atomic_read(&mod->refcnt);
+ return (unsigned long)atomic_read(&mod->refcnt) - MODULE_REF_BASE;
}
EXPORT_SYMBOL(module_refcount);

@@ -921,11 +929,11 @@ bool try_module_get(struct module *module)

if (module) {
preempt_disable();
-
- if (likely(module_is_live(module))) {
- atomic_inc(&module->refcnt);
+ /* Note: here, we can fail to get a reference */
+ if (likely(module_is_live(module) &&
+ atomic_inc_not_zero(&module->refcnt) != 0))
trace_module_get(module, _RET_IP_);
- } else
+ else
ret = false;

preempt_enable();
@@ -936,9 +944,12 @@ EXPORT_SYMBOL(try_module_get);

void module_put(struct module *module)
{
+ int ret;
+
if (module) {
preempt_disable();
- atomic_dec(&module->refcnt);
+ ret = atomic_dec_if_positive(&module->refcnt);
+ WARN_ON(ret < 0); /* Failed to put refcount */
trace_module_put(module, _RET_IP_);
preempt_enable();
}