[PATCH v2 4/5] module: Replace module_ref with atomic_t refcnt

Masami Hiramatsu
2014-10-23T11:30:52Z

Replace module_ref per-cpu complex reference counter with
an atomic_t simple refcnt. This is for code simplification.

Signed-off-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
---
include/linux/module.h | 16 +---------------
include/trace/events/module.h | 2 +-
kernel/module.c | 39 +++++----------------------------------
3 files changed, 7 insertions(+), 50 deletions(-)

diff --git a/include/linux/module.h b/include/linux/module.h
index 71f282a..ebfb0e1 100644
--- a/include/linux/module.h
+++ b/include/linux/module.h
@@ -210,20 +210,6 @@ enum module_state {
MODULE_STATE_UNFORMED, /* Still setting it up. */
};

-/**
- * struct module_ref - per cpu module reference counts
- * @incs: number of module get on this cpu
- * @decs: number of module put on this cpu
- *
- * We force an alignment on 8 or 16 bytes, so that alloc_percpu()
- * put @incs/@decs in same cache line, with no extra memory cost,
- * since alloc_percpu() is fine grained.
- */
-struct module_ref {
- unsigned long incs;
- unsigned long decs;
-} __attribute((aligned(2 * sizeof(unsigned long))));
-
struct module {
enum module_state state;

@@ -367,7 +353,7 @@ struct module {
/* Destruction function. */
void (*exit)(void);

- struct module_ref __percpu *refptr;
+ atomic_t refcnt;
#endif

#ifdef CONFIG_CONSTRUCTORS
diff --git a/include/trace/events/module.h b/include/trace/events/module.h
index 7c5cbfe..81c4c18 100644
--- a/include/trace/events/module.h
+++ b/include/trace/events/module.h
@@ -80,7 +80,7 @@ DECLARE_EVENT_CLASS(module_refcnt,

TP_fast_assign(
__entry->ip = ip;
- __entry->refcnt = __this_cpu_read(mod->refptr->incs) - __this_cpu_read(mod->refptr->decs);
+ __entry->refcnt = atomic_read(&mod->refcnt);
__assign_str(name, mod->name);
),

diff --git a/kernel/module.c b/kernel/module.c
index d596a30..b1d485d 100644
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -631,15 +631,11 @@ EXPORT_TRACEPOINT_SYMBOL(module_get);
/* Init the unload section of the module. */
static int module_unload_init(struct module *mod)
{
- mod->refptr = alloc_percpu(struct module_ref);
- if (!mod->refptr)
- return -ENOMEM;
-
INIT_LIST_HEAD(&mod->source_list);
INIT_LIST_HEAD(&mod->target_list);

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

return 0;
}
@@ -721,8 +717,6 @@ static void module_unload_free(struct module *mod)
kfree(use);
}
mutex_unlock(&module_mutex);
-
- free_percpu(mod->refptr);
}

#ifdef CONFIG_MODULE_FORCE_UNLOAD
@@ -772,28 +766,7 @@ static int try_stop_module(struct module *mod, int flags, int *forced)

unsigned long module_refcount(struct module *mod)
{
- unsigned long incs = 0, decs = 0;
- int cpu;
-
- for_each_possible_cpu(cpu)
- 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;
+ return (unsigned long)atomic_read(&mod->refcnt);
}
EXPORT_SYMBOL(module_refcount);

@@ -935,7 +908,7 @@ void __module_get(struct module *module)
{
if (module) {
preempt_disable();
- __this_cpu_inc(module->refptr->incs);
+ atomic_inc(&module->refcnt);
trace_module_get(module, _RET_IP_);
preempt_enable();
}
@@ -950,7 +923,7 @@ bool try_module_get(struct module *module)
preempt_disable();

if (likely(module_is_live(module))) {
- __this_cpu_inc(module->refptr->incs);
+ atomic_inc(&module->refcnt);
trace_module_get(module, _RET_IP_);
} else
ret = false;
@@ -965,9 +938,7 @@ void module_put(struct module *module)
{
if (module) {
preempt_disable();
- smp_wmb(); /* see comment in module_refcount */
- __this_cpu_inc(module->refptr->decs);
-
+ atomic_dec(&module->refcnt);
trace_module_put(module, _RET_IP_);
preempt_enable();
}