Re: [RFC PATCH 1/1] module: Make wait module's refcount to zero procedure as async

[Rusty Russell <rusty@rustcorp.com.au>]

Lucas De Marchi <lucas.de.marchi@gmail.com> writes:
> On Thu, Sep 12, 2013 at 9:30 PM, Rusty Russell <rusty@rustcorp.com.au> wrote:
>> Peter Chen <peter.chen@freescale.com> writes:
>>> Currently, if module's refcount is not zero during the unload,
>>> it waits there until the user decreases that refcount.
>>
>> Hi Peter,
>>
>>         In practice userspace uses O_NONBLOCK.  In fact, I've been
>> thinking of removing the blocking case altogether, since it's not really
>> what people want.
>>
>> That would solve your problem and make the code simpler.  Thoughts?
>
> I'm all in favor of this. It's been almost 1 year it's deprecated in
> kmod and if anyone tries to use we force a 10s delay on module
> removal. So far nobody complained.
>
> Lucas De Marchi

Here's what I've got in my pending-rebases tree.

Cheers,
Rusty.

module: remove rmmod --wait option.

The option to wait for a module reference count to reach zero was in
the initial module implementation, but it was never supported in
modprobe (you had to use rmmod --wait).  After discussion with Lucas,
It has been deprecated (with a 10 second sleep) in kmod for the last
year.

This finally removes it: the flag will evoke a printk warning and a
normal (non-blocking) remove attempt.

Cc: Lucas De Marchi <lucas.de.marchi@gmail.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

diff --git a/include/linux/module.h b/include/linux/module.h
index 05f2447..15cd6b1 100644
--- a/include/linux/module.h
+++ b/include/linux/module.h
@@ -367,9 +367,6 @@ struct module
 	/* What modules do I depend on? */
 	struct list_head target_list;
 
-	/* Who is waiting for us to be unloaded */
-	struct task_struct *waiter;
-
 	/* Destruction function. */
 	void (*exit)(void);
 
diff --git a/kernel/module.c b/kernel/module.c
index dc58274..947105f 100644
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -644,8 +644,6 @@ static int module_unload_init(struct module *mod)
 
 	/* Hold reference count during initialization. */
 	__this_cpu_write(mod->refptr->incs, 1);
-	/* Backwards compatibility macros put refcount during init. */
-	mod->waiter = current;
 
 	return 0;
 }
@@ -771,16 +769,9 @@ static int __try_stop_module(void *_sref)
 
 static int try_stop_module(struct module *mod, int flags, int *forced)
 {
-	if (flags & O_NONBLOCK) {
-		struct stopref sref = { mod, flags, forced };
+	struct stopref sref = { mod, flags, forced };
 
-		return stop_machine(__try_stop_module, &sref, NULL);
-	} else {
-		/* We don't need to stop the machine for this. */
-		mod->state = MODULE_STATE_GOING;
-		synchronize_sched();
-		return 0;
-	}
+	return stop_machine(__try_stop_module, &sref, NULL);
 }
 
 unsigned long module_refcount(struct module *mod)
@@ -813,21 +804,6 @@ EXPORT_SYMBOL(module_refcount);
 /* This exists whether we can unload or not */
 static void free_module(struct module *mod);
 
-static void wait_for_zero_refcount(struct module *mod)
-{
-	/* Since we might sleep for some time, release the mutex first */
-	mutex_unlock(&module_mutex);
-	for (;;) {
-		pr_debug("Looking at refcount...\n");
-		set_current_state(TASK_UNINTERRUPTIBLE);
-		if (module_refcount(mod) == 0)
-			break;
-		schedule();
-	}
-	current->state = TASK_RUNNING;
-	mutex_lock(&module_mutex);
-}
-
 SYSCALL_DEFINE2(delete_module, const char __user *, name_user,
 		unsigned int, flags)
 {
@@ -842,6 +818,11 @@ SYSCALL_DEFINE2(delete_module, const char __user *, name_user,
 		return -EFAULT;
 	name[MODULE_NAME_LEN-1] = '\0';
 
+	if (!(flags & O_NONBLOCK)) {
+		printk(KERN_WARNING
+		       "waiting module removal not supported: please upgrade");
+	}
+
 	if (mutex_lock_interruptible(&module_mutex) != 0)
 		return -EINTR;
 
@@ -859,8 +840,7 @@ SYSCALL_DEFINE2(delete_module, const char __user *, name_user,
 
 	/* Doing init or already dying? */
 	if (mod->state != MODULE_STATE_LIVE) {
-		/* FIXME: if (force), slam module count and wake up
-                   waiter --RR */
+		/* FIXME: if (force), slam module count damn the torpedoes */
 		pr_debug("%s already dying\n", mod->name);
 		ret = -EBUSY;
 		goto out;
@@ -876,18 +856,11 @@ SYSCALL_DEFINE2(delete_module, const char __user *, name_user,
 		}
 	}
 
-	/* Set this up before setting mod->state */
-	mod->waiter = current;
-
 	/* Stop the machine so refcounts can't move and disable module. */
 	ret = try_stop_module(mod, flags, &forced);
 	if (ret != 0)
 		goto out;
 
-	/* Never wait if forced. */
-	if (!forced && module_refcount(mod) != 0)
-		wait_for_zero_refcount(mod);
-
 	mutex_unlock(&module_mutex);
 	/* Final destruction now no one is using it. */
 	if (mod->exit != NULL)
@@ -1005,9 +978,6 @@ void module_put(struct module *module)
 		__this_cpu_inc(module->refptr->decs);
 
 		trace_module_put(module, _RET_IP_);
-		/* Maybe they're waiting for us to drop reference? */
-		if (unlikely(!module_is_live(module)))
-			wake_up_process(module->waiter);
 		preempt_enable();
 	}
 }