modules: remove modlist_lock

Now we always use stop_machine for module insertion or deletion, we no
longer need the modlist_lock: merely disabling preemption is sufficient to
block against list manipulation.  This avoids deadlock on OOPSen where we
can potentially grab the lock twice.

Bug: 8695
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Tobias Oed <tobiasoed@hotmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/module.c

@@ -61,10 +61,8 @@ extern int module_sysfs_initialized;
 /* If this is set, the section belongs in the init part of the module */
 #define INIT_OFFSET_MASK (1UL << (BITS_PER_LONG-1))
 
-/* Protects module list */
-static DEFINE_SPINLOCK(modlist_lock);
-
-/* List of modules, protected by module_mutex AND modlist_lock */
+/* List of modules, protected by module_mutex or preempt_disable
+ * (add/delete uses stop_machine). */
 static DEFINE_MUTEX(module_mutex);
 static LIST_HEAD(modules);
 
@@ -760,14 +758,13 @@ static void print_unload_info(struct seq_file *m, struct module *mod)
 void __symbol_put(const char *symbol)
 {
 	struct module *owner;
-	unsigned long flags;
 	const unsigned long *crc;
 
-	spin_lock_irqsave(&modlist_lock, flags);
+	preempt_disable();
 	if (!__find_symbol(symbol, &owner, &crc, 1))
 		BUG();
 	module_put(owner);
-	spin_unlock_irqrestore(&modlist_lock, flags);
+	preempt_enable();
 }
 EXPORT_SYMBOL(__symbol_put);
 
@@ -1228,14 +1225,14 @@ static void free_module(struct module *mod)
 void *__symbol_get(const char *symbol)
 {
 	struct module *owner;
-	unsigned long value, flags;
+	unsigned long value;
 	const unsigned long *crc;
 
-	spin_lock_irqsave(&modlist_lock, flags);
+	preempt_disable();
 	value = __find_symbol(symbol, &owner, &crc, 1);
 	if (value && !strong_try_module_get(owner))
 		value = 0;
-	spin_unlock_irqrestore(&modlist_lock, flags);
+	preempt_enable();
 
 	return (void *)value;
 }
@@ -2308,11 +2305,10 @@ const struct seq_operations modules_op = {
 /* Given an address, look for it in the module exception tables. */
 const struct exception_table_entry *search_module_extables(unsigned long addr)
 {
-	unsigned long flags;
 	const struct exception_table_entry *e = NULL;
 	struct module *mod;
 
-	spin_lock_irqsave(&modlist_lock, flags);
+	preempt_disable();
 	list_for_each_entry(mod, &modules, list) {
 		if (mod->num_exentries == 0)
 			continue;
@@ -2323,7 +2319,7 @@ const struct exception_table_entry *search_module_extables(unsigned long addr)
 		if (e)
 			break;
 	}
-	spin_unlock_irqrestore(&modlist_lock, flags);
+	preempt_enable();
 
 	/* Now, if we found one, we are running inside it now, hence
            we cannot unload the module, hence no refcnt needed. */
@@ -2335,25 +2331,24 @@ const struct exception_table_entry *search_module_extables(unsigned long addr)
  */
 int is_module_address(unsigned long addr)
 {
-	unsigned long flags;
 	struct module *mod;
 
-	spin_lock_irqsave(&modlist_lock, flags);
+	preempt_disable();
 
 	list_for_each_entry(mod, &modules, list) {
 		if (within(addr, mod->module_core, mod->core_size)) {
-			spin_unlock_irqrestore(&modlist_lock, flags);
+			preempt_enable();
 			return 1;
 		}
 	}
 
-	spin_unlock_irqrestore(&modlist_lock, flags);
+	preempt_enable();
 
 	return 0;
 }
 
 
-/* Is this a valid kernel address?  We don't grab the lock: we are oopsing. */
+/* Is this a valid kernel address? */
 struct module *__module_text_address(unsigned long addr)
 {
 	struct module *mod;
@@ -2368,11 +2363,10 @@ struct module *__module_text_address(unsigned long addr)
 struct module *module_text_address(unsigned long addr)
 {
 	struct module *mod;
-	unsigned long flags;
 
-	spin_lock_irqsave(&modlist_lock, flags);
+	preempt_disable();
 	mod = __module_text_address(addr);
-	spin_unlock_irqrestore(&modlist_lock, flags);
+	preempt_enable();
 
 	return mod;
 }