Merge tag 'modules-next-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/rusty/linux

Pull module updates from Rusty Russell:
 "The exciting thing here is the getting rid of stop_machine on module
  removal.  This is possible by using a simple atomic_t for the counter,
  rather than our fancy per-cpu counter: it turns out that no one is
  doing a module increment per net packet, so the slowdown should be in
  the noise"

* tag 'modules-next-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/rusty/linux:
  param: do not set store func without write perm
  params: cleanup sysfs allocation
  kernel:module Fix coding style errors and warnings.
  module: Remove stop_machine from module unloading
  module: Replace module_ref with atomic_t refcnt
  lib/bug: Use RCU list ops for module_bug_list
  module: Unlink module with RCU synchronizing instead of stop_machine
  module: Wait for RCU synchronizing before releasing a module

5 files changed, 137 insertions, 168 deletions

diff --git a/include/linux/module.h b/include/linux/module.h
index 71f282a4e30..ebfb0e153c6 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 7c5cbfe3fc4..81c4c183d34 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 e52a8739361..3965511ae13 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);
@@ -158,13 +157,13 @@ static BLOCKING_NOTIFIER_HEAD(module_notify_list);
  * Protected by module_mutex. */
 static unsigned long module_addr_min = -1UL, module_addr_max = 0;
 
-int register_module_notifier(struct notifier_block * nb)
+int register_module_notifier(struct notifier_block *nb)
 {
 	return blocking_notifier_chain_register(&module_notify_list, nb);
 }
 EXPORT_SYMBOL(register_module_notifier);
 
-int unregister_module_notifier(struct notifier_block * nb)
+int unregister_module_notifier(struct notifier_block *nb)
 {
 	return blocking_notifier_chain_unregister(&module_notify_list, nb);
 }
@@ -628,18 +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)
 {
-	mod->refptr = alloc_percpu(struct module_ref);
-	if (!mod->refptr)
-		return -ENOMEM;
+	/*
+	 * 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. */
-	raw_cpu_write(mod->refptr->incs, 1);
+	atomic_inc(&mod->refcnt);
 
 	return 0;
 }
@@ -721,8 +725,6 @@ static void module_unload_free(struct module *mod)
 		kfree(use);
 	}
 	mutex_unlock(&module_mutex);
-
-	free_percpu(mod->refptr);
 }
 
 #ifdef CONFIG_MODULE_FORCE_UNLOAD
@@ -740,60 +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)
 {
-	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) - MODULE_REF_BASE;
 }
 EXPORT_SYMBOL(module_refcount);
 
@@ -877,8 +858,10 @@ static inline void print_unload_info(struct seq_file *m, struct module *mod)
 
 	seq_printf(m, " %lu ", module_refcount(mod));
 
-	/* Always include a trailing , so userspace can differentiate
-           between this and the old multi-field proc format. */
+	/*
+	 * Always include a trailing , so userspace can differentiate
+	 * between this and the old multi-field proc format.
+	 */
 	list_for_each_entry(use, &mod->source_list, source_list) {
 		printed_something = 1;
 		seq_printf(m, "%s,", use->source->name);
@@ -886,11 +869,11 @@ static inline void print_unload_info(struct seq_file *m, struct module *mod)
 
 	if (mod->init != NULL && mod->exit == NULL) {
 		printed_something = 1;
-		seq_printf(m, "[permanent],");
+		seq_puts(m, "[permanent],");
 	}
 
 	if (!printed_something)
-		seq_printf(m, "-");
+		seq_puts(m, "-");
 }
 
 void __symbol_put(const char *symbol)
@@ -935,7 +918,7 @@ 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();
 	}
@@ -948,11 +931,11 @@ bool try_module_get(struct module *module)
 
 	if (module) {
 		preempt_disable();
-
-		if (likely(module_is_live(module))) {
-			__this_cpu_inc(module->refptr->incs);
+		/* 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();
@@ -963,11 +946,12 @@ EXPORT_SYMBOL(try_module_get);
 
 void module_put(struct module *module)
 {
+	int ret;
+
 	if (module) {
 		preempt_disable();
-		smp_wmb(); /* see comment in module_refcount */
-		__this_cpu_inc(module->refptr->decs);
-
+		ret = atomic_dec_if_positive(&module->refcnt);
+		WARN_ON(ret < 0);	/* Failed to put refcount */
 		trace_module_put(module, _RET_IP_);
 		preempt_enable();
 	}
@@ -978,7 +962,7 @@ EXPORT_SYMBOL(module_put);
 static inline void print_unload_info(struct seq_file *m, struct module *mod)
 {
 	/* We don't know the usage count, or what modules are using. */
-	seq_printf(m, " - -");
+	seq_puts(m, " - -");
 }
 
 static inline void module_unload_free(struct module *mod)
@@ -1131,7 +1115,7 @@ static unsigned long maybe_relocated(unsigned long crc,
 static int check_version(Elf_Shdr *sechdrs,
 			 unsigned int versindex,
 			 const char *symname,
-			 struct module *mod, 
+			 struct module *mod,
 			 const unsigned long *crc,
 			 const struct module *crc_owner)
 {
@@ -1165,7 +1149,7 @@ static int check_version(Elf_Shdr *sechdrs,
 	return 0;
 
 bad_version:
-	printk("%s: disagrees about version of symbol %s\n",
+	pr_warn("%s: disagrees about version of symbol %s\n",
 	       mod->name, symname);
 	return 0;
 }
@@ -1200,7 +1184,7 @@ static inline int same_magic(const char *amagic, const char *bmagic,
 static inline int check_version(Elf_Shdr *sechdrs,
 				unsigned int versindex,
 				const char *symname,
-				struct module *mod, 
+				struct module *mod,
 				const unsigned long *crc,
 				const struct module *crc_owner)
 {
@@ -1288,15 +1272,13 @@ static inline bool sect_empty(const Elf_Shdr *sect)
 	return !(sect->sh_flags & SHF_ALLOC) || sect->sh_size == 0;
 }
 
-struct module_sect_attr
-{
+struct module_sect_attr {
 	struct module_attribute mattr;
 	char *name;
 	unsigned long address;
 };
 
-struct module_sect_attrs
-{
+struct module_sect_attrs {
 	struct attribute_group grp;
 	unsigned int nsections;
 	struct module_sect_attr attrs[0];
@@ -1550,7 +1532,8 @@ static int module_add_modinfo_attrs(struct module *mod)
 		    (attr->test && attr->test(mod))) {
 			memcpy(temp_attr, attr, sizeof(*temp_attr));
 			sysfs_attr_init(&temp_attr->attr);
-			error = sysfs_create_file(&mod->mkobj.kobj,&temp_attr->attr);
+			error = sysfs_create_file(&mod->mkobj.kobj,
+					&temp_attr->attr);
 			++temp_attr;
 		}
 	}
@@ -1566,7 +1549,7 @@ static void module_remove_modinfo_attrs(struct module *mod)
 		/* pick a field to test for end of list */
 		if (!attr->attr.name)
 			break;
-		sysfs_remove_file(&mod->mkobj.kobj,&attr->attr);
+		sysfs_remove_file(&mod->mkobj.kobj, &attr->attr);
 		if (attr->free)
 			attr->free(mod);
 	}
@@ -1697,18 +1680,6 @@ static void mod_sysfs_teardown(struct module *mod)
 	mod_sysfs_fini(mod);
 }
 
-/*
- * unlink the module with the whole machine is stopped with interrupts off
- * - this defends against kallsyms not taking locks
- */
-static int __unlink_module(void *_mod)
-{
-	struct module *mod = _mod;
-	list_del(&mod->list);
-	module_bug_cleanup(mod);
-	return 0;
-}
-
 #ifdef CONFIG_DEBUG_SET_MODULE_RONX
 /*
  * LKM RO/NX protection: protect module's text/ro-data
@@ -1860,7 +1831,12 @@ static void free_module(struct module *mod)
 
 	/* Now we can delete it from the lists */
 	mutex_lock(&module_mutex);
-	stop_machine(__unlink_module, mod, NULL);
+	/* Unlink carefully: kallsyms could be walking list. */
+	list_del_rcu(&mod->list);
+	/* Remove this module from bug list, this uses list_del_rcu */
+	module_bug_cleanup(mod);
+	/* Wait for RCU synchronizing before releasing mod->list and buglist. */
+	synchronize_rcu();
 	mutex_unlock(&module_mutex);
 
 	/* This may be NULL, but that's OK */
@@ -1955,7 +1931,7 @@ static int simplify_symbols(struct module *mod, const struct load_info *info)
 			/* We compiled with -fno-common.  These are not
 			   supposed to happen.  */
 			pr_debug("Common symbol: %s\n", name);
-			printk("%s: please compile with -fno-common\n",
+			pr_warn("%s: please compile with -fno-common\n",
 			       mod->name);
 			ret = -ENOEXEC;
 			break;
@@ -2259,7 +2235,7 @@ static char elf_type(const Elf_Sym *sym, const struct load_info *info)
 }
 
 static bool is_core_symbol(const Elf_Sym *src, const Elf_Shdr *sechdrs,
-                           unsigned int shnum)
+			unsigned int shnum)
 {
 	const Elf_Shdr *sec;
 
@@ -2735,7 +2711,7 @@ static int find_module_sections(struct module *mod, struct load_info *info)
 		 * This shouldn't happen with same compiler and binutils
 		 * building all parts of the module.
 		 */
-		printk(KERN_WARNING "%s: has both .ctors and .init_array.\n",
+		pr_warn("%s: has both .ctors and .init_array.\n",
 		       mod->name);
 		return -EINVAL;
 	}
@@ -3023,8 +2999,10 @@ static int do_init_module(struct module *mod)
 	if (mod->init != NULL)
 		ret = do_one_initcall(mod->init);
 	if (ret < 0) {
-		/* Init routine failed: abort.  Try to protect us from
-                   buggy refcounters. */
+		/*
+		 * Init routine failed: abort.  Try to protect us from
+		 * buggy refcounters.
+		 */
 		mod->state = MODULE_STATE_GOING;
 		synchronize_sched();
 		module_put(mod);
@@ -3202,7 +3180,7 @@ out:
 
 static int unknown_module_param_cb(char *param, char *val, const char *modname)
 {
-	/* Check for magic 'dyndbg' arg */ 
+	/* Check for magic 'dyndbg' arg */
 	int ret = ddebug_dyndbg_module_param_cb(param, val, modname);
 	if (ret != 0)
 		pr_warn("%s: unknown parameter '%s' ignored\n", modname, param);
@@ -3352,6 +3330,8 @@ static int load_module(struct load_info *info, const char __user *uargs,
 	/* Unlink carefully: kallsyms could be walking list. */
 	list_del_rcu(&mod->list);
 	wake_up_all(&module_wq);
+	/* Wait for RCU synchronizing before releasing mod->list. */
+	synchronize_rcu();
 	mutex_unlock(&module_mutex);
  free_module:
 	module_deallocate(mod, info);
@@ -3685,8 +3665,8 @@ static int m_show(struct seq_file *m, void *p)
 
 	/* Informative for users. */
 	seq_printf(m, " %s",
-		   mod->state == MODULE_STATE_GOING ? "Unloading":
-		   mod->state == MODULE_STATE_COMING ? "Loading":
+		   mod->state == MODULE_STATE_GOING ? "Unloading" :
+		   mod->state == MODULE_STATE_COMING ? "Loading" :
 		   "Live");
 	/* Used by oprofile and other similar tools. */
 	seq_printf(m, " 0x%pK", mod->module_core);
@@ -3695,7 +3675,7 @@ static int m_show(struct seq_file *m, void *p)
 	if (mod->taints)
 		seq_printf(m, " %s", module_flags(mod, buf));
 
-	seq_printf(m, "\n");
+	seq_puts(m, "\n");
 	return 0;
 }
 
diff --git a/kernel/params.c b/kernel/params.c
index db97b791390..0af9b2c4e56 100644
--- a/kernel/params.c
+++ b/kernel/params.c
@@ -603,74 +603,67 @@ static __modinit int add_sysfs_param(struct module_kobject *mk,
 				     const struct kernel_param *kp,
 				     const char *name)
 {
-	struct module_param_attrs *new;
-	struct attribute **attrs;
-	int err, num;
+	struct module_param_attrs *new_mp;
+	struct attribute **new_attrs;
+	unsigned int i;
 
 	/* We don't bother calling this with invisible parameters. */
 	BUG_ON(!kp->perm);
 
 	if (!mk->mp) {
-		num = 0;
-		attrs = NULL;
-	} else {
-		num = mk->mp->num;
-		attrs = mk->mp->grp.attrs;
+		/* First allocation. */
+		mk->mp = kzalloc(sizeof(*mk->mp), GFP_KERNEL);
+		if (!mk->mp)
+			return -ENOMEM;
+		mk->mp->grp.name = "parameters";
+		/* NULL-terminated attribute array. */
+		mk->mp->grp.attrs = kzalloc(sizeof(mk->mp->grp.attrs[0]),
+					    GFP_KERNEL);
+		/* Caller will cleanup via free_module_param_attrs */
+		if (!mk->mp->grp.attrs)
+			return -ENOMEM;
 	}
 
-	/* Enlarge. */
-	new = krealloc(mk->mp,
-		       sizeof(*mk->mp) + sizeof(mk->mp->attrs[0]) * (num+1),
-		       GFP_KERNEL);
-	if (!new) {
-		kfree(attrs);
-		err = -ENOMEM;
-		goto fail;
-	}
-	/* Despite looking like the typical realloc() bug, this is safe.
-	 * We *want* the old 'attrs' to be freed either way, and we'll store
-	 * the new one in the success case. */
-	attrs = krealloc(attrs, sizeof(new->grp.attrs[0])*(num+2), GFP_KERNEL);
-	if (!attrs) {
-		err = -ENOMEM;
-		goto fail_free_new;
-	}
+	/* Enlarge allocations. */
+	new_mp = krealloc(mk->mp,
+			  sizeof(*mk->mp) +
+			  sizeof(mk->mp->attrs[0]) * (mk->mp->num + 1),
+			  GFP_KERNEL);
+	if (!new_mp)
+		return -ENOMEM;
+	mk->mp = new_mp;
 
-	/* Sysfs wants everything zeroed. */
-	memset(new, 0, sizeof(*new));
-	memset(&new->attrs[num], 0, sizeof(new->attrs[num]));
-	memset(&attrs[num], 0, sizeof(attrs[num]));
-	new->grp.name = "parameters";
-	new->grp.attrs = attrs;
+	/* Extra pointer for NULL terminator */
+	new_attrs = krealloc(mk->mp->grp.attrs,
+			     sizeof(mk->mp->grp.attrs[0]) * (mk->mp->num + 2),
+			     GFP_KERNEL);
+	if (!new_attrs)
+		return -ENOMEM;
+	mk->mp->grp.attrs = new_attrs;
 
 	/* Tack new one on the end. */
-	sysfs_attr_init(&new->attrs[num].mattr.attr);
-	new->attrs[num].param = kp;
-	new->attrs[num].mattr.show = param_attr_show;
-	new->attrs[num].mattr.store = param_attr_store;
-	new->attrs[num].mattr.attr.name = (char *)name;
-	new->attrs[num].mattr.attr.mode = kp->perm;
-	new->num = num+1;
+	sysfs_attr_init(&mk->mp->attrs[mk->mp->num].mattr.attr);
+	mk->mp->attrs[mk->mp->num].param = kp;
+	mk->mp->attrs[mk->mp->num].mattr.show = param_attr_show;
+	/* Do not allow runtime DAC changes to make param writable. */
+	if ((kp->perm & (S_IWUSR | S_IWGRP | S_IWOTH)) != 0)
+		mk->mp->attrs[mk->mp->num].mattr.store = param_attr_store;
+	mk->mp->attrs[mk->mp->num].mattr.attr.name = (char *)name;
+	mk->mp->attrs[mk->mp->num].mattr.attr.mode = kp->perm;
+	mk->mp->num++;
 
 	/* Fix up all the pointers, since krealloc can move us */
-	for (num = 0; num < new->num; num++)
-		new->grp.attrs[num] = &new->attrs[num].mattr.attr;
-	new->grp.attrs[num] = NULL;
-
-	mk->mp = new;
+	for (i = 0; i < mk->mp->num; i++)
+		mk->mp->grp.attrs[i] = &mk->mp->attrs[i].mattr.attr;
+	mk->mp->grp.attrs[mk->mp->num] = NULL;
 	return 0;
-
-fail_free_new:
-	kfree(new);
-fail:
-	mk->mp = NULL;
-	return err;
 }
 
 #ifdef CONFIG_MODULES
 static void free_module_param_attrs(struct module_kobject *mk)
 {
-	kfree(mk->mp->grp.attrs);
+	if (mk->mp)
+		kfree(mk->mp->grp.attrs);
 	kfree(mk->mp);
 	mk->mp = NULL;
 }
@@ -695,8 +688,10 @@ int module_param_sysfs_setup(struct module *mod,
 		if (kparam[i].perm == 0)
 			continue;
 		err = add_sysfs_param(&mod->mkobj, &kparam[i], kparam[i].name);
-		if (err)
+		if (err) {
+			free_module_param_attrs(&mod->mkobj);
 			return err;
+		}
 		params = true;
 	}
 
diff --git a/lib/bug.c b/lib/bug.c
index d1d7c787890..0c3bd9552b6 100644
--- a/lib/bug.c
+++ b/lib/bug.c
@@ -64,16 +64,22 @@ static LIST_HEAD(module_bug_list);
 static const struct bug_entry *module_find_bug(unsigned long bugaddr)
 {
 	struct module *mod;
+	const struct bug_entry *bug = NULL;
 
-	list_for_each_entry(mod, &module_bug_list, bug_list) {
-		const struct bug_entry *bug = mod->bug_table;
+	rcu_read_lock();
+	list_for_each_entry_rcu(mod, &module_bug_list, bug_list) {
 		unsigned i;
 
+		bug = mod->bug_table;
 		for (i = 0; i < mod->num_bugs; ++i, ++bug)
 			if (bugaddr == bug_addr(bug))
-				return bug;
+				goto out;
 	}
-	return NULL;
+	bug = NULL;
+out:
+	rcu_read_unlock();
+
+	return bug;
 }
 
 void module_bug_finalize(const Elf_Ehdr *hdr, const Elf_Shdr *sechdrs,
@@ -99,13 +105,15 @@ void module_bug_finalize(const Elf_Ehdr *hdr, const Elf_Shdr *sechdrs,
 	 * Strictly speaking this should have a spinlock to protect against
 	 * traversals, but since we only traverse on BUG()s, a spinlock
 	 * could potentially lead to deadlock and thus be counter-productive.
+	 * Thus, this uses RCU to safely manipulate the bug list, since BUG
+	 * must run in non-interruptive state.
 	 */
-	list_add(&mod->bug_list, &module_bug_list);
+	list_add_rcu(&mod->bug_list, &module_bug_list);
 }
 
 void module_bug_cleanup(struct module *mod)
 {
-	list_del(&mod->bug_list);
+	list_del_rcu(&mod->bug_list);
 }
 
 #else