Re: [Linux-kernel-mentees] [PATCH] kernel: module: Pass lockdep expression to RCU lists

[Amol Grover <frextrite@gmail.com>]

On Thu, Jan 23, 2020 at 01:10:10PM +0100, Jessica Yu wrote:
> +++ Amol Grover [21/01/20 18:17 +0530]:
> > modules is traversed using list_for_each_entry_rcu outside an
> > RCU read-side critical section but under the protection
> > of module_mutex or with preemption disabled.
> > 
> > Hence, add corresponding lockdep expression to silence false-positive
> > lockdep warnings, and harden RCU lists.
> > 
> > list_for_each_entry_rcu when traversed inside a preempt disabled
> > section, doesn't need an explicit lockdep expression since it is
> > implicitly checked for.
> > 
> > Add macro for the corresponding lockdep expression.
> > 
> > Signed-off-by: Amol Grover <frextrite@gmail.com>
> 
> Hi Amol!
> 
> Masami already submitted a patch for this, it's been in linux-next for
> a while. See commit bf08949cc8b9 ("modules: lockdep: Suppress
> suspicious RCU usage warning").
> 

Hey Jessica,

Thank you for reviewing the patch!

Thanks
Amol

> Thanks!
> 
> Jessica
> 
> > ---
> > kernel/module.c | 12 +++++++-----
> > 1 file changed, 7 insertions(+), 5 deletions(-)
> > 
> > diff --git a/kernel/module.c b/kernel/module.c
> > index b56f3224b161..2425f58159dd 100644
> > --- a/kernel/module.c
> > +++ b/kernel/module.c
> > @@ -84,6 +84,8 @@
> >  * 3) module_addr_min/module_addr_max.
> >  * (delete and add uses RCU list operations). */
> > DEFINE_MUTEX(module_mutex);
> > +#define module_mutex_held() \
> > +	lockdep_is_held(&module_mutex)
> > EXPORT_SYMBOL_GPL(module_mutex);
> > static LIST_HEAD(modules);
> > 
> > @@ -214,7 +216,7 @@ static struct module *mod_find(unsigned long addr)
> > {
> > 	struct module *mod;
> > 
> > -	list_for_each_entry_rcu(mod, &modules, list) {
> > +	list_for_each_entry_rcu(mod, &modules, list, module_mutex_held()) {
> > 		if (within_module(addr, mod))
> > 			return mod;
> > 	}
> > @@ -448,7 +450,7 @@ bool each_symbol_section(bool (*fn)(const struct symsearch *arr,
> > 	if (each_symbol_in_section(arr, ARRAY_SIZE(arr), NULL, fn, data))
> > 		return true;
> > 
> > -	list_for_each_entry_rcu(mod, &modules, list) {
> > +	list_for_each_entry_rcu(mod, &modules, list, module_mutex_held()) {
> > 		struct symsearch arr[] = {
> > 			{ mod->syms, mod->syms + mod->num_syms, mod->crcs,
> > 			  NOT_GPL_ONLY, false },
> > @@ -616,7 +618,7 @@ static struct module *find_module_all(const char *name, size_t len,
> > 
> > 	module_assert_mutex_or_preempt();
> > 
> > -	list_for_each_entry_rcu(mod, &modules, list) {
> > +	list_for_each_entry_rcu(mod, &modules, list, module_mutex_held()) {
> > 		if (!even_unformed && mod->state == MODULE_STATE_UNFORMED)
> > 			continue;
> > 		if (strlen(mod->name) == len && !memcmp(mod->name, name, len))
> > @@ -2040,7 +2042,7 @@ void set_all_modules_text_rw(void)
> > 		return;
> > 
> > 	mutex_lock(&module_mutex);
> > -	list_for_each_entry_rcu(mod, &modules, list) {
> > +	list_for_each_entry_rcu(mod, &modules, list, module_mutex_held()) {
> > 		if (mod->state == MODULE_STATE_UNFORMED)
> > 			continue;
> > 
> > @@ -2059,7 +2061,7 @@ void set_all_modules_text_ro(void)
> > 		return;
> > 
> > 	mutex_lock(&module_mutex);
> > -	list_for_each_entry_rcu(mod, &modules, list) {
> > +	list_for_each_entry_rcu(mod, &modules, list, module_mutex_held()) {
> > 		/*
> > 		 * Ignore going modules since it's possible that ro
> > 		 * protection has already been disabled, otherwise we'll
> > -- 
> > 2.24.1
> > 
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