linux-kernel - Re: [PATCH 1/2] module: use rcu to protect module list read

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Date:	Sat, 10 Mar 2012 07:25:57 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Cong Wang <xiyou.wangcong@...il.com>
Cc:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: [PATCH 1/2] module: use rcu to protect module list read

Le samedi 10 mars 2012 à 22:20 +0800, Cong Wang a écrit :
> Now the read of module list is protected by preempt disable + *_rcu
> list operations, this is odd, as RCU read lock should be able to
> protect it directly. This patch makes the read of module list
> protected by RCU read lock and the write still protected by
> module_mutex.
> 

Problem is that your patch does more than that.

In set_all_modules_text_rw() and set_all_modules_text_ro() you removed
the mutex in favor of rcu_read_lock()

Also, module code uses synchronize_sched(), not synchronize_rcu()

Take a look at Documentation/RCU/whatisRCU.txt and see that
preempt_disable() / preempt_enable() are documented as a right protect
code, in line 333.

You added races in /proc/modules as well.

So I would say your patch is not needed at all : module code already
uses RCU.

What particular problem do you have with current code ?

I would probably do the opposite patch, since its clear that at least in
two points we use the wrong macro :

diff --git a/kernel/module.c b/kernel/module.c
index 2c93276..cacb36e 100644
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -1714,7 +1714,7 @@ void set_all_modules_text_rw(void)
 	struct module *mod;
 
 	mutex_lock(&module_mutex);
-	list_for_each_entry_rcu(mod, &modules, list) {
+	list_for_each_entry(mod, &modules, list) {
 		if ((mod->module_core) && (mod->core_text_size)) {
 			set_page_attributes(mod->module_core,
 						mod->module_core + mod->core_text_size,
@@ -1735,7 +1735,7 @@ void set_all_modules_text_ro(void)
 	struct module *mod;
 
 	mutex_lock(&module_mutex);
-	list_for_each_entry_rcu(mod, &modules, list) {
+	list_for_each_entry(mod, &modules, list) {
 		if ((mod->module_core) && (mod->core_text_size)) {
 			set_page_attributes(mod->module_core,
 						mod->module_core + mod->core_text_size,


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