linux-kernel - [RFC 1/4] module: use rcu_read_lock() while walking over a RCU protected list

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Date:	Fri, 31 Jul 2015 19:08:05 +0200
From:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	linux-kernel@...r.kernel.org,
	Rusty Russell <rusty@...tcorp.com.au>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Anil S Keshavamurthy <anil.s.keshavamurthy@...el.com>,
	tglx@...utronix.de,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: [RFC 1/4] module: use rcu_read_lock() while walking over a RCU protected list

The `modules' list uses list_for_each_entry_rcu() itarator.
rcu_readlock() protects here against against module removal invoked from
another CPU while preempt_disable() does not.

I don't understand what syncs in CONFIG_MODULES_TREE_LOOKUP against
module removal. In the other (RCU) case there is synchronize_sched().

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
---
 kernel/module.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/kernel/module.c b/kernel/module.c
index 4d2b82e610e2..a62d2d48cd63 100644
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -3979,9 +3979,9 @@ bool is_module_address(unsigned long addr)
 {
 	bool ret;
 
-	preempt_disable();
+	rcu_read_lock();
 	ret = __module_address(addr) != NULL;
-	preempt_enable();
+	rcu_read_unlock();
 
 	return ret;
 }
@@ -3990,7 +3990,7 @@ bool is_module_address(unsigned long addr)
  * __module_address - get the module which contains an address.
  * @addr: the address.
  *
- * Must be called with preempt disabled or module mutex held so that
+ * Must be called with RCU read or module mutex held so that
  * module doesn't get freed during this.
  */
 struct module *__module_address(unsigned long addr)
@@ -4024,9 +4024,9 @@ bool is_module_text_address(unsigned long addr)
 {
 	bool ret;
 
-	preempt_disable();
+	rcu_read_lock();
 	ret = __module_text_address(addr) != NULL;
-	preempt_enable();
+	rcu_read_unlock();
 
 	return ret;
 }
@@ -4035,7 +4035,7 @@ bool is_module_text_address(unsigned long addr)
  * __module_text_address - get the module whose code contains an address.
  * @addr: the address.
  *
- * Must be called with preempt disabled or module mutex held so that
+ * Must be called with RCU read lock or module mutex held so that
  * module doesn't get freed during this.
  */
 struct module *__module_text_address(unsigned long addr)
-- 
2.4.6

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