[patch 2/4] sched: Use rcu in sched_get/set_affinity()

[Thomas Gleixner <tglx@linutronix.de>]

[-- Attachment #1: sched-use-rcu-in-affinity-get-set.patch --]
[-- Type: text/plain, Size: 1675 bytes --]

tasklist_lock is held read locked to protect the find_task_by_vpid()
call and to prevent the task going away. sched_setaffinity acquires a
task struct ref and drops tasklist lock right away. The access to the
cpus_allowed mask is protected by rq->lock.

rcu_read_lock() provides the same protection here.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
---
 kernel/sched.c |   16 ++++++----------
 1 file changed, 6 insertions(+), 10 deletions(-)

Index: linux-2.6-tip/kernel/sched.c
===================================================================
--- linux-2.6-tip.orig/kernel/sched.c
+++ linux-2.6-tip/kernel/sched.c
@@ -6535,22 +6535,18 @@ long sched_setaffinity(pid_t pid, const 
 	int retval;
 
 	get_online_cpus();
-	read_lock(&tasklist_lock);
+	rcu_read_lock();
 
 	p = find_process_by_pid(pid);
 	if (!p) {
-		read_unlock(&tasklist_lock);
+		rcu_read_unlock();
 		put_online_cpus();
 		return -ESRCH;
 	}
 
-	/*
-	 * It is not safe to call set_cpus_allowed with the
-	 * tasklist_lock held. We will bump the task_struct's
-	 * usage count and then drop tasklist_lock.
-	 */
+	/* Prevent p going away */
 	get_task_struct(p);
-	read_unlock(&tasklist_lock);
+	rcu_read_unlock();
 
 	if (!alloc_cpumask_var(&cpus_allowed, GFP_KERNEL)) {
 		retval = -ENOMEM;
@@ -6636,7 +6632,7 @@ long sched_getaffinity(pid_t pid, struct
 	int retval;
 
 	get_online_cpus();
-	read_lock(&tasklist_lock);
+	rcu_read_lock();
 
 	retval = -ESRCH;
 	p = find_process_by_pid(pid);
@@ -6652,7 +6648,7 @@ long sched_getaffinity(pid_t pid, struct
 	task_rq_unlock(rq, &flags);
 
 out_unlock:
-	read_unlock(&tasklist_lock);
+	rcu_read_unlock();
 	put_online_cpus();
 
 	return retval;