2006-08-13 12:39:56

by Oleg Nesterov

Subject: [PATCH] do_sched_setscheduler: don't take tasklist_lock

We don't need to take tasklist_lock or disable irqs for
find_task_by_pid() + get_task_struct(). Use RCU locks
instead.

Signed-off-by: Oleg Nesterov <[email protected]>

--- 2.6.18-rc3/kernel/sched.c~1_dss 2006-07-16 01:53:08.000000000 +0400
+++ 2.6.18-rc3/kernel/sched.c 2006-08-13 20:19:02.000000000 +0400
@@ -4156,14 +4156,15 @@ do_sched_setscheduler(pid_t pid, int pol
return -EINVAL;
if (copy_from_user(&lparam, param, sizeof(struct sched_param)))
return -EFAULT;
- read_lock_irq(&tasklist_lock);
+
+ rcu_read_lock();
p = find_process_by_pid(pid);
- if (!p) {
- read_unlock_irq(&tasklist_lock);
+ if (p)
+ get_task_struct(p);
+ rcu_read_unlock();
+ if (!p)
return -ESRCH;
- }
- get_task_struct(p);
- read_unlock_irq(&tasklist_lock);
+
retval = sched_setscheduler(p, policy, &lparam);
put_task_struct(p);

2006-08-13 12:58:11

by Oleg Nesterov

Subject: Re: [PATCH] do_sched_setscheduler: don't take tasklist_lock

On 08/13, Oleg Nesterov wrote:
>
> We don't need to take tasklist_lock or disable irqs for
> find_task_by_pid() + get_task_struct(). Use RCU locks
> instead.

On the other hand, I think sched_setscheduler() does need tasklist_lock!

It is unsafe do dereference ->signal unless tasklist_lock or ->siglock
is held (or p == current). Yes, we pin the task structure, but this can't
prevent from release_task()->__exit_signal() which sets ->signal = NULL.

So, I think this patch

[PATCH] Drop tasklist lock in do_sched_setscheduler
commit e74c69f46d93d29eea0ad8647863d1c6488f0f55

is not correct.

Am I missed something?

Oleg.