2006-08-21 12:42:30

by Oleg Nesterov

Subject: [PATCH 2/3] futex_find_get_task: don't take tasklist_lock

It is ok to do find_task_by_pid() + get_task_struct() under rcu_read_lock(),
we cand drop tasklist_lock.

Note that testing of ->exit_state is racy with or without tasklist anyway.

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

--- 2.6.18-rc4/kernel/futex.c~2_ffgt 2006-08-21 20:32:48.000000000 +0400
+++ 2.6.18-rc4/kernel/futex.c 2006-08-21 20:41:15.000000000 +0400
@@ -389,7 +389,7 @@ static struct task_struct * futex_find_g
{
struct task_struct *p;

- read_lock(&tasklist_lock);
+ rcu_read_lock();
p = find_task_by_pid(pid);
if (!p)
goto out_unlock;
@@ -403,7 +403,7 @@ static struct task_struct * futex_find_g
}
get_task_struct(p);
out_unlock:
- read_unlock(&tasklist_lock);
+ rcu_read_unlock();

return p;
}

2006-08-22 10:52:27

by Ingo Molnar

Subject: Re: [PATCH 2/3] futex_find_get_task: don't take tasklist_lock

* Oleg Nesterov <[email protected]> wrote:

> It is ok to do find_task_by_pid() + get_task_struct() under
> rcu_read_lock(), we cand drop tasklist_lock.
>
> Note that testing of ->exit_state is racy with or without tasklist
> anyway.
>
> Signed-off-by: Oleg Nesterov <[email protected]>

yeah. (We did this in the -rt tree for some time too, never saw any
problems.)

Acked-by: Ingo Molnar <[email protected]>

Ingo