linux-kernel - Re: [PATCH, RFC] kthread: (possibly) a missing memory barrier in kthread

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Date:	Tue, 19 Feb 2008 23:52:11 +0100
From:	"Dmitry Adamushko" <dmitry.adamushko@...il.com>
To:	linux-kernel@...r.kernel.org
Cc:	"Nick Piggin" <nickpiggin@...oo.com.au>,
	"Ingo Molnar" <mingo@...e.hu>,
	"Andrew Morton" <akpm@...ux-foundation.org>,
	"Rusty Russel" <rusty@...tcorp.com.au>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	"Andy Whitcroft" <apw@...dowen.org>,
	"Peter Zijlstra" <a.p.zijlstra@...llo.nl>,
	"Linus Torvalds" <torvalds@...ux-foundation.org>
Subject: Re: [PATCH, RFC] kthread: (possibly) a missing memory barrier in kthread_stop()

humm... following the same logic, there is also a problem in kthread.c.

(1) the main loop of kthreadd() :

                set_current_state(TASK_INTERRUPTIBLE);
                if (list_empty(&kthread_create_list))
                        schedule();

and

(2) kthread_create() does:

spin_lock(&kthread_create_lock);
list_add_tail(&create.list, &kthread_create_list);
wake_up_process(kthreadd_task);
spin_unlock(&kthread_create_lock);

provided,

- (1) doesn't want to take the 'kthread_create_lock' in order to do a
check for list_empty(&kthread_create_list)
[ which can be possible if list_empty() results in a single word-size
aligned read op. -- which is guaranteed to be atomic on any arch, iirc
]

and

- (1) and (2) can run in parallel.

then it's crucial that a modification of the list (i.e.
list_add_tail()) is completed by the moment a state of the task
(kthreadd_task->state) is checked in try_to_wake_up(). i.e. they must
not be re-ordered.

which makes me think that try_to_wake_up() could be better off just
acting as a full mb.

otherwise, a possible fix would be:

this way we get a pair of UNLOCK/LOCK which is guaranteed to be a full mb
(the LOCK is in try_to_wake_up())

[ moreover, there seems to be no need to call wake_up_process() with
'kthread_create_lock' being held ]

--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -145,8 +145,8 @@ struct task_struct *kthread_create(int
(*threadfn)(void *data),

        spin_lock(&kthread_create_lock);
        list_add_tail(&create.list, &kthread_create_list);
-       wake_up_process(kthreadd_task);
        spin_unlock(&kthread_create_lock);
+       wake_up_process(kthreadd_task);

        wait_for_completion(&create.done);


-- 
Best regards,
Dmitry Adamushko
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