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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