On 03/14, liguang wrote:
>
> Signed-off-by: liguang <lig.f...@cn.fujitsu.com>
Changelog please...
> ---
> kernel/task_work.c | 15 +++------------
> 1 files changed, 3 insertions(+), 12 deletions(-)
>
> diff --git a/kernel/task_work.c b/kernel/task_work.c
> index 65bd3c9..0bf4258 100644
> --- a/kernel/task_work.c
> +++ b/kernel/task_work.c
> @@ -13,11 +13,12 @@ task_work_add(struct task_struct *task, struct
> callback_head *work, bool notify)
> head = ACCESS_ONCE(task->task_works);
> if (unlikely(head == &work_exited))
> return -ESRCH;
> - work->next = head;
> - } while (cmpxchg(&task->task_works, head, work) != head);
> + head = head->next;
> + } while (cmpxchg(&head, NULL, work) == head);
I simply can't understand how this can work... The patch assumes
that head->next == NULL after head = head->next, why? And then
compares the result with head and succeeds if not equal.
Could you please explain how it was supposed to work? If nothing
else, Suppose we have task->task_works -> W1 -> W2 -> W3. How this
code can add W4 after W3?
And cmpxchg(&head) should be cmpxchg(&head->next)....
Anyway, whatever I missed this is racy.
head = head->next;
nothing protects "head" after this. Say, it can be task_work_cancel'ed
and freed. So,
cmpxchg(&head, ...)
can modify the freed and reused memory.
Oleg.
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