在 2013-03-14四的 15:40 +0100，Oleg Nesterov写道：
> On 03/14, liguang wrote:
> >
> > Signed-off-by: liguang <lig.f...@cn.fujitsu.com>
>
> Changelog please...
>
OK.
> > ---
> > 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.
>
then ->next filed was not initialized, so I think it will
be 0'ed by compiler, is it unreliable?.
> 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?
>
1. head = task_works
2. head = head->next
3. if head == NULL
/* it's next node of list tail (w3->next) */
head = work
else
goto 1
> 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.
>
Thanks Oleg,
Hmm, at first, I think even it was changed, it can't happened to be
NULL, but ... maybe it need more deliberation.
The motivation it make the list FIFO at task_work_add, so you don't
need to reverse it at task_work_run, and it's a time-saver if the list
doesn't have too many nodes I think.
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majord...@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
