Messages in this thread | | | Subject | Re: [PATCH 1/2] task_work: make FIFO task_work list | From | li guang <> | Date | Fri, 15 Mar 2013 08:16:31 +0800 |
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在 2013-03-14四的 15:40 +0100,Oleg Nesterov写道: > On 03/14, liguang wrote: > > > > Signed-off-by: liguang <lig.fnst@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.
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