Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx> writes:
> On 2022/09/05 17:24, Schspa Shi wrote:
>>
>> Tejun Heo <tj@xxxxxxxxxx> writes:
>>
>>> Hello,
>>>
>>> On Sat, Sep 03, 2022 at 07:11:18PM -0700, Luiz Augusto von Dentz wrote:
>>>> And we can check for __WQ_DRAINING? Anyway checking
>>>
>>> Please don't do that. That's an internal flag. It shouldn't be *that*
>>> difficult to avoid this without peeking into wq internal state.
>>>
>>> Thanks.
>>
>> It seems we only need to change hdev->{cmd,ncmd}_timer to
>> hdev->workqueue, there will be no race because drain_workqueue will
>> flush all pending work internally.
>
> True for queue_work(), not always true for queue_delayed_work(). Explained below.
>
Ok, you are right, got it now.
>> Any new timeout work will see HCI_CMD_DRAIN_WORKQUEUE flags after we
>> cancel and flushed all the delayed work.
>>
>
> If you don't mind calling
>
> queue_work(&hdev->cmd_work) followed by hci_cmd_work() (case A below)
>
> and/or
>
> queue_delayed_work(&hdev->ncmd_timer) potentially followed by hci_ncmd_timeout()/hci_reset_dev() (case B and C below)
>
> after observing HCI_CMD_DRAIN_WORKQUEUE flag.
> We need to use RCU protection if you mind one of these.
>
>
>
> Case A:
>
> hci_dev_do_reset() {
> hci_cmd_work() {
> if (test_bit(HCI_RESET, &hdev->flags) ||
> hci_dev_test_flag(hdev, HCI_CMD_DRAIN_WORKQUEUE))
> cancel_delayed_work(&hdev->cmd_timer);
> else
> queue_delayed_work(hdev->workqueue, &hdev->cmd_timer, HCI_CMD_TIMEOUT);
> } else {
> skb_queue_head(&hdev->cmd_q, skb);
> hci_dev_set_flag(hdev, HCI_CMD_DRAIN_WORKQUEUE);
> cancel_delayed_work(&hdev->cmd_timer);
> cancel_delayed_work(&hdev->ncmd_timer);
> queue_work(hdev->workqueue, &hdev->cmd_work); // Queuing after setting HCI_CMD_DRAIN_WORKQUEUE despite the intent of HCI_CMD_DRAIN_WORKQUEUE...
> drain_workqueue(hdev->workqueue); // Will wait for hci_cmd_timeout() queued by queue_work() to complete.
>
> }
>
> // Actual flush() happens here.
>
> hci_dev_clear_flag(hdev, HCI_CMD_DRAIN_WORKQUEUE);
> }
>
>
>
> Case B:
>
> hci_dev_do_reset() {
> handle_cmd_cnt_and_timer(struct hci_dev *hdev, u8 ncmd) {
> if (!hci_dev_test_flag(hdev, HCI_CMD_DRAIN_WORKQUEUE))
> hci_dev_set_flag(hdev, HCI_CMD_DRAIN_WORKQUEUE);
> cancel_delayed_work(&hdev->cmd_timer);
> queue_delayed_work(hdev->workqueue, &hdev->ncmd_timer, HCI_NCMD_TIMEOUT);
> cancel_delayed_work(&hdev->ncmd_timer); // May or may not cancel hci_ncmd_timeout() queued by queue_delayed_work().
> drain_workqueue(hdev->workqueue); // Will wait for hci_ncmd_timeout() queued by queue_delayed_work() to complete if cancel_delayed_work() failed to cancel.
>
> }
>
> // Actual flush() happens here.
>
> hci_dev_clear_flag(hdev, HCI_CMD_DRAIN_WORKQUEUE);
> }
>
>
>
> Case C:
>
> hci_dev_do_reset() {
> handle_cmd_cnt_and_timer(struct hci_dev *hdev, u8 ncmd) {
> if (!hci_dev_test_flag(hdev, HCI_CMD_DRAIN_WORKQUEUE))
> hci_dev_set_flag(hdev, HCI_CMD_DRAIN_WORKQUEUE);
> cancel_delayed_work(&hdev->cmd_timer);
> cancel_delayed_work(&hdev->ncmd_timer); // Does nothing.
> queue_delayed_work(hdev->workqueue, &hdev->ncmd_timer, HCI_NCMD_TIMEOUT);
> drain_workqueue(hdev->workqueue); // Will wait for hci_ncmd_timeout() queued by queue_delayed_work() to complete if delay timer has expired.
>
> }
>
> // Actual flush() happens here, but hci_ncmd_timeout() queued by queue_delayed_work() can be running if delay timer has not expired as of calling drain_workqueue().
>
> hci_dev_clear_flag(hdev, HCI_CMD_DRAIN_WORKQUEUE);
> }
--
BRs
Schspa Shi
