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SubjectRe: [PATCH 3/3] Bluetooth: hidp: fix possible might sleep error in hidp_session_thread
Hi Jeffy,

I'm really not an expert on bluetooth or HIDP, but I can't bring myself
to say that this is correct. I still think you have a problem.

On Tue, Jan 24, 2017 at 12:07:51PM +0800, Jeffy Chen wrote:
> It looks like hidp_session_thread has same pattern as the issue reported in
> old rfcomm:
>
> while (1) {
> set_current_state(TASK_INTERRUPTIBLE);
> if (condition)
> break;
> // may call might_sleep here
> schedule();
> }
> __set_current_state(TASK_RUNNING);
>
> Which fixed at:
> dfb2fae Bluetooth: Fix nested sleeps
>
> So let's fix it at the same way, also follow the suggestion of:
> https://lwn.net/Articles/628628/
>
> Signed-off-by: Jeffy Chen <jeffy.chen@rock-chips.com>
> ---
>
> net/bluetooth/hidp/core.c | 23 +++++++++++++++--------
> 1 file changed, 15 insertions(+), 8 deletions(-)
>
> diff --git a/net/bluetooth/hidp/core.c b/net/bluetooth/hidp/core.c
> index 0bec458..43d6e6a 100644
> --- a/net/bluetooth/hidp/core.c
> +++ b/net/bluetooth/hidp/core.c
> @@ -36,6 +36,7 @@
> #define VERSION "1.2"
>
> static DECLARE_RWSEM(hidp_session_sem);
> +static DECLARE_WAIT_QUEUE_HEAD(hidp_session_wq);
> static LIST_HEAD(hidp_session_list);
>
> static unsigned char hidp_keycode[256] = {
> @@ -1068,12 +1069,15 @@ static int hidp_session_start_sync(struct hidp_session *session)
> * Wake up session thread and notify it to stop. This is asynchronous and
> * returns immediately. Call this whenever a runtime error occurs and you want
> * the session to stop.
> - * Note: wake_up_process() performs any necessary memory-barriers for us.
> */
> static void hidp_session_terminate(struct hidp_session *session)
> {
> atomic_inc(&session->terminate);
> - wake_up_process(session->task);
> +
> + /* Ensure session->terminate is updated */
> + smp_mb__after_atomic();
> +
> + wake_up_interruptible(&hidp_session_wq);

So, you're adding a whole new wait queue here.

> }
>
> /*
> @@ -1180,7 +1184,9 @@ static void hidp_session_run(struct hidp_session *session)
> struct sock *ctrl_sk = session->ctrl_sock->sk;
> struct sock *intr_sk = session->intr_sock->sk;
> struct sk_buff *skb;
> + DEFINE_WAIT_FUNC(wait, woken_wake_function);
>
> + add_wait_queue(&hidp_session_wq, &wait);
> for (;;) {
> /*
> * This thread can be woken up two ways:
> @@ -1188,12 +1194,10 @@ static void hidp_session_run(struct hidp_session *session)
> * session->terminate flag and wakes this thread up.
> * - Via modifying the socket state of ctrl/intr_sock. This
> * thread is woken up by ->sk_state_changed().
> - *
> - * Note: set_current_state() performs any necessary
> - * memory-barriers for us.
> */
> - set_current_state(TASK_INTERRUPTIBLE);
>
> + /* Ensure session->terminate is updated */
> + smp_mb__before_atomic();
> if (atomic_read(&session->terminate))
> break;
>
> @@ -1227,11 +1231,14 @@ static void hidp_session_run(struct hidp_session *session)
> hidp_process_transmit(session, &session->ctrl_transmit,
> session->ctrl_sock);
>
> - schedule();
> + wait_woken(&wait, TASK_INTERRUPTIBLE, MAX_SCHEDULE_TIMEOUT);

And you're waiting on it here.

But you're already on two other wait queues (hidp_session_thread()). So
the nice WQ_FLAG_WOKEN handling will only happen if you get woken via
the new hidp_session_wq queue. But what about the other two? Seems like
again you might have a race condition that would lead you to
(temporarily, at least?) missing a wake-up attempt.

I'm not really sure what the best way to resolve this would be. My best
guess would be to either consolidate the use of these wait queues, or
lese roll a version of wait_woken() to handle 2 or more wait heads...

Am I wrong? I easily could be.

Brian

> }
> + remove_wait_queue(&hidp_session_wq, &wait);
>
> atomic_inc(&session->terminate);
> - set_current_state(TASK_RUNNING);
> +
> + /* Ensure session->terminate is updated */
> + smp_mb__after_atomic();
> }
>
> /*
> --
> 2.1.4
>
>

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