Re: [PATCH 2/2] usb: typec: tcpm: Don't handle two continuous same state

[Guenter Roeck <linux@roeck-us.net>]

On 8/24/21 4:30 AM, Xu Yang wrote:
> When two tcpm_set_state are successively executed in non-worker context,
> such as tcpm_init, an abnormal sequence may be exist which causes two
> continuous same state to be handled.
> 
> tcpm_init:
> [    1.686293] CC1: 0 -> 0, CC2: 0 -> 0 [state SNK_UNATTACHED, polarity 0, disconnected]
> [    1.686300] state change SNK_UNATTACHED -> PORT_RESET [rev1 NONE_AMS]
> [    1.686309] 1-0050: registered
> [    1.686315] Setting usb_comm capable false
> [    1.687417] Setting voltage/current limit 0 mV 0 mA
> [    1.687425] polarity 0
> [    1.690709] Requesting mux state 0, usb-role 0, orientation 0
> [    1.691599] cc:=0
> [    1.691871] pending state change PORT_RESET -> PORT_RESET_WAIT_OFF @ 100 ms [rev1 NONE_AMS]
> [    1.691880] Setting usb_comm capable false
> [    1.692973] Setting voltage/current limit 0 mV 0 mA
> [    1.692980] polarity 0
> [    1.696843] Requesting mux state 0, usb-role 0, orientation 0
> [    1.697729] cc:=0
> [    1.697994] pending state change PORT_RESET -> PORT_RESET_WAIT_OFF @ 100 ms [rev1 NONE_AMS]
> 
> abnormal sequence:
>              [tcpm_init]                 [tcpm_state_machine_work]
> 1       tcpm_set_state(A)
> 2           port->state = A
> 3           kthread_queue_work
> 4           queue work to worker_list
> 5                                       dequeue work from worker_list
> 6       tcpm_set_state(B)
> 7           port->state = B
> 8           kthread_queue_work
> 9           queue work to worker_list
> 10                                      tcpm_state_machine_work(B)
> 11                                      port->state not change
> 12                                      dequeue work from worker_list
> 13                                      tcpm_state_machine_work(B)
> 
> state B is handled twice.
> 
> Fixes: 4b4e02c83167 ("typec: tcpm: Move out of staging")
> cc: <stable@vger.kernel.org>
> Signed-off-by: Xu Yang <xu.yang_2@nxp.com>
> 
> diff --git a/drivers/usb/typec/tcpm/tcpm.c b/drivers/usb/typec/tcpm/tcpm.c
> index 4bdf119b1306..e0a319e00b12 100644
> --- a/drivers/usb/typec/tcpm/tcpm.c
> +++ b/drivers/usb/typec/tcpm/tcpm.c
> @@ -4825,6 +4825,10 @@ static void tcpm_state_machine_work(struct kthread_work *work)
>   		port->delayed_state = INVALID_STATE;
>   	}
>   
> +	/* If target state is the same as the last entered state, skip it */
> +	if (port->enter_state == port->state)
> +		goto done;
> +

I don't think that really solves the problem; it looks more like a bandage around
a race condition between setting a state in one context and executing the state
machine in another. I think it would be better to solve the underlying race condition.

The problem is ultimately that tcpm_init() ends up calling tcpm_set_state() several
times, the last time being PORT_RESET. The worker starts running and waits for the
port lock to be released. Since the worker is already running, but state_machine_running
is not yet set, tcpm_set_state() triggers it again. Maybe we need a second flag to detect
and handle that situation, or a better means to determine if the worker is running.

Guenter