linux-kernel - Re: [PATCH] USB: serial: mos7720: defer state restore to a workqueue

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Date:   Mon, 16 Nov 2020 14:31:02 -0800
From:   Davidlohr Bueso <dave@...olabs.net>
To:     Johan Hovold <johan@...nel.org>
Cc:     linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
        Davidlohr Bueso <dbueso@...e.de>
Subject: Re: [PATCH] USB: serial: mos7720: defer state restore to a workqueue

On Mon, 16 Nov 2020, Johan Hovold wrote:

>On Fri, Nov 13, 2020 at 08:27:25PM -0800, Davidlohr Bueso wrote:
>> @@ -1883,21 +1724,17 @@ static void mos7720_release(struct usb_serial *serial)
>>		if (mos_parport->msg_pending)
>>			wait_for_completion_timeout(&mos_parport->syncmsg_compl,
>>					    msecs_to_jiffies(MOS_WDR_TIMEOUT));
>> +		/*
>> +		 * If delayed work is currently scheduled, wait for it to
>> +		 * complete. This also implies barriers that ensure the
>> +		 * below serial clearing is not hoisted above the ->work.
>> +		 */
>> +		cancel_work_sync(&mos_parport->work);
>
>As I mentioned, this needs to be done *after* deregistering the port or
>you could theoretically end up with the work item being requeued.

Hmm sorry yes I forgot to mention this. I was counting on the private_data
already being null to prevent any new work being actually scheduled, so an
incoming restore state, for example, would be a nop.

>
>Yes, the same applies for the "synchronous" requests, but that's a
>preexisting issue.

Per the above I also assumed sync requests were also safe as is.

But I can certainly re-order things, how about:

mos7720_release():
    mos_parport->pp->private_data = NULL;
    parport_remove_port(mos_parport->pp);

    wait_for_completion_timeout();
    cancel_work_sync();

    usb_set_serial_data(serial, NULL);
    mos_parport->serial = NULL;

    parport_del_port(mos_parport->pp);
    kref_put();

Thanks,
Davidlohr