On Fri, Nov 13, 2020 at 08:27:25PM -0800, Davidlohr Bueso wrote:
> The parallel port restore operation currently defers writes
> to a tasklet, if it sees a locked disconnect mutex. The
> driver goes to a lot of trouble to ensure writes happen
> in a non-blocking context, but things can be greatly
> simplified if it's done in regular process context and
> this is not a system performance critical path. As such,
> instead of doing the async state restore writes in irq
> context, use a workqueue and just do regular synchronous
> writes.
>
> In addition to the cleanup, this also imposes less on the
> overall system as tasklets have been deprecated because
> of it's BH implications, potentially blocking a higher
> priority task from running. We also get rid of hacks
> such as trylocking a mutex in irq, something which does
> not play nice with priority boosting in PREEMPT_RT.
>
> Signed-off-by: Davidlohr Bueso <dbueso@xxxxxxx>
> -/*
> - * This is the the common top part of all parallel port callback operations that
> + * This is the common top part of all parallel port callback operations that
> * send synchronous messages to the device. This implements convoluted locking
> * that avoids two scenarios: (1) a port operation is called after usbserial
> * has called our release function, at which point struct mos7715_parport has
This is an unrelated change, but ok.
> @@ -641,10 +488,9 @@ static void parport_mos7715_restore_state(struct parport *pp,
> }
> mos_parport->shadowDCR = s->u.pc.ctr;
> mos_parport->shadowECR = s->u.pc.ecr;
> - write_parport_reg_nonblock(mos_parport, MOS7720_DCR,
> - mos_parport->shadowDCR);
> - write_parport_reg_nonblock(mos_parport, MOS7720_ECR,
> - mos_parport->shadowECR);
> +
> + /* defer synchronous writes outside of irq */
This one isn't called in interrupt context, but with interrupts
disabled and a spin lock held.
> + schedule_work(&mos_parport->work);
> spin_unlock(&release_lock);
> }
> @@ -1869,8 +1712,6 @@ static void mos7720_release(struct usb_serial *serial)
>
> if (le16_to_cpu(serial->dev->descriptor.idProduct)
> == MOSCHIP_DEVICE_ID_7715) {
> - struct urbtracker *urbtrack;
> - unsigned long flags;
> struct mos7715_parport *mos_parport =
> usb_get_serial_data(serial);
>
> @@ -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.
Yes, the same applies for the "synchronous" requests, but that's a
preexisting issue.
> parport_remove_port(mos_parport->pp);
> usb_set_serial_data(serial, NULL);
> mos_parport->serial = NULL;
Johan
