Re: dvb usb issues since kernel 4.9

[Linus Torvalds]

On Mon, Jan 8, 2018 at 11:15 AM, Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> wrote:
>
> Both dwc2_hsotg and ehci-hcd use the tasklets embedded in the
> giveback_urb_bh member of struct usb_hcd.  See usb_hcd_giveback_urb()
> in drivers/usb/core/hcd.c; the calls are
>
>         else if (high_prio_bh)
>                 tasklet_hi_schedule(&bh->bh);
>         else
>                 tasklet_schedule(&bh->bh);
>
> As it turns out, high_prio_bh gets set for interrupt and isochronous
> URBs but not for bulk and control URBs.  The DVB driver in question
> uses bulk transfers.

Ok, so we could try out something like the appended?

NOTE! I have not tested this at all. It LooksObvious(tm), but...

                    Linus
 kernel/softirq.c | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/kernel/softirq.c b/kernel/softirq.c
index 2f5e87f1bae2..97b080956fea 100644
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -79,12 +79,16 @@ static void wakeup_softirqd(void)
 
 /*
  * If ksoftirqd is scheduled, we do not want to process pending softirqs
- * right now. Let ksoftirqd handle this at its own rate, to get fairness.
+ * right now. Let ksoftirqd handle this at its own rate, to get fairness,
+ * unless we're doing some of the synchronous softirqs.
  */
-static bool ksoftirqd_running(void)
+#define SOFTIRQ_NOW_MASK ((1 << HI_SOFTIRQ) | (1 << TASKLET_SOFTIRQ))
+static bool ksoftirqd_running(unsigned long pending)
 {
 	struct task_struct *tsk = __this_cpu_read(ksoftirqd);
 
+	if (pending & SOFTIRQ_NOW_MASK)
+		return false;
 	return tsk && (tsk->state == TASK_RUNNING);
 }
 
@@ -325,7 +329,7 @@ asmlinkage __visible void do_softirq(void)
 
 	pending = local_softirq_pending();
 
-	if (pending && !ksoftirqd_running())
+	if (pending && !ksoftirqd_running(pending))
 		do_softirq_own_stack();
 
 	local_irq_restore(flags);
@@ -352,7 +356,7 @@ void irq_enter(void)
 
 static inline void invoke_softirq(void)
 {
-	if (ksoftirqd_running())
+	if (ksoftirqd_running(local_softirq_pending()))
 		return;
 
 	if (!force_irqthreads) {