[PATCH v3] USB: HCD: Fix URB giveback issue in tasklet function

[Weitao Wang]

Usb core introduce the mechanism of giveback of URB in tasklet context to
reduce hardware interrupt handling time. On some test situation(such as
FIO with 4KB block size), when tasklet callback function called to
giveback URB, interrupt handler add URB node to the bh->head list also.
If check bh->head list again after finish all URB giveback of local_list,
then it may introduce a "dynamic balance" between giveback URB and add URB
to bh->head list. This tasklet callback function may not exit for a long
time, which will cause other tasklet function calls to be delayed. Some
real-time applications(such as KB and Mouse) will see noticeable lag.

In order to prevent the tasklet function from occupying the cpu for a long
time at a time, new URBS will not be added to the local_list even though
the bh->head list is not empty. But also need to ensure the left URB
giveback to be processed in time, so add a member high_prio for structure
giveback_urb_bh to prioritize tasklet and schelule this tasklet again if
bh->head list is not empty.

At the same time, we are able to prioritize tasklet through structure
member high_prio. So, replace the local high_prio_bh variable with this
structure member in usb_hcd_giveback_urb.

Fixes: 94dfd7edfd5c ("USB: HCD: support giveback of URB in tasklet context")
Signed-off-by: Weitao Wang <WeitaoWang-oc@xxxxxxxxxxx>
---
v2->v3
 - Add more detail info about how to patch this issue.
 - Change initial value of boolean variable high_prio from 1 to true.

 drivers/usb/core/hcd.c  | 26 +++++++++++++++-----------
 include/linux/usb/hcd.h |  1 +
 2 files changed, 16 insertions(+), 11 deletions(-)

diff --git a/drivers/usb/core/hcd.c b/drivers/usb/core/hcd.c
index 06eea8848ccc..11c8ea0cccc8 100644
--- a/drivers/usb/core/hcd.c
+++ b/drivers/usb/core/hcd.c
@@ -1691,7 +1691,6 @@ static void usb_giveback_urb_bh(struct tasklet_struct *t)
 
 	spin_lock_irq(&bh->lock);
 	bh->running = true;
- restart:
 	list_replace_init(&bh->head, &local_list);
 	spin_unlock_irq(&bh->lock);
 
@@ -1705,10 +1704,17 @@ static void usb_giveback_urb_bh(struct tasklet_struct *t)
 		bh->completing_ep = NULL;
 	}
 
-	/* check if there are new URBs to giveback */
+	/*
+	 * giveback new URBs next time to prevent this function
+	 * from not exiting for a long time.
+	 */
 	spin_lock_irq(&bh->lock);
-	if (!list_empty(&bh->head))
-		goto restart;
+	if (!list_empty(&bh->head)) {
+		if (bh->high_prio)
+			tasklet_hi_schedule(&bh->bh);
+		else
+			tasklet_schedule(&bh->bh);
+	}
 	bh->running = false;
 	spin_unlock_irq(&bh->lock);
 }
@@ -1737,7 +1743,7 @@ static void usb_giveback_urb_bh(struct tasklet_struct *t)
 void usb_hcd_giveback_urb(struct usb_hcd *hcd, struct urb *urb, int status)
 {
 	struct giveback_urb_bh *bh;
-	bool running, high_prio_bh;
+	bool running;
 
 	/* pass status to tasklet via unlinked */
 	if (likely(!urb->unlinked))
@@ -1748,13 +1754,10 @@ void usb_hcd_giveback_urb(struct usb_hcd *hcd, struct urb *urb, int status)
 		return;
 	}
 
-	if (usb_pipeisoc(urb->pipe) || usb_pipeint(urb->pipe)) {
+	if (usb_pipeisoc(urb->pipe) || usb_pipeint(urb->pipe))
 		bh = &hcd->high_prio_bh;
-		high_prio_bh = true;
-	} else {
+	else
 		bh = &hcd->low_prio_bh;
-		high_prio_bh = false;
-	}
 
 	spin_lock(&bh->lock);
 	list_add_tail(&urb->urb_list, &bh->head);
@@ -1763,7 +1766,7 @@ void usb_hcd_giveback_urb(struct usb_hcd *hcd, struct urb *urb, int status)
 
 	if (running)
 		;
-	else if (high_prio_bh)
+	else if (bh->high_prio)
 		tasklet_hi_schedule(&bh->bh);
 	else
 		tasklet_schedule(&bh->bh);
@@ -2959,6 +2962,7 @@ int usb_add_hcd(struct usb_hcd *hcd,
 
 	/* initialize tasklets */
 	init_giveback_urb_bh(&hcd->high_prio_bh);
+	hcd->high_prio_bh.high_prio = true;
 	init_giveback_urb_bh(&hcd->low_prio_bh);
 
 	/* enable irqs just before we start the controller,
diff --git a/include/linux/usb/hcd.h b/include/linux/usb/hcd.h
index 2c1fc9212cf2..98d1921f02b1 100644
--- a/include/linux/usb/hcd.h
+++ b/include/linux/usb/hcd.h
@@ -66,6 +66,7 @@
 
 struct giveback_urb_bh {
 	bool running;
+	bool high_prio;
 	spinlock_t lock;
 	struct list_head  head;
 	struct tasklet_struct bh;
-- 
2.32.0