Forwarded: [PATCH] ath9k: defer reg_in URB resubmission to workqueue

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Subject: [PATCH] ath9k: defer reg_in URB resubmission to workqueue
Author: kartikey406@xxxxxxxxx

#syz test: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master

ath9k_hif_usb_reg_in_cb() is a URB completion callback that
runs in softirq context via dummy_hcd's hrtimer which is
registered with HRTIMER_MODE_REL_SOFT.

Calling usb_submit_urb() directly from this softirq context
triggers a long synchronous chain:

  dummy_urb_enqueue()
    hrtimer_start(HRTIMER_MODE_REL_SOFT)
      dummy_timer()
        __usb_hcd_giveback_urb()
          ath9k_hif_usb_reg_in_cb()
            usb_submit_urb()  <- back to start

This keeps CPU busy in softirq context indefinitely, starving
the rcu_preempt kthread and causing an RCU stall:

  rcu: rcu_preempt kthread starved for 3053 jiffies!
  rcu: Unless rcu_preempt kthread gets sufficient CPU time,
       OOM is now expected behavior.

Fix this by deferring URB resubmission to a workqueue via
schedule_work(), allowing the softirq to exit quickly and
giving rcu_preempt kthread sufficient CPU time to process
the grace period.

Reported-by: syzbot+9b95da55ba5146a60734@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspot.com/bug?extid=9b95da55ba5146a60734
Link: https://syzkaller.appspot.com/bug?extid=9b95da55ba5146a60734
Signed-off-by: Deepanshu Kartikey <kartikey406@xxxxxxxxx>
---
 drivers/net/wireless/ath/ath9k/hif_usb.c | 38 +++++++++++++++++++-----
 drivers/net/wireless/ath/ath9k/hif_usb.h |  2 ++
 2 files changed, 32 insertions(+), 8 deletions(-)

diff --git a/drivers/net/wireless/ath/ath9k/hif_usb.c b/drivers/net/wireless/ath/ath9k/hif_usb.c
index 8533b88974b2..38c0cabe52bf 100644
--- a/drivers/net/wireless/ath/ath9k/hif_usb.c
+++ b/drivers/net/wireless/ath/ath9k/hif_usb.c
@@ -731,12 +731,38 @@ static void ath9k_hif_usb_rx_cb(struct urb *urb)
 	kfree(rx_buf);
 }
 
+static void ath9k_hif_usb_reg_in_resubmit(struct work_struct *work)
+{
+	struct rx_buf *rx_buf = container_of(work,
+					     struct rx_buf,
+					     work);
+	struct hif_device_usb *hif_dev = rx_buf->hif_dev;
+	struct urb *urb = rx_buf->urb;
+	int ret;
+
+	if (!hif_dev || !urb)
+		goto free_rx_buf;
+
+	usb_anchor_urb(urb, &hif_dev->reg_in_submitted);
+	ret = usb_submit_urb(urb, GFP_KERNEL);
+	if (ret) {
+		usb_unanchor_urb(urb);
+		goto free_skb;
+	}
+	return;
+
+free_skb:
+	kfree_skb(rx_buf->skb);
+free_rx_buf:
+	kfree(rx_buf);
+	urb->context = NULL;
+}
+
 static void ath9k_hif_usb_reg_in_cb(struct urb *urb)
 {
 	struct rx_buf *rx_buf = urb->context;
 	struct hif_device_usb *hif_dev = rx_buf->hif_dev;
 	struct sk_buff *skb = rx_buf->skb;
-	int ret;
 
 	if (!skb)
 		return;
@@ -786,13 +812,9 @@ static void ath9k_hif_usb_reg_in_cb(struct urb *urb)
 	}
 
 resubmit:
-	usb_anchor_urb(urb, &hif_dev->reg_in_submitted);
-	ret = usb_submit_urb(urb, GFP_ATOMIC);
-	if (ret) {
-		usb_unanchor_urb(urb);
-		goto free_skb;
-	}
-
+	rx_buf->urb = urb;
+	INIT_WORK(&rx_buf->work, ath9k_hif_usb_reg_in_resubmit);
+	schedule_work(&rx_buf->work);
 	return;
 free_skb:
 	kfree_skb(skb);
diff --git a/drivers/net/wireless/ath/ath9k/hif_usb.h b/drivers/net/wireless/ath/ath9k/hif_usb.h
index b3e66b0485a5..7c2a8d2c1cca 100644
--- a/drivers/net/wireless/ath/ath9k/hif_usb.h
+++ b/drivers/net/wireless/ath/ath9k/hif_usb.h
@@ -89,6 +89,8 @@ struct tx_buf {
 struct rx_buf {
 	struct sk_buff *skb;
 	struct hif_device_usb *hif_dev;
+	struct urb *urb;
+	struct work_struct work;
 };
 
 #define HIF_USB_TX_STOP  BIT(0)
-- 
2.43.0