Subject: [PATCH RFC] r8152: Ensure that napi_schedule() is handled

From: Thomas Gleixner <[email-masked]>
Date: Fri, 14 May 2021 12:17:19 +0200

From: Thomas Gleixner <[email-masked]>
Date: Fri, 14 May 2021 11:46:08 +0200

The driver invokes napi_schedule() in several places from task
context. napi_schedule() raises the NET_RX softirq bit and relies on the
calling context to ensure that the softirq is handled. That's usually on
return from interrupt or on the outermost local_bh_enable().

But that's not the case here which causes the soft interrupt handling to be
delayed to the next interrupt or local_bh_enable(). If the task in which
context this is invoked is the last runnable task on a CPU and the CPU goes
idle before an interrupt arrives or a local_bh_disable/enable() pair
handles the pending soft interrupt then the NOHZ idle code emits the
following warning.

  NOHZ tick-stop error: Non-RCU local softirq work is pending, handler #08!!!

Prevent this by wrapping the napi_schedule() invocation from task context
into a local_bh_disable/enable() pair.

Reported-by: Michal Svec <[email-masked]>
Signed-off-by: Thomas Gleixner <[email-masked]>
---

Note: That's not the first incident of this. Shouldn't napi_schedule()
      have a debug check (under lockdep) to catch this?

---
 drivers/net/usb/r8152.c |   19 +++++++++++++++----
 1 file changed, 15 insertions(+), 4 deletions(-)

--- a/drivers/net/usb/r8152.c
+++ b/drivers/net/usb/r8152.c
@@ -1543,6 +1543,17 @@ void write_mii_word(struct net_device *n
 	r8152_mdio_write(tp, reg, val);
 }
 
+/*
+ * Wrapper around napi_schedule() to ensure that the raised network softirq
+ * is actually handled.
+ */
+static void r8152_napi_schedule(struct napi_struct *napi)
+{
+	local_bh_disable();
+	napi_schedule(napi);
+	local_bh_enable();
+}
+
 static int
 r8152_submit_rx(struct r8152 *tp, struct rx_agg *agg, gfp_t mem_flags);
 
@@ -1753,7 +1764,7 @@ static void read_bulk_callback(struct ur
 		spin_lock_irqsave(&tp->rx_lock, flags);
 		list_add_tail(&agg->list, &tp->rx_done);
 		spin_unlock_irqrestore(&tp->rx_lock, flags);
-		napi_schedule(&tp->napi);
+		r8152_napi_schedule(&tp->napi);
 		return;
 	case -ESHUTDOWN:
 		rtl_set_unplug(tp);
@@ -2640,7 +2651,7 @@ int r8152_submit_rx(struct r8152 *tp, st
 		netif_err(tp, rx_err, tp->netdev,
 			  "Couldn't submit rx[%p], ret = %d\n", agg, ret);
 
-		napi_schedule(&tp->napi);
+		r8152_napi_schedule(&tp->napi);
 	}
 
 	return ret;
@@ -8202,7 +8213,7 @@ static int rtl8152_post_reset(struct usb
 	usb_submit_urb(tp->intr_urb, GFP_KERNEL);
 
 	if (!list_empty(&tp->rx_done))
-		napi_schedule(&tp->napi);
+		r8152_napi_schedule(&tp->napi);
 
 	return 0;
 }
@@ -8256,7 +8267,7 @@ static int rtl8152_runtime_resume(struct
 		smp_mb__after_atomic();
 
 		if (!list_empty(&tp->rx_done))
-			napi_schedule(&tp->napi);
+			r8152_napi_schedule(&tp->napi);
 
 		usb_submit_urb(tp->intr_urb, GFP_NOIO);
 	} else {