[PATCH net-next v2 1/3] netconsole: do not schedule skb pool refill from NMI

[Breno Leitao <leitao@debian.org>]

When alloc_skb() fails in find_skb(), the fallback path dequeues an skb
from np->skb_pool and unconditionally calls schedule_work() to top the
pool back up. schedule_work() ends up taking the workqueue pool locks,
which are not NMI-safe.

netconsole_write() is registered as the nbcon write_atomic callback and
is explicitly marked CON_NBCON_ATOMIC_UNSAFE, meaning it is invoked from
emergency/panic contexts including NMIs. If the NMI interrupts a thread
already holding the workqueue pool lock, calling schedule_work()
self-deadlocks and the panic message that was being printed is lost.

Introduce netcons_skb_pop() to fold the pool dequeue and the refill
request into a single helper. The helper skips schedule_work() when
called from NMI context; the pool is best-effort, and the next non-NMI
invocation of find_skb() will refill it. This keeps the fast path
untouched, the panic path NMI-safe, and the locking rules around the
fallback pool documented in one place.

Signed-off-by: Breno Leitao <leitao@debian.org>
---
 drivers/net/netconsole.c | 23 +++++++++++++++++++----
 1 file changed, 19 insertions(+), 4 deletions(-)

diff --git a/drivers/net/netconsole.c b/drivers/net/netconsole.c
index 8ecc2c71c699..918e4a9f4456 100644
--- a/drivers/net/netconsole.c
+++ b/drivers/net/netconsole.c
@@ -1654,6 +1654,23 @@ static struct notifier_block netconsole_netdev_notifier = {
 	.notifier_call  = netconsole_netdev_event,
 };
 
+/* Pop a pre-allocated skb from the pool and request a refill.
+ *
+ * The refill is requested via schedule_work(), which takes the workqueue
+ * pool locks and is therefore not NMI-safe. Skip the refill when called
+ * from NMI context; the next non-NMI caller will top the pool back up.
+ */
+static struct sk_buff *netcons_skb_pop(struct netpoll *np)
+{
+	struct sk_buff *skb;
+
+	skb = skb_dequeue(&np->skb_pool);
+	if (!in_nmi())
+		schedule_work(&np->refill_wq);
+
+	return skb;
+}
+
 static struct sk_buff *find_skb(struct netpoll *np, int len, int reserve)
 {
 	int count = 0;
@@ -1663,10 +1680,8 @@ static struct sk_buff *find_skb(struct netpoll *np, int len, int reserve)
 repeat:
 
 	skb = alloc_skb(len, GFP_ATOMIC);
-	if (!skb) {
-		skb = skb_dequeue(&np->skb_pool);
-		schedule_work(&np->refill_wq);
-	}
+	if (!skb)
+		skb = netcons_skb_pop(np);
 
 	if (!skb) {
 		if (++count < 10) {

-- 
2.54.0