netdev - [PATCH net v2] net: core: make napi

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Date:   Wed, 14 Apr 2021 03:08:45 -0500
From:   Lijun Pan <lijunp213@...il.com>
To:     netdev@...r.kernel.org
Cc:     Lijun Pan <lijunp213@...il.com>
Subject: [PATCH net v2] net: core: make napi_disable more robust

There are chances that napi_disable can be called twice by NIC driver.
This could generate deadlock. For example,
the first napi_disable will spin until NAPI_STATE_SCHED is cleared
by napi_complete_done, then set it again.
When napi_disable is called the second time, it will loop infinitely
because no dev->poll will be running to clear NAPI_STATE_SCHED.

Though it is driver writer's responsibility to make sure it being
called only once, making napi_disable more robust does not hurt, not
to say it can prevent a buggy driver from crashing a system.
So, we check the napi state bit to make sure that if napi is already
disabled, we exit the call early enough to avoid spinning infinitely.

Fixes: bea3348eef27 ("[NET]: Make NAPI polling independent of struct net_device objects.")
Signed-off-by: Lijun Pan <lijunp213@...il.com>
---
v2: justify that this patch makes napi_disable more robust.

 net/core/dev.c | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/net/core/dev.c b/net/core/dev.c
index 1f79b9aa9a3f..fa0aa212b7bb 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -6830,6 +6830,24 @@ EXPORT_SYMBOL(netif_napi_add);
 void napi_disable(struct napi_struct *n)
 {
 	might_sleep();
+
+	/* make sure napi_disable() runs only once,
+	 * When napi is disabled, the state bits are like:
+	 * NAPI_STATE_SCHED (set by previous napi_disable)
+	 * NAPI_STATE_NPSVC (set by previous napi_disable)
+	 * NAPI_STATE_DISABLE (cleared by previous napi_disable)
+	 * NAPI_STATE_PREFER_BUSY_POLL (cleared by previous napi_complete_done)
+	 * NAPI_STATE_MISSED (cleared by previous napi_complete_done)
+	 */
+
+	if (napi_disable_pending(n))
+		return;
+	if (test_bit(NAPI_STATE_SCHED, &n->state) &&
+	    test_bit(NAPI_STATE_NPSVC, &n->state) &&
+	    !test_bit(NAPI_STATE_MISSED, &n->state) &&
+	    !test_bit(NAPI_STATE_PREFER_BUSY_POLL, &n->state))
+		return;
+
 	set_bit(NAPI_STATE_DISABLE, &n->state);

 	while (test_and_set_bit(NAPI_STATE_SCHED, &n->state))
-- 
2.23.0