[PATCH net] net: core: avoid napi_disable to cause deadlock

[Lijun Pan]

There are chances that napi_disable is 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.

CPU0                            CPU1
 napi_disable
  test_and_set_bit
  (napi_complete_done clears
   NAPI_STATE_SCHED, ret 0,
   and set NAPI_STATE_SCHED)
                                napi_disable
                                 test_and_set_bit
                                 (ret 1 and loop infinitely because
                                  no napi instance is scheduled to
                                  clear NAPI_STATE_SCHED bit)

Checking the napi state bit to make sure if napi is already disabled,
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 <l...@linux.ibm.com>
---
 net/core/dev.c | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/net/core/dev.c b/net/core/dev.c
index 6c5967e80132..eb3c0ddd4fd7 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -6809,6 +6809,24 @@ 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