netdev - [PATCH net] dev: Delay the free of the percpu refcount

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Date:   Thu, 29 Aug 2019 23:23:16 -0600
From:   Subash Abhinov Kasiviswanathan <subashab@...eaurora.org>
To:     eric.dumazet@...il.com, davem@...emloft.net, netdev@...r.kernel.org
Cc:     Subash Abhinov Kasiviswanathan <subashab@...eaurora.org>,
        Sean Tranchetti <stranche@...eaurora.org>
Subject: [PATCH net] dev: Delay the free of the percpu refcount

While running stress-ng on an ARM64 kernel, the following oops
was observedi -

44837.761523:   <6> Unable to handle kernel paging request at
                     virtual address 0000004a88287000
44837.761651:   <2> pc : in_dev_finish_destroy+0x4c/0xc8
44837.761654:   <2> lr : in_dev_finish_destroy+0x2c/0xc8
44837.762393:   <2> Call trace:
44837.762398:   <2>  in_dev_finish_destroy+0x4c/0xc8
44837.762404:   <2>  in_dev_rcu_put+0x24/0x30
44837.762412:   <2>  rcu_nocb_kthread+0x43c/0x468
44837.762418:   <2>  kthread+0x118/0x128
44837.762424:   <2>  ret_from_fork+0x10/0x1c

Prior to this, it appeared as if some of the inet6_dev allocations
were failing. From the memory dump, the last operation performed
was dev_put(), however the pcpu_refcnt was NULL while the
reg_state = NETREG_RELEASED. Effectively, the refcount memory was
freed in free_netdev() before the last reference was dropped.

Fix this by freeing the memory after all references are dropped and
before the dev memory itself is freed.

Fixes: 29b4433d991c ("net: percpu net_device refcount")
Cc: Sean Tranchetti <stranche@...eaurora.org>
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@...eaurora.org>
---
 net/core/dev.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/net/core/dev.c b/net/core/dev.c
index 49589ed..bce40d8 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -9128,6 +9128,9 @@ void netdev_freemem(struct net_device *dev)
 {
 	char *addr = (char *)dev - dev->padded;
 
+	free_percpu(dev->pcpu_refcnt);
+	dev->pcpu_refcnt = NULL;
+
 	kvfree(addr);
 }
 
@@ -9272,9 +9275,6 @@ void free_netdev(struct net_device *dev)
 	list_for_each_entry_safe(p, n, &dev->napi_list, dev_list)
 		netif_napi_del(p);
 
-	free_percpu(dev->pcpu_refcnt);
-	dev->pcpu_refcnt = NULL;
-
 	/*  Compatibility with error handling in drivers */
 	if (dev->reg_state == NETREG_UNINITIALIZED) {
 		netdev_freemem(dev);
-- 
1.9.1