gtp: fix Illegal context switch in RCU read-side critical section. [Linux 4.9.187]

This Linux kernel change "gtp: fix Illegal context switch in RCU read-side critical section" is included in the Linux 4.9.187 release. This change is authored by Taehee Yoo <ap420073 [at] gmail.com> on Wed Jul 3 00:23:13 2019 +0900. The commit for this change in Linux stable tree is 1f9ec64 (patch) which is from upstream commit 3f167e1. The same Linux upstream change may have been applied to various maintained Linux releases and you can find all Linux releases containing changes from upstream 3f167e1.

gtp: fix Illegal context switch in RCU read-side critical section.

[ Upstream commit 3f167e1921865b379a9becf03828e7202c7b4917 ]

ipv4_pdp_add() is called in RCU read-side critical section.
So GFP_KERNEL should not be used in the function.
This patch make ipv4_pdp_add() to use GFP_ATOMIC instead of GFP_KERNEL.

Test commands:
gtp-link add gtp1 &
gtp-tunnel add gtp1 v1 100 200 1.1.1.1 2.2.2.2

Splat looks like:
[  130.618881] =============================
[  130.626382] WARNING: suspicious RCU usage
[  130.626994] 5.2.0-rc6+ #50 Not tainted
[  130.627622] -----------------------------
[  130.628223] ./include/linux/rcupdate.h:266 Illegal context switch in RCU read-side critical section!
[  130.629684]
[  130.629684] other info that might help us debug this:
[  130.629684]
[  130.631022]
[  130.631022] rcu_scheduler_active = 2, debug_locks = 1
[  130.632136] 4 locks held by gtp-tunnel/1025:
[  130.632925]  #0: 000000002b93c8b7 (cb_lock){++++}, at: genl_rcv+0x15/0x40
[  130.634159]  #1: 00000000f17bc999 (genl_mutex){+.+.}, at: genl_rcv_msg+0xfb/0x130
[  130.635487]  #2: 00000000c644ed8e (rtnl_mutex){+.+.}, at: gtp_genl_new_pdp+0x18c/0x1150 [gtp]
[  130.636936]  #3: 0000000007a1cde7 (rcu_read_lock){....}, at: gtp_genl_new_pdp+0x187/0x1150 [gtp]
[  130.638348]
[  130.638348] stack backtrace:
[  130.639062] CPU: 1 PID: 1025 Comm: gtp-tunnel Not tainted 5.2.0-rc6+ #50
[  130.641318] Call Trace:
[  130.641707]  dump_stack+0x7c/0xbb
[  130.642252]  ___might_sleep+0x2c0/0x3b0
[  130.642862]  kmem_cache_alloc_trace+0x1cd/0x2b0
[  130.643591]  gtp_genl_new_pdp+0x6c5/0x1150 [gtp]
[  130.644371]  genl_family_rcv_msg+0x63a/0x1030
[  130.645074]  ? mutex_lock_io_nested+0x1090/0x1090
[  130.645845]  ? genl_unregister_family+0x630/0x630
[  130.646592]  ? debug_show_all_locks+0x2d0/0x2d0
[  130.647293]  ? check_flags.part.40+0x440/0x440
[  130.648099]  genl_rcv_msg+0xa3/0x130
[ ... ]

Fixes: 459aa660eb1d ("gtp: add initial driver for datapath of GPRS Tunneling Protocol (GTP-U)")
Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>

There are 2 lines of Linux source code added/deleted in this change. Code changes to Linux kernel are as follows.

 drivers/net/gtp.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/net/gtp.c b/drivers/net/gtp.c
index cb206e5..60df6e3 100644
--- a/drivers/net/gtp.c
+++ b/drivers/net/gtp.c
@@ -952,7 +952,7 @@ static int ipv4_pdp_add(struct net_device *dev, struct genl_info *info)

    }

-   pctx = kmalloc(sizeof(struct pdp_ctx), GFP_KERNEL);
+   pctx = kmalloc(sizeof(*pctx), GFP_ATOMIC);
    if (pctx == NULL)
        return -ENOMEM;

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