[PATCH] SUNRPC: Fix a race in xs_reset_transport

[Suzuki K. Poulose]

From: "Suzuki K. Poulose" <suzuki.poulose@xxxxxxx>

Encountered the following BUG() with 4.3-rc1 on a fast model
for arm64 with NFS root filesystem.

------------[ cut here ]------------
kernel BUG at fs/inode.c:1493!

Internal error: Oops - BUG: 0 [#1] PREEMPT SMP
Modules linked in:
CPU: 2 PID: 1 Comm: swapper/0 Not tainted 4.3.0-rc1+ #855
Hardware name: FVP Base (DT)
task: ffffc000760b0000 ti: ffffc00076070000 task.ti: ffffc00076070000
PC is at iput+0x144/0x170
LR is at sock_release+0xbc/0xdc
pc : [<ffffc000001b4920>] lr : [<ffffc000004d1974>] pstate: 40000045
sp : ffffc00076073790
x29: ffffc00076073790 x28: ffffc00076073b40
x27: 00000000000003e8 x26: ffffc00076955000
x25: 000000000000000c x24: ffffc00076637200
x23: ffffc00076073930 x22: ffffc000769b8180
x21: ffffc000740500a8 x20: ffffc00074050158
x19: ffffc00074050030 x18: 000000009fcef6bf
x17: 00000000593e3df5 x16: 00000000b597f71d
x15: 00000000e2f9d3f6 x14: 0ffffffffffffffe
x13: 0000000000000020 x12: 0101010101010101
x11: 00000000000001c9 x10: 0000000000000750
x9 : ffffc00076073670 x8 : ffffc000760b07b0
x7 : 0000000000000001 x6 : 0000000000000001
x5 : 0000000000000000 x4 : 00000000ffffffff
x3 : 0000000000000000 x2 : ffffffffffffffff
x1 : ffffc00076070000 x0 : 0000000000000060

Process swapper/0 (pid: 1, stack limit = 0xffffc00076070020)
Stack: (0xffffc00076073790 to 0xffffc00076074000)

 [ stack contents stripped ]

Call trace:
[<ffffc000001b4920>] iput+0x144/0x170
[<ffffc000004d1970>] sock_release+0xb8/0xdc
[<ffffc00000578df0>] xs_reset_transport+0x8c/0xac
[<ffffc00000578e60>] xs_close+0x50/0x6c
[<ffffc00000578e9c>] xs_destroy+0x20/0x5c
[<ffffc00000575f70>] xprt_destroy+0x68/0x8c
[<ffffc0000057777c>] xprt_put+0x24/0x30
[<ffffc000005726c4>] rpc_free_client+0x78/0xd8
[<ffffc0000057288c>] rpc_release_client+0x94/0xec
[<ffffc00000572aac>] rpc_shutdown_client+0x58/0x118
[<ffffc00000278588>] nfs_mount+0x100/0x234
[<ffffc0000026cc88>] nfs_request_mount+0xa8/0x12c
[<ffffc0000026e564>] nfs_try_mount+0x54/0x2b4
[<ffffc0000026f140>] nfs_fs_mount+0x5cc/0xac0
[<ffffc0000019f1a0>] mount_fs+0x38/0x158
[<ffffc000001b81a8>] vfs_kern_mount+0x48/0x11c
[<ffffc000001bb390>] do_mount+0x208/0xc04
[<ffffc000001bc0b0>] SyS_mount+0x78/0xd0
[<ffffc000007f0fa8>] mount_root+0x80/0x148
[<ffffc000007f11a8>] prepare_namespace+0x138/0x184
[<ffffc000007f0b20>] kernel_init_freeable+0x1cc/0x1f4
[<ffffc000005a2914>] kernel_init+0xc/0xd8
Code: b5fffc00 17ffffed d4210000 17ffffd7 (d4210000)
---[ end trace 02951451f1831f54 ]---

With rpc_debug enabled here is the log :

RPC:       shutting down mount client for your.nfs.server
RPC:       rpc_release_client(ffffc00076637800)
RPC:       destroying UNIX authenticator ffffc000008f48c8
RPC:       destroying mount client for your.nfs.server
RPC:       destroying transport ffffc00076226000
RPC:       xs_destroy xprt ffffc00076226000
RPC:       xs_close xprt ffffc00076226000
RPC:       xs_tcp_state_change client ffffc00076226000...
RPC:       state 4 conn 1 dead 0 zapped 1 sk_shutdown 3
RPC:       xs_tcp_state_change client ffffc00076226000...
RPC:       state 5 conn 0 dead 0 zapped 1 sk_shutdown 3
RPC:       xs_tcp_state_change client ffffc00076226000...
RPC:       state 7 conn 0 dead 0 zapped 1 sk_shutdown 3
RPC:       disconnected transport ffffc00076226000
RPC:       xs_tcp_state_change client ffffc00076226000...
RPC:       state 7 conn 0 dead 0 zapped 1 sk_shutdown 3
RPC:       disconnected transport ffffc00076226000
RPC:       xs_tcp_data_ready...
RPC:       xs_tcp_state_change client ffffc00076226000...
RPC:       state 7 conn 0 dead 0 zapped 1 sk_shutdown 3
RPC:       disconnected transport ffffc00076226000
RPC:       wake_up_first(ffffc00076226170 "xprt_sending")

So it looks like just before we lock the callbacks in xs_reset_transport,
a few of the callbacks got through and issued the reset before we could
lock it. And we end up repeating the cleanups, ending up in the above
BUG() due to multiple sock_release().

This patch fixes the race by confirming that somebody else hasn't performed
the reset while we were waiting for the lock. Also, the kernel_shutdown()
is performed only if the sock is non-NULL to avoid a possible race.

Signed-off-by: Suzuki K. Poulose <suzuki.poulose@xxxxxxx>
---
 net/sunrpc/xprtsock.c |    9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/net/sunrpc/xprtsock.c b/net/sunrpc/xprtsock.c
index 7be90bc..6f4789d 100644
--- a/net/sunrpc/xprtsock.c
+++ b/net/sunrpc/xprtsock.c
@@ -822,9 +822,16 @@ static void xs_reset_transport(struct sock_xprt *transport)
 	if (atomic_read(&transport->xprt.swapper))
 		sk_clear_memalloc(sk);
 
-	kernel_sock_shutdown(sock, SHUT_RDWR);
+	if (sock)
+		kernel_sock_shutdown(sock, SHUT_RDWR);
 
 	write_lock_bh(&sk->sk_callback_lock);
+	/* Check someone has already done the job, while we were waiting */
+	if (!transport->inet) {
+		write_unlock_bh(&sk->sk_callback_lock);
+		return;
+	}
+
 	transport->inet = NULL;
 	transport->sock = NULL;
 
-- 
1.7.9.5

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