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BUG: KCSAN: data-race in p9_client_cb / p9_client_rpc

write to 0xffff88811eb8c7f8 of 4 bytes by task 12 on cpu 0:
 p9_client_cb+0x14/0x40 net/9p/client.c:428
 p9_conn_cancel+0x3a2/0x400 net/9p/trans_fd.c:213
 p9_poll_mux net/9p/trans_fd.c:622 [inline]
 p9_poll_workfn+0x14c/0x410 net/9p/trans_fd.c:1122
 process_one_work+0x3e1/0x9a0 kernel/workqueue.c:2269
 worker_thread+0x665/0xbe0 kernel/workqueue.c:2415
 kthread+0x20d/0x230 kernel/kthread.c:291
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:293

read to 0xffff88811eb8c7f8 of 4 bytes by task 15286 on cpu 1:
 p9_client_rpc+0x23f/0x650 net/9p/client.c:757
 p9_client_version+0x8e/0x360 net/9p/client.c:943
 p9_client_create+0x601/0x6f0 net/9p/client.c:1053
 v9fs_session_init+0xf5/0xd90 fs/9p/v9fs.c:406
 v9fs_mount+0x57/0x530 fs/9p/vfs_super.c:124
 legacy_get_tree+0x70/0xc0 fs/fs_context.c:592
 vfs_get_tree+0x4d/0x1a0 fs/super.c:1547
 do_new_mount fs/namespace.c:2874 [inline]
 do_mount+0x115c/0x1bd0 fs/namespace.c:3199
 __do_sys_mount fs/namespace.c:3409 [inline]
 __se_sys_mount+0xd3/0x100 fs/namespace.c:3386
 __x64_sys_mount+0x63/0x70 fs/namespace.c:3386
 do_syscall_64+0x51/0xb0 arch/x86/entry/common.c:384
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 15286 Comm: syz-executor.3 Not tainted 5.8.0-rc5-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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