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BUG: KCSAN: data-race in p9_client_cb / p9_client_rpc

write to 0xffff88812e1be908 of 4 bytes by task 42 on cpu 0:
 p9_client_cb+0x14/0x40 net/9p/client.c:429
 p9_conn_cancel+0x3d0/0x460 net/9p/trans_fd.c:215
 p9_poll_mux net/9p/trans_fd.c:628 [inline]
 p9_poll_workfn+0x14c/0x410 net/9p/trans_fd.c:1145
 process_one_work+0x3e9/0x8f0 kernel/workqueue.c:2275
 worker_thread+0x636/0xae0 kernel/workqueue.c:2421
 kthread+0x1d0/0x1f0 kernel/kthread.c:313
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:294

read to 0xffff88812e1be908 of 4 bytes by task 6802 on cpu 1:
 p9_client_rpc+0x205/0x630 net/9p/client.c:759
 p9_client_flush+0x155/0x220 net/9p/client.c:667
 p9_client_rpc+0x55f/0x630 net/9p/client.c:784
 p9_client_version+0x8e/0x300 net/9p/client.c:945
 p9_client_create+0x601/0x6f0 net/9p/client.c:1055
 v9fs_session_init+0x100/0xcf0 fs/9p/v9fs.c:406
 v9fs_mount+0x57/0x4c0 fs/9p/vfs_super.c:126
 legacy_get_tree+0x70/0xc0 fs/fs_context.c:592
 vfs_get_tree+0x4a/0x1a0 fs/super.c:1498
 do_new_mount fs/namespace.c:2905 [inline]
 path_mount+0x11d4/0x1ce0 fs/namespace.c:3235
 do_mount fs/namespace.c:3248 [inline]
 __do_sys_mount fs/namespace.c:3456 [inline]
 __se_sys_mount+0x23d/0x2e0 fs/namespace.c:3433
 __x64_sys_mount+0x63/0x70 fs/namespace.c:3433
 do_syscall_64+0x4a/0x90 arch/x86/entry/common.c:47
 entry_SYSCALL_64_after_hwframe+0x44/0xae

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 6802 Comm: syz-executor.2 Not tainted 5.13.0-rc2-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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