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BUG: KCSAN: data-race in p9_conn_cancel / p9_write_work

write to 0xffff888025096e28 of 4 bytes by task 16252 on cpu 0:
 p9_conn_cancel+0x88/0x400 net/9p/trans_fd.c:201
 p9_poll_mux net/9p/trans_fd.c:628 [inline]
 p9_poll_workfn+0x14c/0x410 net/9p/trans_fd.c:1145
 process_one_work+0x3e1/0x950 kernel/workqueue.c:2275
 worker_thread+0x616/0xa70 kernel/workqueue.c:2421
 kthread+0x20b/0x230 kernel/kthread.c:292
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:296

read to 0xffff888025096e28 of 4 bytes by task 3858 on cpu 1:
 p9_write_work+0x23/0x770 net/9p/trans_fd.c:451
 process_one_work+0x3e1/0x950 kernel/workqueue.c:2275
 worker_thread+0x616/0xa70 kernel/workqueue.c:2421
 kthread+0x20b/0x230 kernel/kthread.c:292
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:296

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 3858 Comm: kworker/1:2 Not tainted 5.11.0-rc6-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Workqueue: events p9_write_work
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