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BUG: KCSAN: data-race in p9_conn_cancel / p9_write_work

write to 0xffff8880407ca428 of 4 bytes by task 10791 on cpu 1:
 p9_conn_cancel+0x88/0x400 net/9p/trans_fd.c:199
 p9_poll_mux net/9p/trans_fd.c:626 [inline]
 p9_poll_workfn+0x14c/0x410 net/9p/trans_fd.c:1143
 process_one_work+0x3e1/0x950 kernel/workqueue.c:2272
 worker_thread+0x635/0xb90 kernel/workqueue.c:2418
 kthread+0x1fd/0x220 kernel/kthread.c:292
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:296

read to 0xffff8880407ca428 of 4 bytes by task 9763 on cpu 0:
 p9_write_work+0x23/0x770 net/9p/trans_fd.c:449
 process_one_work+0x3e1/0x950 kernel/workqueue.c:2272
 worker_thread+0x635/0xb90 kernel/workqueue.c:2418
 kthread+0x1fd/0x220 kernel/kthread.c:292
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:296

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 9763 Comm: kworker/0:4 Not tainted 5.10.0-rc5-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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