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BUG: KCSAN: data-race in p9_conn_cancel / p9_write_work

write to 0xffff8881348f8a28 of 4 bytes by task 5825 on cpu 0:
 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:1146
 process_one_work+0x3d3/0x720 kernel/workqueue.c:2289
 worker_thread+0x618/0xa70 kernel/workqueue.c:2436
 kthread+0x1a9/0x1e0 kernel/kthread.c:376
 ret_from_fork+0x1f/0x30

read to 0xffff8881348f8a28 of 4 bytes by task 1923 on cpu 1:
 p9_write_work+0x23/0x770 net/9p/trans_fd.c:449
 process_one_work+0x3d3/0x720 kernel/workqueue.c:2289
 worker_thread+0x618/0xa70 kernel/workqueue.c:2436
 kthread+0x1a9/0x1e0 kernel/kthread.c:376
 ret_from_fork+0x1f/0x30

value changed: 0x00000000 -> 0xffffff98

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 1923 Comm: kworker/1:5 Not tainted 5.18.0-rc3-syzkaller-00006-gb7f73403a3e9-dirty #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Workqueue: events p9_write_work
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