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BUG: KCSAN: data-race in p9_poll_workfn / p9_write_work

write to 0xffff888107699894 of 4 bytes by task 4516 on cpu 0:
 p9_write_work+0x4b1/0x7c0 net/9p/trans_fd.c:496
 process_one_work+0x3e9/0x8f0 kernel/workqueue.c:2276
 worker_thread+0x636/0xae0 kernel/workqueue.c:2422
 kthread+0x262/0x280 kernel/kthread.c:319
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:295

read to 0xffff888107699894 of 4 bytes by task 718 on cpu 1:
 p9_poll_mux net/9p/trans_fd.c:643 [inline]
 p9_poll_workfn+0x202/0x410 net/9p/trans_fd.c:1145
 process_one_work+0x3e9/0x8f0 kernel/workqueue.c:2276
 worker_thread+0x636/0xae0 kernel/workqueue.c:2422
 kthread+0x262/0x280 kernel/kthread.c:319
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:295

value changed: 0x00000009 -> 0x00000000

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 718 Comm: kworker/1:2 Tainted: G        W         5.14.0-rc2-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Workqueue: events p9_poll_workfn
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