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BUG: KCSAN: data-race in p9_client_cb / p9_client_rpc

write to 0xffff88810c59a870 of 4 bytes by task 28391 on cpu 1:
 p9_client_cb+0x22/0x50 net/9p/client.c:428
 p9_read_work+0x766/0x860 net/9p/trans_fd.c:364
 process_one_work+0x3d4/0x890 kernel/workqueue.c:2269
 worker_thread+0xa0/0x800 kernel/workqueue.c:2415
 kthread+0x1d4/0x200 drivers/block/aoe/aoecmd.c:1253
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:352

read to 0xffff88810c59a870 of 4 bytes by task 28433 on cpu 0:
 p9_client_rpc+0x27e/0xae0 net/9p/client.c:757
 p9_client_write+0xe8/0x430 net/9p/client.c:1656
 v9fs_fid_xattr_set+0x10e/0x1a0 fs/9p/xattr.c:128
 v9fs_xattr_set fs/9p/xattr.c:100 [inline]
 v9fs_xattr_handler_set+0x55/0x70 fs/9p/xattr.c:156
 __vfs_setxattr+0xc3/0x100 fs/xattr.c:150
 __vfs_setxattr_noperm+0xe1/0x2f0 fs/xattr.c:181
 vfs_setxattr+0xd2/0xf0 fs/xattr.c:224
 setxattr+0x243/0x340 fs/xattr.c:451
 path_setxattr+0x12c/0x150 fs/xattr.c:470
 __do_sys_setxattr fs/xattr.c:485 [inline]
 __se_sys_setxattr fs/xattr.c:481 [inline]
 __x64_sys_setxattr+0x76/0x90 fs/xattr.c:481
 do_syscall_64+0xcc/0x370 arch/x86/entry/common.c:290
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 28433 Comm: syz-executor.3 Not tainted 5.4.0-rc7+ #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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