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BUG: KCSAN: data-race in bdev_disk_changed / nbd_size_update

write to 0xffff88812982d040 of 8 bytes by task 11046 on cpu 0:
 set_capacity include/linux/genhd.h:336 [inline]
 nbd_size_update+0x1a4/0x240 drivers/block/nbd.c:311
 __nbd_ioctl drivers/block/nbd.c:330 [inline]
 nbd_ioctl+0x3c0/0x770 drivers/block/nbd.c:1447
 __blkdev_driver_ioctl block/ioctl.c:224 [inline]
 blkdev_ioctl+0x1cf/0x410 block/ioctl.c:620
 block_ioctl+0x6d/0x80 fs/block_dev.c:1871
 vfs_ioctl fs/ioctl.c:48 [inline]
 __do_sys_ioctl fs/ioctl.c:753 [inline]
 __se_sys_ioctl+0xcb/0x140 fs/ioctl.c:739
 __x64_sys_ioctl+0x3f/0x50 fs/ioctl.c:739
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff88812982d040 of 8 bytes by task 11009 on cpu 1:
 get_capacity include/linux/genhd.h:332 [inline]
 check_disk_size_change fs/block_dev.c:1293 [inline]
 bdev_disk_changed+0x198/0x320 fs/block_dev.c:1413
 __blkdev_get+0x477/0xe30 fs/block_dev.c:1559
 blkdev_get fs/block_dev.c:1639 [inline]
 blkdev_open+0x123/0x170 fs/block_dev.c:1753
 do_dentry_open+0x52d/0x890 fs/open.c:817
 vfs_open+0x43/0x50 fs/open.c:931
 do_open fs/namei.c:3251 [inline]
 path_openat+0x1844/0x20a0 fs/namei.c:3368
 do_filp_open+0xbd/0x1d0 fs/namei.c:3395
 do_sys_openat2+0x33b/0x500 fs/open.c:1168
 do_sys_open fs/open.c:1184 [inline]
 __do_sys_open fs/open.c:1192 [inline]
 __se_sys_open fs/open.c:1188 [inline]
 __x64_sys_open+0xe2/0x110 fs/open.c:1188
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 11009 Comm: systemd-udevd Not tainted 5.9.0-rc3-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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