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BUG: KCSAN: data-race in __blkdev_get / blkdev_get

write to 0xffff88812abbe7c0 of 8 bytes by task 26934 on cpu 1:
 __blkdev_get+0x12c/0xc10 fs/block_dev.c:1591
 blkdev_get+0x50/0x210 fs/block_dev.c:1736
 blkdev_open+0x195/0x1e0 fs/block_dev.c:1875
 do_dentry_open+0x301/0x970 fs/open.c:797
 vfs_open+0x62/0x80 fs/open.c:914
 do_last fs/namei.c:3420 [inline]
 path_openat+0xf9f/0x3580 fs/namei.c:3537
 do_filp_open+0x11e/0x1b0 fs/namei.c:3567
 do_sys_open+0x3b3/0x4f0 fs/open.c:1097
 __do_sys_open fs/open.c:1115 [inline]
 __se_sys_open fs/open.c:1110 [inline]
 __x64_sys_open+0x55/0x70 fs/open.c:1110
 do_syscall_64+0xcc/0x3a0 arch/x86/entry/common.c:294
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff88812abbe7c0 of 8 bytes by task 26898 on cpu 0:
 blkdev_get+0xd7/0x210 fs/block_dev.c:1739
 blkdev_get_by_path+0x9e/0x110 fs/block_dev.c:1795
 btrfs_get_bdev_and_sb+0x41/0x1a0 fs/btrfs/volumes.c:496
 btrfs_open_one_device fs/btrfs/volumes.c:613 [inline]
 open_fs_devices+0x3ae/0x610 fs/btrfs/volumes.c:1156
 btrfs_open_devices+0xc5/0xd0 fs/btrfs/volumes.c:1201
 btrfs_mount_root+0x4b8/0x9c0 fs/btrfs/super.c:1533
 legacy_get_tree+0x7e/0xf0 fs/fs_context.c:647
 vfs_get_tree+0x56/0x1a0 fs/super.c:1545
 fc_mount+0x20/0x80 fs/namespace.c:962
 vfs_kern_mount.part.0+0xe6/0xf0 fs/namespace.c:992
 vfs_kern_mount+0x49/0x70 fs/namespace.c:979
 btrfs_mount+0x250/0xf37 fs/btrfs/super.c:1621
 legacy_get_tree+0x7e/0xf0 fs/fs_context.c:647
 vfs_get_tree+0x56/0x1a0 fs/super.c:1545
 do_new_mount fs/namespace.c:2822 [inline]
 do_mount+0x1004/0x14f0 fs/namespace.c:3142
 ksys_mount+0xe8/0x160 fs/namespace.c:3351
 __do_sys_mount fs/namespace.c:3365 [inline]
 __se_sys_mount fs/namespace.c:3362 [inline]
 __x64_sys_mount+0x70/0x90 fs/namespace.c:3362
 do_syscall_64+0xcc/0x3a0 arch/x86/entry/common.c:294
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 26898 Comm: syz-executor.0 Not tainted 5.5.0-rc1-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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