syzbot

BTRFS warning (device loop5): failed to read tree root
BTRFS info (device loop5): enabling ssd optimizations
BTRFS info (device loop5): checking UUID tree
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BUG: KCSAN: data-race in btrfs_iget_path / join_transaction

write to 0xffff888090b2e250 of 8 bytes by task 18552 on cpu 1:
 join_transaction+0x63c/0x6d0 fs/btrfs/transaction.c:342
 start_transaction+0x4d1/0xb40 fs/btrfs/transaction.c:630
 btrfs_start_transaction+0x26/0x30 fs/btrfs/transaction.c:715
 btrfs_uuid_scan_kthread+0x38e/0x5b0 fs/btrfs/volumes.c:4456
 btrfs_uuid_rescan_kthread+0x2e/0x80 fs/btrfs/disk-io.c:2856
 kthread+0x20d/0x230 kernel/kthread.c:292
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:294

read to 0xffff888090b2e250 of 8 bytes by task 18523 on cpu 0:
 btrfs_read_locked_inode fs/btrfs/inode.c:3310 [inline]
 btrfs_iget_path+0x5e1/0xc00 fs/btrfs/inode.c:5386
 btrfs_iget+0x23/0x30 fs/btrfs/inode.c:5408
 btrfs_fill_super fs/btrfs/super.c:1322 [inline]
 btrfs_mount_root+0x76f/0x830 fs/btrfs/super.c:1672
 legacy_get_tree+0x70/0xc0 fs/fs_context.c:592
 vfs_get_tree+0x4d/0x1a0 fs/super.c:1547
 fc_mount fs/namespace.c:978 [inline]
 vfs_kern_mount+0xc6/0x130 fs/namespace.c:1008
 btrfs_mount+0x27b/0x790 fs/btrfs/super.c:1732
 legacy_get_tree+0x70/0xc0 fs/fs_context.c:592
 vfs_get_tree+0x4d/0x1a0 fs/super.c:1547
 do_new_mount fs/namespace.c:2875 [inline]
 path_mount+0x118d/0x1c70 fs/namespace.c:3192
 do_mount fs/namespace.c:3205 [inline]
 __do_sys_mount fs/namespace.c:3413 [inline]
 __se_sys_mount+0x126/0x180 fs/namespace.c:3390
 __x64_sys_mount+0x63/0x70 fs/namespace.c:3390
 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: 0 PID: 18523 Comm: syz-executor.5 Not tainted 5.9.0-rc6-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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