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BUG: KCSAN: data-race in __xa_set_mark / writeback_sb_inodes

write to 0xffff8881415ca97c of 4 bytes by task 32414 on cpu 0:
 xa_mark_set lib/xarray.c:69 [inline]
 xas_set_mark lib/xarray.c:889 [inline]
 __xa_set_mark+0x166/0x190 lib/xarray.c:1898
 __set_page_dirty+0x17e/0x1d0 fs/buffer.c:609
 mark_buffer_dirty+0x126/0x230 fs/buffer.c:1137
 __ext4_handle_dirty_metadata+0x20a/0x590 fs/ext4/ext4_jbd2.c:362
 __ext4_new_inode+0x1290/0x3070 fs/ext4/ialloc.c:1121
 ext4_symlink+0x2bc/0x770 fs/ext4/namei.c:3341
 vfs_symlink+0x216/0x2d0 fs/namei.c:3960
 do_symlinkat+0xee/0x280 fs/namei.c:3987
 __do_sys_symlink fs/namei.c:4006 [inline]
 __se_sys_symlink fs/namei.c:4004 [inline]
 __x64_sys_symlink+0x32/0x40 fs/namei.c:4004
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff8881415ca97c of 4 bytes by task 3051 on cpu 1:
 xa_marked include/linux/xarray.h:416 [inline]
 mapping_tagged include/linux/fs.h:488 [inline]
 requeue_inode fs/fs-writeback.c:1409 [inline]
 writeback_sb_inodes+0x91d/0x1020 fs/fs-writeback.c:1748
 __writeback_inodes_wb+0xb0/0x2a0 fs/fs-writeback.c:1790
 wb_writeback+0x290/0x660 fs/fs-writeback.c:1896
 wb_check_old_data_flush fs/fs-writeback.c:1998 [inline]
 wb_do_writeback+0x520/0x5d0 fs/fs-writeback.c:2051
 wb_workfn+0xb8/0x410 fs/fs-writeback.c:2080
 process_one_work+0x3e1/0x950 kernel/workqueue.c:2272
 worker_thread+0x635/0xb90 kernel/workqueue.c:2418
 kthread+0x1fa/0x220 kernel/kthread.c:292
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:296

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 3051 Comm: kworker/u4:4 Not tainted 5.10.0-rc1-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Workqueue: writeback wb_workfn (flush-8:0)
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