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BUG: KCSAN: data-race in filemap_flush / xas_set_mark

write to 0xffff88810041dca4 of 4 bytes by task 14185 on cpu 0:
 xa_mark_set lib/xarray.c:69 [inline]
 xas_set_mark+0x13e/0x150 lib/xarray.c:889
 __test_set_page_writeback+0x27f/0x500 mm/page-writeback.c:2787
 set_page_writeback include/linux/page-flags.h:562 [inline]
 __block_write_full_page+0x5d4/0xa10 fs/buffer.c:1805
 block_write_full_page+0x240/0x260 fs/buffer.c:2981
 blkdev_writepage+0x20/0x30 fs/block_dev.c:639
 __writepage+0x32/0xc0 mm/page-writeback.c:2311
 write_cache_pages+0x4d7/0x810 mm/page-writeback.c:2246
 generic_writepages+0x64/0xa0 mm/page-writeback.c:2337
 blkdev_writepages+0x19/0x20 fs/block_dev.c:1748
 do_writepages+0x7b/0x150 mm/page-writeback.c:2352
 __writeback_single_inode+0x80/0x450 fs/fs-writeback.c:1467
 writeback_sb_inodes+0x564/0xf10 fs/fs-writeback.c:1732
 __writeback_inodes_wb fs/fs-writeback.c:1801 [inline]
 wb_writeback+0x327/0x700 fs/fs-writeback.c:1907
 wb_check_background_flush fs/fs-writeback.c:1975 [inline]
 wb_do_writeback+0x4ce/0x510 fs/fs-writeback.c:2063
 wb_workfn+0xc8/0x520 fs/fs-writeback.c:2091
 process_one_work+0x3e9/0x8f0 kernel/workqueue.c:2275
 worker_thread+0x636/0xae0 kernel/workqueue.c:2421
 kthread+0x1d0/0x1f0 kernel/kthread.c:292
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:294

read to 0xffff88810041dca4 of 4 bytes by task 546 on cpu 1:
 xa_marked include/linux/xarray.h:418 [inline]
 mapping_tagged include/linux/fs.h:490 [inline]
 __filemap_fdatawrite_range mm/filemap.c:420 [inline]
 __filemap_fdatawrite mm/filemap.c:432 [inline]
 filemap_flush+0xfc/0x160 mm/filemap.c:459
 __sync_blockdev+0x69/0x70 fs/block_dev.c:525
 __sync_filesystem fs/sync.c:40 [inline]
 sync_filesystem+0xcb/0x160 fs/sync.c:64
 __do_sys_syncfs fs/sync.c:172 [inline]
 __se_sys_syncfs+0x5f/0xe0 fs/sync.c:161
 __x64_sys_syncfs+0x1b/0x20 fs/sync.c:161
 do_syscall_64+0x34/0x50 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xae

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 546 Comm: syz-executor.0 Not tainted 5.12.0-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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