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BUG: KCSAN: data-race in writeback_sb_inodes / xas_set_mark

write to 0xffff888128eefd54 of 4 bytes by task 13989 on cpu 0:
 xa_mark_set lib/xarray.c:69 [inline]
 xas_set_mark lib/xarray.c:886 [inline]
 xas_set_mark+0x140/0x150 lib/xarray.c:870
 __xa_set_mark+0xdd/0x110 lib/xarray.c:1706
 __set_page_dirty+0x151/0x270 fs/buffer.c:624
 mark_buffer_dirty+0x26e/0x2b0 fs/buffer.c:1152
 __block_commit_write.isra.0+0x11d/0x170 fs/buffer.c:2081
 block_write_end+0x6d/0x140 fs/buffer.c:2159
 generic_write_end+0x8c/0x1f0 fs/buffer.c:2173
 ext4_da_write_end+0x162/0x670 fs/ext4/inode.c:3092
 generic_perform_write+0x1d7/0x320 mm/filemap.c:3298
 ext4_buffered_write_iter+0x14e/0x280 fs/ext4/file.c:270
 ext4_file_write_iter+0xf4/0xd30 fs/ext4/file.c:642
 call_write_iter include/linux/fs.h:1901 [inline]
 new_sync_write+0x303/0x400 fs/read_write.c:483
 __vfs_write+0x9e/0xb0 fs/read_write.c:496
 vfs_write fs/read_write.c:558 [inline]
 vfs_write+0x189/0x380 fs/read_write.c:542
 ksys_write+0xc5/0x1a0 fs/read_write.c:611
 __do_sys_write fs/read_write.c:623 [inline]
 __se_sys_write fs/read_write.c:620 [inline]
 __x64_sys_write+0x49/0x60 fs/read_write.c:620
 do_syscall_64+0xc7/0x390 arch/x86/entry/common.c:294
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff888128eefd54 of 4 bytes by task 22257 on cpu 1:
 mapping_tagged include/linux/fs.h:521 [inline]
 requeue_inode fs/fs-writeback.c:1401 [inline]
 writeback_sb_inodes+0x6c9/0xa10 fs/fs-writeback.c:1743
 __writeback_inodes_wb+0xe9/0x1b0 fs/fs-writeback.c:1785
 wb_writeback+0x557/0x6a0 fs/fs-writeback.c:1894
 wb_check_background_flush fs/fs-writeback.c:1962 [inline]
 wb_do_writeback fs/fs-writeback.c:2050 [inline]
 wb_workfn+0x66b/0x970 fs/fs-writeback.c:2078
 process_one_work+0x424/0x930 kernel/workqueue.c:2264
 worker_thread+0x9a/0x7e0 kernel/workqueue.c:2410
 kthread+0x1cb/0x1f0 kernel/kthread.c:255
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:352

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 22257 Comm: kworker/u4:1 Not tainted 5.6.0-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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