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BUG: KCSAN: data-race in __xa_set_mark / writeback_sb_inodes

write to 0xffff888124f053ec of 4 bytes by task 8221 on cpu 0:
 xa_mark_set lib/xarray.c:69 [inline]
 xas_set_mark lib/xarray.c:886 [inline]
 __xa_set_mark+0x317/0x330 lib/xarray.c:1708
 __set_page_dirty+0x17e/0x1d0 fs/buffer.c:609
 mark_buffer_dirty+0x12e/0x260 fs/buffer.c:1137
 __block_commit_write fs/buffer.c:2084 [inline]
 block_commit_write fs/buffer.c:2476 [inline]
 block_page_mkwrite+0x25d/0x2e0 fs/buffer.c:2525
 ext4_page_mkwrite+0x572/0xae0 fs/ext4/inode.c:5985
 do_page_mkwrite mm/memory.c:2527 [inline]
 wp_page_shared+0x2b9/0x760 mm/memory.c:2855
 do_wp_page+0xb67/0xd30 mm/memory.c:2973
 handle_pte_fault mm/memory.c:4243 [inline]
 __handle_mm_fault mm/memory.c:4357 [inline]
 handle_mm_fault+0x170c/0x1950 mm/memory.c:4394
 do_user_addr_fault+0x393/0x810 arch/x86/mm/fault.c:1295
 handle_page_fault arch/x86/mm/fault.c:1365 [inline]
 exc_page_fault+0xb8/0x330 arch/x86/mm/fault.c:1418
 asm_exc_page_fault+0x1e/0x30 arch/x86/include/asm/idtentry.h:549

read to 0xffff888124f053ec of 4 bytes by task 12872 on cpu 1:
 xa_marked include/linux/xarray.h:416 [inline]
 mapping_tagged include/linux/fs.h:493 [inline]
 requeue_inode fs/fs-writeback.c:1401 [inline]
 writeback_sb_inodes+0x818/0xe40 fs/fs-writeback.c:1743
 wb_writeback+0x2b8/0x730 fs/fs-writeback.c:1892
 wb_do_writeback+0x108/0x610 fs/fs-writeback.c:2037
 wb_workfn+0xc8/0x460 fs/fs-writeback.c:2078
 process_one_work+0x3e1/0x9a0 kernel/workqueue.c:2269
 worker_thread+0x665/0xbe0 kernel/workqueue.c:2415
 kthread+0x20d/0x230 kernel/kthread.c:292
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:293

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 12872 Comm: kworker/u4:6 Not tainted 5.8.0-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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