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BUG: KCSAN: data-race in __xa_set_mark / sync_inodes_sb

write to 0xffff8880bb84932c of 4 bytes by task 20408 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:2730 [inline]
 wp_page_shared+0x2d8/0x820 mm/memory.c:3059
 do_wp_page+0x6eb/0x740 mm/memory.c:3152
 handle_pte_fault mm/memory.c:4418 [inline]
 __handle_mm_fault mm/memory.c:4535 [inline]
 handle_mm_fault+0x1899/0x1af0 mm/memory.c:4633
 do_user_addr_fault+0x380/0x770 arch/x86/mm/fault.c:1372
 handle_page_fault arch/x86/mm/fault.c:1429 [inline]
 exc_page_fault+0xb8/0x330 arch/x86/mm/fault.c:1482
 asm_exc_page_fault+0x1e/0x30 arch/x86/include/asm/idtentry.h:538

read to 0xffff8880bb84932c of 4 bytes by task 8717 on cpu 1:
 xa_marked include/linux/xarray.h:416 [inline]
 mapping_tagged include/linux/fs.h:493 [inline]
 wait_sb_inodes fs/fs-writeback.c:2418 [inline]
 sync_inodes_sb+0x30e/0x450 fs/fs-writeback.c:2561
 __sync_filesystem fs/sync.c:34 [inline]
 sync_filesystem+0xe4/0x160 fs/sync.c:67
 ovl_sync_fs+0x74/0x90 fs/overlayfs/super.c:281
 __sync_filesystem fs/sync.c:39 [inline]
 sync_filesystem+0x112/0x160 fs/sync.c:67
 generic_shutdown_super+0x42/0x210 fs/super.c:448
 kill_anon_super+0x23/0x40 fs/super.c:1108
 deactivate_locked_super+0x6a/0xb0 fs/super.c:335
 deactivate_super+0x85/0x90 fs/super.c:366
 cleanup_mnt+0x277/0x2e0 fs/namespace.c:1118
 __cleanup_mnt+0x15/0x20 fs/namespace.c:1125
 task_work_run+0x8e/0x110 kernel/task_work.c:141
 tracehook_notify_resume include/linux/tracehook.h:188 [inline]
 exit_to_user_mode_loop kernel/entry/common.c:165 [inline]
 exit_to_user_mode_prepare+0x1b2/0x210 kernel/entry/common.c:192
 syscall_exit_to_user_mode+0x16/0x30 kernel/entry/common.c:267
 do_syscall_64+0x45/0x80 arch/x86/entry/common.c:56
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 8717 Comm: syz-executor.1 Not tainted 5.9.0-rc7-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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