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BUG: KCSAN: data-race in __mark_inode_dirty / writeback_sb_inodes

write to 0xffff8880b9f4cac0 of 8 bytes by task 21 on cpu 0:
 writeback_sb_inodes+0x3a7/0xe40 fs/fs-writeback.c:1705
 __writeback_inodes_wb+0xb2/0x240 fs/fs-writeback.c:1785
 wb_writeback+0x2e0/0x730 fs/fs-writeback.c:1894
 wb_check_start_all fs/fs-writeback.c:2018 [inline]
 wb_do_writeback+0x433/0x610 fs/fs-writeback.c:2044
 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:294

read to 0xffff8880b9f4cac0 of 8 bytes by task 32000 on cpu 1:
 __mark_inode_dirty+0xb9/0x7e0 fs/fs-writeback.c:2272
 mark_buffer_dirty+0x14b/0x260 fs/buffer.c:1141
 ext4_block_write_begin+0x7c6/0xb60 fs/ext4/inode.c:1068
 ext4_da_write_begin+0x4e8/0xa30 fs/ext4/inode.c:2997
 generic_perform_write+0x196/0x390 mm/filemap.c:3397
 ext4_buffered_write_iter+0x2cc/0x3b0 fs/ext4/file.c:270
 ext4_file_write_iter+0x57b/0x1000 fs/ext4/file.c:498
 call_write_iter include/linux/fs.h:1882 [inline]
 do_iter_readv_writev+0x32e/0x3d0 fs/read_write.c:713
 do_iter_write+0x112/0x4b0 fs/read_write.c:1018
 vfs_iter_write+0x4c/0x70 fs/read_write.c:1059
 iter_file_splice_write+0x41a/0x770 fs/splice.c:750
 do_splice_from fs/splice.c:846 [inline]
 direct_splice_actor+0x95/0x160 fs/splice.c:1016
 splice_direct_to_actor+0x365/0x660 fs/splice.c:971
 do_splice_direct+0xf2/0x170 fs/splice.c:1059
 do_sendfile+0x56a/0xba0 fs/read_write.c:1540
 __do_sys_sendfile64 fs/read_write.c:1601 [inline]
 __se_sys_sendfile64 fs/read_write.c:1587 [inline]
 __x64_sys_sendfile64+0xf2/0x130 fs/read_write.c:1587
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 32000 Comm: syz-executor.3 Not tainted 5.8.0-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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