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BUG: KCSAN: data-race in __writeback_single_inode / ext4_sync_file

write to 0xffff8880b6dc7740 of 8 bytes by task 21992 on cpu 1:
 __writeback_single_inode+0x168/0x610 fs/fs-writeback.c:1486
 writeback_single_inode+0x121/0x530 fs/fs-writeback.c:1565
 sync_inode fs/fs-writeback.c:2602 [inline]
 sync_inode_metadata+0x52/0x70 fs/fs-writeback.c:2622
 ext4_fsync_nojournal fs/ext4/fsync.c:92 [inline]
 ext4_sync_file+0x335/0x6e0 fs/ext4/fsync.c:170
 vfs_fsync_range+0x107/0x120 fs/sync.c:200
 generic_write_sync include/linux/fs.h:2882 [inline]
 ext4_buffered_write_iter+0x369/0x3b0 fs/ext4/file.c:277
 ext4_file_write_iter+0x76f/0x1010 include/linux/fs.h:805
 call_write_iter include/linux/fs.h:1908 [inline]
 do_iter_readv_writev+0x321/0x3c0 fs/read_write.c:713
 do_iter_write+0x10e/0x470 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+0x562/0xb10 fs/read_write.c:1540
 __do_sys_sendfile64 fs/read_write.c:1595 [inline]
 __se_sys_sendfile64 fs/read_write.c:1587 [inline]
 __x64_sys_sendfile64+0xa9/0x130 fs/read_write.c:1587
 do_syscall_64+0x51/0xb0 arch/x86/entry/common.c:384
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff8880b6dc7740 of 8 bytes by task 22028 on cpu 0:
 ext4_fsync_nojournal fs/ext4/fsync.c:87 [inline]
 ext4_sync_file+0x26c/0x6e0 fs/ext4/fsync.c:170
 vfs_fsync_range+0x107/0x120 fs/sync.c:200
 generic_write_sync include/linux/fs.h:2882 [inline]
 ext4_buffered_write_iter+0x369/0x3b0 fs/ext4/file.c:277
 ext4_file_write_iter+0x76f/0x1010 include/linux/fs.h:805
 call_write_iter include/linux/fs.h:1908 [inline]
 do_iter_readv_writev+0x321/0x3c0 fs/read_write.c:713
 do_iter_write+0x10e/0x470 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+0x562/0xb10 fs/read_write.c:1540
 __do_sys_sendfile64 fs/read_write.c:1595 [inline]
 __se_sys_sendfile64 fs/read_write.c:1587 [inline]
 __x64_sys_sendfile64+0xa9/0x130 fs/read_write.c:1587
 do_syscall_64+0x51/0xb0 arch/x86/entry/common.c:384
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 22028 Comm: syz-executor.3 Not tainted 5.8.0-rc6-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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