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BUG: KCSAN: data-race in generic_file_buffered_read / generic_write_end

write to 0xffff8880bb4f0238 of 8 bytes by task 26891 on cpu 1:
 i_size_write include/linux/fs.h:897 [inline]
 generic_write_end+0x99/0x220 fs/buffer.c:2187
 ext4_da_write_end+0x57e/0x760 fs/ext4/inode.c:3092
 generic_perform_write+0x23b/0x390 mm/filemap.c:3329
 ext4_buffered_write_iter+0x2cc/0x3b0 fs/ext4/file.c:270
 ext4_file_write_iter+0x76f/0x1010 include/linux/fs.h:805
 call_write_iter include/linux/fs.h:1908 [inline]
 new_sync_write fs/read_write.c:503 [inline]
 vfs_write+0x607/0x690 fs/read_write.c:578
 ksys_write+0xce/0x180 fs/read_write.c:631
 __do_sys_write fs/read_write.c:643 [inline]
 __se_sys_write fs/read_write.c:640 [inline]
 __x64_sys_write+0x3e/0x50 fs/read_write.c:640
 do_syscall_64+0x51/0xb0 arch/x86/entry/common.c:384
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff8880bb4f0238 of 8 bytes by task 26901 on cpu 0:
 i_size_read include/linux/fs.h:875 [inline]
 generic_file_buffered_read+0xf9d/0x19e0 mm/filemap.c:2092
 generic_file_read_iter+0x7d/0x3e0 mm/filemap.c:2326
 ext4_file_read_iter+0x2d6/0x420 fs/ext4/file.c:74
 call_read_iter include/linux/fs.h:1902 [inline]
 generic_file_splice_read+0x22a/0x310 fs/splice.c:312
 do_splice_to fs/splice.c:870 [inline]
 splice_direct_to_actor+0x2a8/0x660 fs/splice.c:950
 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:1601 [inline]
 __se_sys_sendfile64 fs/read_write.c:1587 [inline]
 __x64_sys_sendfile64+0xf2/0x130 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: 26901 Comm: syz-executor.0 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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