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BUG: KCSAN: data-race in do_sendfile / generic_file_llseek_size

write to 0xffff88813b440c68 of 8 bytes by task 20015 on cpu 1:
 do_sendfile+0x749/0x950 fs/read_write.c:1269
 __do_sys_sendfile64 fs/read_write.c:1322 [inline]
 __se_sys_sendfile64 fs/read_write.c:1308 [inline]
 __x64_sys_sendfile64+0x110/0x150 fs/read_write.c:1308
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x41/0xc0 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x63/0xcd

read to 0xffff88813b440c68 of 8 bytes by task 20014 on cpu 0:
 vfs_setpos fs/read_write.c:61 [inline]
 generic_file_llseek_size+0x17a/0x2a0 fs/read_write.c:130
 ext4_llseek+0x166/0x190 fs/ext4/file.c:924
 vfs_llseek fs/read_write.c:289 [inline]
 ksys_lseek fs/read_write.c:302 [inline]
 __do_sys_lseek fs/read_write.c:313 [inline]
 __se_sys_lseek fs/read_write.c:311 [inline]
 __x64_sys_lseek+0xe3/0x140 fs/read_write.c:311
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x41/0xc0 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x63/0xcd

value changed: 0x0000000000000200 -> 0x0000000000000800

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 20014 Comm: syz-executor.4 Not tainted 6.4.0-rc7-syzkaller-00204-g61dabacdad4e #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/27/2023
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