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BUG: KCSAN: data-race in do_fcntl / generic_write_checks

write to 0xffff888108b67540 of 4 bytes by task 8688 on cpu 0:
 setfl fs/fcntl.c:81 [inline]
 do_fcntl+0xc4c/0xc80 fs/fcntl.c:355
 __do_sys_fcntl fs/fcntl.c:471 [inline]
 __se_sys_fcntl+0xc6/0x180 fs/fcntl.c:456
 __x64_sys_fcntl+0x3f/0x50 fs/fcntl.c:456
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xae

read to 0xffff888108b67540 of 4 bytes by task 8696 on cpu 1:
 generic_write_check_limits fs/read_write.c:1624 [inline]
 generic_write_checks+0x1e7/0x290 fs/read_write.c:1663
 ext4_generic_write_checks fs/ext4/file.c:220 [inline]
 ext4_write_checks fs/ext4/file.c:243 [inline]
 ext4_buffered_write_iter+0xb7/0x3e0 fs/ext4/file.c:264
 ext4_file_write_iter+0x48a/0x10b0 fs/ext4/file.c:502
 call_write_iter include/linux/fs.h:1977 [inline]
 new_sync_write fs/read_write.c:518 [inline]
 vfs_write+0x6f9/0x7e0 fs/read_write.c:605
 ksys_write+0xce/0x180 fs/read_write.c:658
 __do_sys_write fs/read_write.c:670 [inline]
 __se_sys_write fs/read_write.c:667 [inline]
 __x64_sys_write+0x3e/0x50 fs/read_write.c:667
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xae

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 8696 Comm: syz-executor.1 Not tainted 5.12.0-rc4-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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