concurrent access to file linux

Question

I was looking at how a syscall read/write was done in linux, and i found this :

....
loff_t pos = file_pos_read(f.file);
ret = vfs_read(f.file, buf, count, &pos);
file_pos_write(f.file, pos);
fdput(f);
...`

My questions are :

Where did the locking go? I would have imaginated something like :

....
lock(f.file);  // <-- lock file struct
loff_t pos = file_pos_read(f.file);
ret = vfs_read(f.file, buf, count, &pos);
file_pos_write(f.file, pos);
fdput(f);
unlock(f.file);  // <-- unlock file struct
...

If multiple threads try to read/write at the same time, they could read/write at the same offset ?

If my understanding is correct, linux doesn't use any locking mechanism to protect the offset, is this POSIX compliant ?

I did look at the POSIX specification, and found nothing about this case.

Answer 1

Linux doesn't use any locking mechanism to protect multithread writing to a file.

You have to use your own mutex to protect your file.