On Sat, Jan 23, 2021 at 03:50:55PM -0800, Andres Freund wrote:

As there's only a shared lock, seems like both would end up with the
same ctx->pos and end up updating f_pos to the same offset (assuming the
same count).

Am I missing something?

This:
        f = fdget_pos(fd);
        if (!f.file)
                return -EBADF;
in the callers.  Protection of struct file contents belongs to struct file,
*not* struct inode.  Specifically, file->f_pos_lock.  *IF* struct file
in question happens to be shared and the file is a regular or directory
(sockets don't need any exclusion on read(2), etc.)