>>> frame 5
    #5  0x00007f7430627733 in pthread_mutex_lock () from /home/rusty/futex/sysroot/lib/x86_64-linux-gnu/libpthread.so.0

    >>> disassemble pthread_mutex_lock
    ...
       0x00007f7430627723 <+211>: mov    %rdi,0x8(%rsp)
       0x00007f7430627728 <+216>: and    $0x80,%esi
       0x00007f743062772e <+222>: call   0x7f743062efc0 <__lll_lock_wait>
    => 0x00007f7430627733 <+227>: mov    0x8(%rsp),%rdi
       0x00007f7430627738 <+232>: jmp    0x7f7430627689 <pthread_mutex_lock+57>
    ...

    >>> x/w ($rsp + 8)
    0x7f7371ff1ce8: 0x08001ea1

I love reading these articles, but sometimes they skip over things that the reader might not understand.

In my case I don't understand where x and w come from, and how the result of 0x08001ea1 is invalid (I can see it's not aligned to a 4 byte boundary).