Re: [PATCH] Alternate futex non-page-pinning and COW fix

[Jamie Lokier]

Linus Torvalds wrote:
> >   * I contend that the user-visible behaviour of a mapping should
> >   * _not_ depend on whether the file was opened with O_RDWR or O_RDONLY.
> 
> And I violently agree. But I also add the _other_ requirement:
> 
>  * I contend that user-visible behaviour of a mapping should be 100% the 
>  * same for a unwritable MAP_SHARED and a unwritten MAP_PRIVATE
> 
> Put the two together, and see what you get. You get the requirement that 
> if MAP_SHARED works, then MAP_PRIVATE also has to work.

I'll add three more conditions to be explicit:

    * A futex on a MAP_PRIVATE must be mm-local: the canonical
    * example being MAP_PRIVATE of /dev/zero.

    * A FUTEX_WAIT on an unwritten mapping should be woken by a
    * FUTEX_WAKE to the same address after writing.

    * A FUTEX_WAIT on a read-only mapping should wait for the same
    * thing from other processes as if it were a writable mapping.

> That's my requirement. Consistency.

Unfortunately I think the above 5 conditions do not have a consistent
solution.  Please prove me wrong :)

-- Jamie
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