[PATCH v2] futex: remove rw parameter from get_futex_key()

[KOSAKI Motohiro]

Hi

sorry very delayed responce, I've review futex code again.


> > Hm. probably we need to discuss more.
> > 
> > Firstly, if we assume current glibc implimentation, you are right,
> > we can assume userland always initialize the page explicitly before using
> > futex. then we never seen zero page in futex.
> > 
> > but, I don't think futex itself assume it now. at least man page
> > doesn't describe such limilation. so, if you prefer bail and man fix,
> > I'm acceptable maybe.
> 
> Here's another worry with the current futex implementation,
> which might help me to decide which way to jump on the ZERO_PAGE.
> 
> Originally, a futex on a MAP_PRIVATE (!VM_MAYSHARE) area was necessarily
> FUT_OFF_MMSHARED.  Nowadays, with the get_user_pages_fast implementation,
> we have no idea whether the vma is VM_MAYSHARE or not.  So if a futex is
> placed in a private area backed by a file, then it could be regarded as
> FUT_OFF_INODE at futex_wait() time, but FUT_OFF_MMSHARED at futex_wake()
> time.

very true!


> Perhaps that's no problem at all, it's a long time since I was involved
> with futexes, I think you and Peter will grasp the consequences quicker
> than I shall.
> 
> But it seems no more improbable than the ZERO_PAGE case: some app
> might place its futexes in the .data section of the executable,
> which is a private mapping of the executable file.
> 
> If this case is also an issue, then perhaps we just need to update
> the man page to say that whatever is responsible for initializing a
> futex does need to write to it (or the page it's in) before it's used,
> otherwise behaviour is undefined.  (But we should then use the -EFAULT
> patch above, we'd all prefer an error to busylooping.)

I have a question. Why can't we use write mode get_user_pages_fast()?
I mean glibc always mekes write access before calling futex. it mean
write mode get_user_pages() doesn't mekes cow on practical usage.

Following patch is implemented such policy. What do you think?