Re: process 'stuck' at exit.

[Darren Hart]

On Tue, 2013-12-10 at 14:33 -0800, Linus Torvalds wrote:
> On Tue, Dec 10, 2013 at 2:19 PM, Linus Torvalds
> <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> >
> > Shouldn't we do something like the attached?
> 
> So I think that kernel/futex.c part of the patch might be a good idea,
> but on x86-64 (which is what Dave is running), the
> 
>         if (end >> __VIRTUAL_MASK_SHIFT)
> 
> test in get_user_pages_fast() should have been equivalent. And even on
> 32-bit, we do check the _PAGE_USER bits in the page tables, so I guess
> it's all good on a get_user_pages_fast() side.
> 
> So never mind. It's not the address checking.
> 
> And I think I see what's up.
> 
> I think what happens is:
>  - get_user_pages_fast(address, 1, 1, &page) fails (because it's read-only)
>  - get_user_pages_fast(address, 1, 0, &page) succeeds and gets a large-page
>  - __get_user_pages_fast(address, 1, 1, &page) fails (because it's read-only).
> 
> so what triggers this is likely that Dave now does large-pages, and
> one of them is a read-only mapping.
> 
> So I would suggest replacing the second "1" in the
> __get_user_pages_fast() call with a "!ro" instead. So how about this
> second patch instead (the access_ok() move remains).
> 
> Comments?
> 

You're too fast for me, but I'm trying to keep up.

It was my understanding that access_ok() was an optimization for private
futexes, but something more heavy weight was required for shared. I
believe that was find_vma() in the past, but Peter Z changed that with
fast gup. Trying to page that all in now and get the exact details....

-- 
Darren Hart
Intel Open Source Technology Center
Yocto Project - Linux Kernel


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