On Tue, Dec 10, 2013 at 02:48:52PM -0800, Linus Torvalds wrote:
> On Tue, Dec 10, 2013 at 2:42 PM, Thomas Gleixner <t...@linutronix.de> wrote:
> >
> > And yes, I remember that we do not do an extra check for the fshared
> > case, because get_user_pages_fast() should do it for us already. If
> > not we are fubared not only in the futex code.
>
> Yeah. It turns out we do do the access check indirectly - by looking
> at the PAGE_USER bit, even if we don't necessarily check the actual
> limits. So get_user_pages_fast() is fine.
>
> > But there is a subtle detail:
>
> Yup, see my email from ten minutes ago, we found the same thing. And
> that would seem to explain the endless loop, and also the timing
> (since Dave mentions he started doing large-pages lately).
>
> So I think the "__get_user_pages_fast(address, 1, !ro, &page)" thing
> should work.
>
> Dave, can you re-create that trinity run and test that patch? I think
> we've got this, but it might be nice to leave the hung machine up and
> running until it's verified.. Although I don't really see what else we
> could need or get out of it, so..
The only thing I'm still unclear on, is how that pid allegedly wasn't doing
a futex call as part of its run. The only thing I can think of is that
the other pid that _did_ do a futex call did it on a page that was MAP_SHARED
between all the other children, and this 'spin forever' thing only
happens when the last process with a reference on that page exits ?
does that make sense?
Dave
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