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SubjectRe: process 'stuck' at exit.
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On Tue, 2013-12-10 at 23:42 +0100, Thomas Gleixner wrote:
> On Tue, 10 Dec 2013, Linus Torvalds wrote:
>
> > On Tue, Dec 10, 2013 at 1:57 PM, Linus Torvalds
> > <torvalds@linux-foundation.org> wrote:
> > >
> > > So it looks like __get_user_pages_fast() fails, and keeps failing.
> >
> > Hmm.. Is any of the addresses unchecked, perhaps?
> > __get_user_pages_fast() does an access_ok() check, while
> > get_user_pages_fast() does *not* seem to do one.
> >
> > That looks a bit dangerous. Yeah, users should have checked the
> > address range, but there really is no reason not to do it in
> > get_user_pages_fast().
> >
> > And it looks like the futex code is actually seriously buggered. It
> > only does the access_ok() check for the non-shared case.
> >
> > Why?
>
> The !fshared case is the fast path which does not even reach
> get_user_pages_fast().
>
> We had this discussion some time ago already, where the access_ok()
> check was missing in the !fshared case or the check was buggered for
> some reason. Need to dig up the gory details.
>
> 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.
>
> But there is a subtle detail:
>
> err = get_user_pages_fast(address, 1, 1, &page);
>
> So we ask for write access as the write argument is 1. In case that
> fails we have that fallback path:
>
> /*
> * If write access is not required (eg. FUTEX_WAIT), try
> * and get read-only access.
> */
> if (err == -EFAULT && rw == VERIFY_READ) {
> err = get_user_pages_fast(address, 1, 0, &page);
>
> That's a legitimate use case. And futex_requeue only requests
> VERIFY_READ for the !requeue_pi case.
>
> Now, if that map is RO, i.e. we took the fallback path then the THP
> one will fail as it has write=1 unconditionally.
>
> if (likely(__get_user_pages_fast(address, 1, 1, &page) == 1))
>

Is there a reason THP requires unconditional rw? Andrea?

Or is the following actually the answer here?

diff --git a/kernel/futex.c b/kernel/futex.c
index 80ba086..02febad 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -288,7 +288,7 @@ again:
put_page(page);
/* serialize against __split_huge_page_splitting() */
local_irq_disable();
- if (likely(__get_user_pages_fast(address, 1, 1, &page) == 1)) {
+ if (likely(__get_user_pages_fast(address, 1, !ro, &page) == 1)) {
page_head = compound_head(page);
/*
* page_head is valid pointer but we must pin



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




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