Re: [PATCH 0/4] futex: get_user_pages_fast() for shared futexes

Peter Zijlstra
2008-09-30T08:51:31Z

On Tue, 2008-09-30 at 17:21 +1000, Nick Piggin wrote:
> On Sunday 28 September 2008 02:17, Ingo Molnar wrote:
> > * Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:
> > > Since get_user_pages_fast() made it in, I thought to give this another
> > > try. Lightly tested by disabling the private futexes and running some
> > > java proglets.
> >
> > hm, very interesting. Since this is an important futex usecase i started
> > testing it in tip/core/futexes:
> >
> > cd33272: futex: cleanup fshared
> > a135356: futex: use fast_gup()
> > 39ce77b: futex: reduce mmap_sem usage
> > 0d7a336: futex: rely on get_user_pages() for shared futexes
> >
> > Nick, it would be nice to get an Acked-by/Reviewed-by from you, before
> > we think about whether it should go upstream.
>
> Yeah, these all look pretty good. It's nice to get rid of mmap sem here.
>
> Which reminds me, we need to put a might_lock mmap_sem into
> get_user_pages_fast...

Yeah..

> But these patches look good to me (last time we discussed them I thought
> there was a race with page truncate, but it looks like you've closed that
> by holding page lock over the whole operation...)

Just to be sure, I only hold the page lock over the get_futex_key() op,
and drop it after getting a ref on the futex key.

I then drop the futex key ref after the futex op is complete.

This assumes the futex key ref is suffucient to guarantee whatever is
needed - which is the point I'm still not quite sure about myself.

The futex key ref was used between futex ops, with I assume the intent
to ensure the futex backing stays valid. However, the key ref only takes
a ref on either the inode or the mm, neither which avoid the specific
address of the futex to get unmapped between ops.

So in that respect we're not worse off than before, and any application
doing: futex_wait(), munmap(), futex_wake() is going to suffer. And as
far as I understand it get the waiting task stuck in D state for
ever-more or somesuch.

By now not holding the mmap_sem over the full futex op, but only over
the get_futex_key(), that munmap() race gets larger and the actual futex
could disappear while we're working on it, but in all cases I looked at
that will make the futex op return -EFAULT, so we should be good there.

Gah, now that I look at it, it looks like I made get_futex_key()
asymetric wrt private futexes, they don't take a ref on the key, but
then do drop one... ouch.. Patch below.

> Nice work, Peter.

Thanks!

> BTW. what kinds of things use inter-process futexes as of now?

On a regular modern Linux system, not much. But I've been told there are
applications out there that do indeed make heavy use of them - as
they're part of POSIX etc.. blah blah :-)

Also some legacy stuff that's stuck on an ancient glibc (but somehow did
manage to upgrade the kernel) might benefit.

---
Subject: futex: fixup get_futex_key() for private futexes
From: Peter Zijlstra <a.p.zijlstra@chello.nl>

With the get_user_pages_fast() patches we made get_futex_key() obtain a
reference on the returned key, but failed to do so for private futexes.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
---
diff --git a/kernel/futex.c b/kernel/futex.c
index 197fdab..beee9af 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -227,6 +227,7 @@ static int get_futex_key(u32 __user *uaddr, int
fshared, union futex_key *key)
return -EFAULT;
key->private.mm = mm;
key->private.address = address;
+ get_futex_key_refs(key);
return 0;
}