linux-kernel - Re: [PATCH] futex: Ensure get_futex_key

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Date:	Mon, 20 Oct 2014 17:00:09 +0100
From:	Catalin Marinas <catalin.marinas@....com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Davidlohr Bueso <dave@...olabs.net>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Matteo Franchin <Matteo.Franchin@....com>,
	Darren Hart <dvhart@...ux.intel.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Mike Galbraith <umgwanakikbuti@...il.com>
Subject: Re: [PATCH] futex: Ensure get_futex_key_refs() always implies a
 barrier

On Mon, Oct 20, 2014 at 04:32:00PM +0100, Linus Torvalds wrote:
> On Mon, Oct 20, 2014 at 3:49 AM, Catalin Marinas
> <catalin.marinas@....com> wrote:
> > Since you mention symmetry, something like below makes the barriers more
> > explicit.
> 
> Borken, for two reasons:
> 
> > diff --git a/kernel/futex.c b/kernel/futex.c
> > index f3a3a071283c..5b9d857d0816 100644
> > --- a/kernel/futex.c
> > +++ b/kernel/futex.c
> > @@ -143,9 +143,7 @@
> >  static inline void futex_get_mm(union futex_key *key)
> >  {
> >         atomic_inc(&key->private.mm->mm_count);
> > -       /*
> > -        * Ensure futex_get_mm() implies a full barrier such that
> > -        * get_futex_key() implies a full barrier. This is relied upon
> > -        * as full barrier (B), see the ordering comment above.
> > -        */
> > -       smp_mb__after_atomic();
> >  }
> 
> So the thing is, this means that we can't take advantage of the fact
> that "atomic_inc" is already an atomic. So this is just a performance
> breakage. But:

OK, I looked at this from an ARM perspective only and it would not make
any difference. But it seems that MIPS makes a distinction between
"before" and "after" barriers with "before" defined as wmb in some
configuration, so the hunk below would break it.

> >  static inline int hb_waiters_pending(struct futex_hash_bucket *hb)
> >  {
> > +       /*
> > +        * Full barrier (B), see the ordering comment above.
> > +        */
> > +       smp_mb__before_atomic();
> >  #ifdef CONFIG_SMP
> >         return atomic_read(&hb->waiters);
> 
> This is just entirely broken.
> 
> "atomic_read()" isn't really an "atomic op" at all. despite the name,
> it's just a read that is basically ACCESS_ONCE.
> 
> So smp_mb__before_atomic() doesn't work for atomic_read(), and the
> code is nonsensical and doesn't work. It would need to be a full
> memory barrier.

Looking at the semantics of smp_mb__*_atomic(), it would indeed have to
be a full smp_mb() here.

-- 
Catalin
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