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Date: Mon, 20 Oct 2014 08:32:00 -0700
Message-ID: <CA+55aFxjFpPWMn=w1orFk3k7-ZaWwFoLsv7y37X5zamd428BsQ@mail.gmail.com>
Subject: Re: [PATCH] futex: Ensure get_futex_key_refs() always implies a barrier
From: Linus Torvalds <torvalds@linux-foundation.org>
To: Catalin Marinas <catalin.marinas@arm.com>
Cc: Thomas Gleixner <tglx@linutronix.de>, Davidlohr Bueso <dave@stgolabs.net>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Matteo Franchin <Matteo.Franchin@arm.com>,
        Darren Hart <dvhart@linux.intel.com>,
        Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@kernel.org>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Mike Galbraith <umgwanakikbuti@gmail.com>
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Sender: linux-kernel-owner@vger.kernel.org

On Mon, Oct 20, 2014 at 3:49 AM, Catalin Marinas
<catalin.marinas@arm.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:

>
>  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.

                   Linus
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