linux-kernel - Re: [PATCH] vsprintf: Fix memory barriers of ptr_key to have_filed_random_ptr

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Date:   Tue, 15 May 2018 09:55:13 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Kees Cook <keescook@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        tcharding <me@...in.cc>
Subject: Re: [PATCH] vsprintf: Fix memory barriers of ptr_key to have_filed_random_ptr_key

On Tue, May 15, 2018 at 7:06 AM Steven Rostedt <rostedt@...dmis.org> wrote:
> -       smp_mb();
> +       smp_wmb();
>          WRITE_ONCE(have_filled_random_ptr_key, true);

> +       /* Read ptr_key after reading have_filled_random_ptr_key */
> +       smp_rmb();
> +
>   #ifdef CONFIG_64BIT
>          hashval = (unsigned long)siphash_1u64((u64)ptr, &ptr_key);

Hmm. smp_wmb/rmb are basically free on x86, but on some architectures
smp_rmb() in particular can be pretty expensive.

So when you have a "handoff" situation like this, it's _probably_ better to
use use "smp_store_release()" and "smp_load_acquire()". To some degree that
might also be better for documentation purposes, because that's exactly the
"release-acquire" pattern.

That said, I'm not convinced this really matters all that much for a
boot-time flag like this. The race is pretty theoretical.

                    Linus