linux-kernel - Re: [GIT PULL] locking fixes

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Date:	Fri, 20 Feb 2015 16:03:57 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [GIT PULL] locking fixes

How does this work for you at all?

On Fri, Feb 20, 2015 at 5:37 AM, Ingo Molnar <mingo@...nel.org> wrote:
> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
> index 94f643484300..e354cc6446ab 100644
> --- a/arch/x86/kernel/kvm.c
> +++ b/arch/x86/kernel/kvm.c
> @@ -803,8 +808,8 @@ static void kvm_unlock_kick(struct arch_spinlock *lock, __ticket_t ticket)
>         add_stats(RELEASED_SLOW, 1);
>         for_each_cpu(cpu, &waiting_cpus) {
>                 const struct kvm_lock_waiting *w = &per_cpu(klock_waiting, cpu);
> -               if (ACCESS_ONCE(w->lock) == lock &&
> -                   ACCESS_ONCE(w->want) == ticket) {
> +               if (READ_ONCE(w->lock) == lock &&
> +                   READ_ONCE(w->want) == ticket) {
>                         add_stats(RELEASED_SLOW_KICKED, 1);
>                         kvm_kick_cpu(cpu);
>                         break;

I get horrible compile warnings from this, because of how 'w' is a
pointer to a 'const' structure, which then causes things like

    include/linux/compiler.h:262:39: warning: passing argument 1 of
‘__read_once_size’ discards ‘const’ qualifier from pointer target type
      ({ typeof(x) __val; __read_once_size(&x, &__val, sizeof(__val)); __val; })

which is fairly hard to avoid (looks like it might need a union)

                       Linus
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