linux-kernel - Re: [RFC patch] cmpxchg_double: remove local variables to get better performance

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Date:	Fri, 02 Mar 2012 09:11:47 +0000
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"Alex Shi" <alex.shi@...el.com>
Cc:	<jeremy@...p.org>,
	"asit.k.mallick@...el.com" <asit.k.mallick@...el.com>,
	"x86@...nel.org" <x86@...nel.org>, <tglx@...utronix.de>,
	"Andi Kleen" <ak@...ux.intel.com>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"hpa@...or.com" <hpa@...or.com>
Subject: Re: [RFC patch] cmpxchg_double: remove local variables to get
 better performance

>>> On 02.03.12 at 10:00, Alex Shi <alex.shi@...el.com> wrote:
> Yes, we can use cast for intermediate data. And actually, current kernel
> has live mis-used case on cmpxchg(), that I plan to point out too. 
> 
> -- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -203,12 +203,12 @@ static bool make_all_cpus_request(struct kvm *kvm, 
> unsigned int req)
> 
>  void kvm_flush_remote_tlbs(struct kvm *kvm)
>  {
> -       int dirty_count = kvm->tlbs_dirty;
> +       long dirty_count = kvm->tlbs_dirty;
> 
>         smp_mb();
>         if (make_all_cpus_request(kvm, KVM_REQ_TLB_FLUSH))
>                 ++kvm->stat.remote_tlb_flush;
> -       cmpxchg(&kvm->tlbs_dirty, dirty_count, 0);
> +       cmpxchg(&kvm->tlbs_dirty, dirty_count, 0L);

Indeed - the cmpxchg would fail if the value doesn't fit. But this is not
to say that in certain cases it isn't valid to pass an int for the second
and/or third argument. (And quite likely the issue here is theoretical
only anyway.)

In particular, requiring an L suffix here on literals should be avoided.

Jan

>  }


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