linux-kernel - Re: [PATCH] KVM: Remove redundant smp_mb() in the kvm_mmu_commit_zap

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Message-ID: <56E19224.9090906@redhat.com>
Date:	Thu, 10 Mar 2016 16:26:28 +0100
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Xiao Guangrong <guangrong.xiao@...ux.intel.com>,
	Lan Tianyu <tianyu.lan@...el.com>,
	Thomas Gleixner <tglx@...utronix.de>
Cc:	gleb@...nel.org, mingo@...hat.com, hpa@...or.com, x86@...nel.org,
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: Remove redundant smp_mb() in the
 kvm_mmu_commit_zap_page()



On 10/03/2016 15:40, Xiao Guangrong wrote:
>      long dirty_count = kvm->tlbs_dirty;
> 
> +        /*
> +         * read tlbs_dirty before doing tlb flush to make sure not tlb
> request is
> +         * lost.
> +         */
>      smp_mb();
> +
>      if (kvm_make_all_cpus_request(kvm, KVM_REQ_TLB_FLUSH))
>          ++kvm->stat.remote_tlb_flush;
>      cmpxchg(&kvm->tlbs_dirty, dirty_count, 0);
> 
> 
> Any comment?

Compared to smp_load_acquire(), smp_mb() adds an ordering between stores
and loads.  Is the

The load of kvm->tlbs_dirty should then be

	/*
	 * Read tlbs_dirty before setting KVM_REQ_TLB_FLUSH in
	 * kvm_make_all_cpus_request.  This
	 */
	long dirty_count = smp_load_acquire(kvm->tlbs_dirty);

Tianyu, I think Xiao provided the information that I was missing.  Would
you like to prepare the patch?

Thanks,

Paolo