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

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Date:	Thu, 10 Mar 2016 17:04:49 +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 16:45, Xiao Guangrong wrote:
>>
>>> Compared to smp_load_acquire(), smp_mb() adds an ordering between stores
>>> and loads.
>>
>> Here, the ordering is load-store, hence...
> 
> Yes, this is why i put smp_mb() in the code. :)

Here is a table of barriers:


    '. after|                   |
before '.   |    load           |    store
__________'.|___________________|________________________
            |                   |
            |  smp_rmb          | smp_load_acquire
load        |  smp_load_acquire | smp_store_release    XX
            |  smp_mb           | smp_mb
____________|___________________|________________________
            |                   |
            |                   | smp_wmb
store       |  smp_mb           | smp_store_release
            |                   | smp_mb
            |                   |

Your case is the one marked with XX, so a smp_load_acquire() is
enough---and it's preferrable, because it's cheaper than smp_mb() and
more self-documenting.

Paolo