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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <56E19B21.8090501@redhat.com>
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