linux-kernel - Re: [PATCH 19/24] kvm: x86/mmu: Protect tdp_mmu

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Message-ID: <14147680-740d-6e7f-e00d-aa7698fd2ba6@redhat.com>
Date:   Tue, 26 Jan 2021 14:37:32 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Ben Gardon <bgardon@...gle.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     Peter Xu <peterx@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Peter Shier <pshier@...gle.com>,
        Peter Feiner <pfeiner@...gle.com>,
        Junaid Shahid <junaids@...gle.com>,
        Jim Mattson <jmattson@...gle.com>,
        Yulei Zhang <yulei.kernel@...il.com>,
        Wanpeng Li <kernellwp@...il.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Xiao Guangrong <xiaoguangrong.eric@...il.com>
Subject: Re: [PATCH 19/24] kvm: x86/mmu: Protect tdp_mmu_pages with a lock

On 12/01/21 19:10, Ben Gardon wrote:
> +	 *  May be acquired under the MMU lock in read mode or non-overlapping
> +	 *  with the MMU lock.
> +	 */
> +	spinlock_t tdp_mmu_pages_lock;

Is this correct?  My understanding is that:

- you can take tdp_mmu_pages_lock from a shared MMU lock critical section

- you don't need to take tdp_mmu_pages_lock from an exclusive MMU lock 
critical section, because you can't be concurrent with a shared critical 
section

- but then, you can't take tdp_mmu_pages_lock outside the MMU lock, 
because you could have

    write_lock(mmu_lock)
                                      spin_lock(tdp_mmu_pages_lock)
    do tdp_mmu_pages_lock stuff  !!!  do tdp_mmu_pages_lock stuff
    write_unlock(mmu_lock)
                                      spin_unlock(tdp_mmu_pages_lock)

Paolo