linux-kernel - Re: [PATCH v4 4/7] KVM: mmu: Add slots_arch

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Message-ID: <ef473b52-0b71-a524-ed36-c2c1a9d0fb03@redhat.com>
Date:   Tue, 11 May 2021 21:29:19 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Ben Gardon <bgardon@...gle.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Peter Xu <peterx@...hat.com>, Peter Shier <pshier@...gle.com>,
        Yulei Zhang <yulei.kernel@...il.com>,
        Wanpeng Li <kernellwp@...il.com>,
        Xiao Guangrong <xiaoguangrong.eric@...il.com>,
        Kai Huang <kai.huang@...el.com>,
        Keqian Zhu <zhukeqian1@...wei.com>,
        David Hildenbrand <david@...hat.com>
Subject: Re: [PATCH v4 4/7] KVM: mmu: Add slots_arch_lock for memslot arch
 fields

On 11/05/21 21:21, Sean Christopherson wrote:
>> +	/* Released in install_new_memslots. */
> 
> This needs a much more comprehensive comment, either here or above the declaration
> of slots_arch_lock.  I can't find anything that explicitly states the the lock
> must be held from the time the previous memslots are duplicated/copied until the
> new memslots are set.  Without that information, the rules/expecations are not
> clear.

Indeed, this needs basically a description of the races that can happen, 
as you explained them in the v1 review.

> Unfortunately I'm just whining at this point since I
> don't have a better idea 

Yeah, the synchronize_srcu call in install_new_memslots throws a wrench 
in most alternatives.

Paolo