linux-kernel - Re: [PATCH v2] KVM: x86: Handle SRCU initialization failure during page track init

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Message-ID: <af562685-673a-489d-6a00-5e77329dc2a4@redhat.com>
Date:   Tue, 21 Sep 2021 19:47:28 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>, tcs.kernel@...il.com
Cc:     vkuznets@...hat.com, wanpengli@...cent.com, jmattson@...gle.com,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        jarkko@...nel.org, Haimin Zhang <tcs_kernel@...cent.com>
Subject: Re: [PATCH v2] KVM: x86: Handle SRCU initialization failure during
 page track init

On 03/09/21 18:18, Sean Christopherson wrote:
> Before moving forward with a fix, I'd like to get Paolo's input on dropping
> track_srcu in favor of kvm->srcu and avoiding this altogheter.  Note, Paolo is on
> vacation at the moment, so this won't get attention for a week or more.

The reason for track_srcu's existence is to avoid complications in 
kvm_arch_flush_shadow_memslot, which is called from the _write_ side of 
kvm->srcu but is on the _read_ side of track_srcu.

I think this should be fixed easily by taking slots_lock in 
kvm_page_track_register_notifier and kvm_page_track_unregister_notifier, 
however it's a bit more complicated from the point of view of the lock 
hierarchy and possible deadlocks.

So I'm open to patches that drop track_srcu, but for now I applied this 
patch.

Paolo