linux-kernel - Re: [PATCH 3/4] KVM: introduce memory transaction semaphore

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Message-ID: <62500f94-b95b-1e16-4aa2-f67905fab01a@redhat.com>
Date:   Sun, 23 Oct 2022 19:50:45 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Emanuele Giuseppe Esposito <eesposit@...hat.com>,
        kvm@...r.kernel.org
Cc:     Jonathan Corbet <corbet@....net>,
        Maxim Levitsky <mlevitsk@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        David Hildenbrand <david@...hat.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/4] KVM: introduce memory transaction semaphore

On 10/22/22 17:48, Emanuele Giuseppe Esposito wrote:
> +static DECLARE_RWSEM(memory_transaction);

This cannot be global, it must be per-struct kvm.  Otherwise one VM can 
keep the rwsem indefinitely while a second VM hangs in 
KVM_KICK_ALL_RUNNING_VCPUS.

It can also be changed to an SRCU (with the down_write+up_write sequence 
changed to synchronize_srcu_expedited) which has similar characteristics 
to your use of the rwsem.

Paolo