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Date:   Mon, 9 Jan 2023 15:50:32 +0100
From:   Heiko Carstens <hca@linux.ibm.com>
To:     Heiko Carstens <hca@linux.ibm.com>
Cc:     Christian Borntraeger <borntraeger@linux.ibm.com>,
        Janosch Frank <frankja@linux.ibm.com>,
        Claudio Imbrenda <imbrenda@linux.ibm.com>,
        David Hildenbrand <david@redhat.com>, kvm@vger.kernel.org,
        linux-s390@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] KVM: s390: interrupt: use READ_ONCE() before cmpxchg()
Message-ID: <Y7wpuAArn1Al6c9v@osiris>
References: <20230109125135.393784-1-hca@linux.ibm.com>
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On Mon, Jan 09, 2023 at 01:51:35PM +0100, Heiko Carstens wrote:
> Use READ_ONCE() before cmpxchg() to prevent that the compiler generates
> code that fetches the to be compared old value several times from memory.
> 
> Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
> ---
>  arch/s390/kvm/interrupt.c | 12 ++++++++----
>  1 file changed, 8 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/s390/kvm/interrupt.c b/arch/s390/kvm/interrupt.c
> index 1dae78deddf2..a3bf1b6a7962 100644
> --- a/arch/s390/kvm/interrupt.c
> +++ b/arch/s390/kvm/interrupt.c
> @@ -83,8 +83,9 @@ static int sca_inject_ext_call(struct kvm_vcpu *vcpu, int src_id)
>  		struct esca_block *sca = vcpu->kvm->arch.sca;
>  		union esca_sigp_ctrl *sigp_ctrl =
>  			&(sca->cpu[vcpu->vcpu_id].sigp_ctrl);
> -		union esca_sigp_ctrl new_val = {0}, old_val = *sigp_ctrl;
> +		union esca_sigp_ctrl new_val = {0};
>  
> +		old_val = READ_ONCE(*sigp_ctrl);

FWIW, Claudio pointed out that this doesn't even compile.
Sending a new version - sorry about this!