linux-kernel - Re: [PATCH v3 2/4] KVM: s390: remove unneeded srcu lock

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Message-Id: <DA2QFYNT6G6B.3JVSDMJGGBL7N@linux.ibm.com>
Date: Thu, 22 May 2025 15:44:14 +0200
From: "Christoph Schlameuss" <schlameuss@...ux.ibm.com>
To: "Claudio Imbrenda" <imbrenda@...ux.ibm.com>,
        <linux-kernel@...r.kernel.org>
Cc: <kvm@...r.kernel.org>, <linux-s390@...r.kernel.org>,
        <frankja@...ux.ibm.com>, <borntraeger@...ibm.com>,
        <seiden@...ux.ibm.com>, <nsg@...ux.ibm.com>, <nrb@...ux.ibm.com>,
        <david@...hat.com>, <hca@...ux.ibm.com>, <agordeev@...ux.ibm.com>,
        <svens@...ux.ibm.com>, <gor@...ux.ibm.com>
Subject: Re: [PATCH v3 2/4] KVM: s390: remove unneeded srcu lock

On Thu May 22, 2025 at 3:22 PM CEST, Claudio Imbrenda wrote:
> All paths leading to handle_essa() already hold the kvm->srcu.
> Remove unneeded srcu locking from handle_essa().
> Add lockdep assertion to make sure we will always be holding kvm->srcu
> when entering handle_essa().
>
> Signed-off-by: Claudio Imbrenda <imbrenda@...ux.ibm.com>
> Reviewed-by: Nina Schoetterl-Glausch <nsg@...ux.ibm.com>
> Reviewed-by: Christian Borntraeger <borntraeger@...ux.ibm.com>

Reviewed-by: Christoph Schlameuss <schlameuss@...ux.ibm.com>

> ---
>  arch/s390/kvm/priv.c | 6 ++----
>  1 file changed, 2 insertions(+), 4 deletions(-)
>
> diff --git a/arch/s390/kvm/priv.c b/arch/s390/kvm/priv.c
> index 1a49b89706f8..9253c70897a8 100644
> --- a/arch/s390/kvm/priv.c
> +++ b/arch/s390/kvm/priv.c
> @@ -1248,6 +1248,8 @@ static inline int __do_essa(struct kvm_vcpu *vcpu, const int orc)
>  
>  static int handle_essa(struct kvm_vcpu *vcpu)
>  {
> +	lockdep_assert_held(&vcpu->kvm->srcu);
> +
>  	/* entries expected to be 1FF */
>  	int entries = (vcpu->arch.sie_block->cbrlo & ~PAGE_MASK) >> 3;
>  	unsigned long *cbrlo;
> @@ -1297,12 +1299,8 @@ static int handle_essa(struct kvm_vcpu *vcpu)
>  		/* Retry the ESSA instruction */
>  		kvm_s390_retry_instr(vcpu);
>  	} else {
> -		int srcu_idx;
> -
>  		mmap_read_lock(vcpu->kvm->mm);
> -		srcu_idx = srcu_read_lock(&vcpu->kvm->srcu);
>  		i = __do_essa(vcpu, orc);
> -		srcu_read_unlock(&vcpu->kvm->srcu, srcu_idx);
>  		mmap_read_unlock(vcpu->kvm->mm);
>  		if (i < 0)
>  			return i;