linux-kernel - Re: [PATCH v1 2/5] KVM: s390: remove unneeded srcu lock

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Message-ID: <0b90cd0ad24727c9d7b110f09fd79b2525b4fbe1.camel@linux.ibm.com>
Date: Tue, 20 May 2025 16:34:30 +0200
From: Nina Schoetterl-Glausch <nsg@...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, 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 v1 2/5] KVM: s390: remove unneeded srcu lock

On Mon, 2025-05-19 at 16:42 +0200, Nina Schoetterl-Glausch wrote:
> On Wed, 2025-05-14 at 18:38 +0200, Claudio Imbrenda wrote:
> > All paths leading to handle_essa() already hold the kvm->srcu.
> > Remove unneeded srcu locking from handle_essa().
> > 
> > Signed-off-by: Claudio Imbrenda <imbrenda@...ux.ibm.com>
> 
> Reviewed-by: Nina Schoetterl-Glausch <nsg@...ux.ibm.com>
> 
> Why are you removing it tho?
> It should be very low cost and it makes the code more robust,
> since handle_essa itself ensures that it has the lock.
> It is also easier to understand which synchronization the function does.
> You could of course add a comment stating that the kvm srcu read side needs
> to be held. I think this would be good to have if you really don't want the
> srcu_read_lock here.
> But then you might also want that documented up the call chain.

Actually, can we use __must_hold or have some assert?
> 
> > ---
> >  arch/s390/kvm/priv.c | 4 ----
> >  1 file changed, 4 deletions(-)
> > 
> > diff --git a/arch/s390/kvm/priv.c b/arch/s390/kvm/priv.c
> > index 1a49b89706f8..758cefb5bac7 100644
> > --- a/arch/s390/kvm/priv.c
> > +++ b/arch/s390/kvm/priv.c
> > @@ -1297,12 +1297,8 @@ 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;

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