Re: [PATCH v1 1/1] KVM: s390x: fix SCK locking

[Claudio Imbrenda <imbrenda@linux.ibm.com>]

On Wed, 2 Mar 2022 11:15:23 +0100
Janis Schoetterl-Glausch <scgl@linux.ibm.com> wrote:

> On 3/1/22 15:33, Claudio Imbrenda wrote:
> > When handling the SCK instruction, the kvm lock is taken, even though
> > the vcpu lock is already being held. The normal locking order is kvm
> > lock first and then vcpu lock. This is can (and in some circumstances
> > does) lead to deadlocks.
> > 
> > The function kvm_s390_set_tod_clock is called both by the SCK handler
> > and by some IOCTLs to set the clock. The IOCTLs will not hold the vcpu
> > lock, so they can safely take the kvm lock. The SCK handler holds the
> > vcpu lock, but will also somehow need to acquire the kvm lock without
> > relinquishing the vcpu lock.
> > 
> > The solution is to factor out the code to set the clock, and provide
> > two wrappers. One is called like the original function and does the
> > locking, the other is called kvm_s390_try_set_tod_clock and uses
> > trylock to try to acquire the kvm lock. This new wrapper is then used
> > in the SCK handler. If locking fails, -EAGAIN is returned, which is
> > eventually propagated to userspace, thus also freeing the vcpu lock and
> > allowing for forward progress.
> > 
> > This is not the most efficient or elegant way to solve this issue, but
> > the SCK instruction is deprecated and its performance is not critical.
> > 
> > The goal of this patch is just to provide a simple but correct way to
> > fix the bug.
> > 
> > Fixes: 6a3f95a6b04c ("KVM: s390: Intercept SCK instruction")
> > Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>  
> 
> Reviewed-by: Janis Schoetterl-Glausch <scgl@linux.ibm.com>
> > ---
> >  arch/s390/kvm/kvm-s390.c | 19 ++++++++++++++++---
> >  arch/s390/kvm/kvm-s390.h |  4 ++--
> >  arch/s390/kvm/priv.c     | 14 +++++++++++++-
> >  3 files changed, 31 insertions(+), 6 deletions(-)
> > 
> > diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
> > index 2296b1ff1e02..4e3db4004bfd 100644
> > --- a/arch/s390/kvm/kvm-s390.c
> > +++ b/arch/s390/kvm/kvm-s390.c
> > @@ -3869,14 +3869,12 @@ static int kvm_s390_handle_requests(struct kvm_vcpu *vcpu)
> >  	return 0;
> >  }
> > 
> > -void kvm_s390_set_tod_clock(struct kvm *kvm,
> > -			    const struct kvm_s390_vm_tod_clock *gtod)
> > +static void __kvm_s390_set_tod_clock(struct kvm *kvm, const struct kvm_s390_vm_tod_clock *gtod)
> >  {
> >  	struct kvm_vcpu *vcpu;
> >  	union tod_clock clk;
> >  	unsigned long i;
> > 
> > -	mutex_lock(&kvm->lock);
> >  	preempt_disable();
> > 
> >  	store_tod_clock_ext(&clk);
> > @@ -3897,7 +3895,22 @@ void kvm_s390_set_tod_clock(struct kvm *kvm,
> > 
> >  	kvm_s390_vcpu_unblock_all(kvm);
> >  	preempt_enable();
> > +}
> > +
> > +void kvm_s390_set_tod_clock(struct kvm *kvm, const struct kvm_s390_vm_tod_clock *gtod)
> > +{
> > +	mutex_lock(&kvm->lock);
> > +	__kvm_s390_set_tod_clock(kvm, gtod);
> > +	mutex_unlock(&kvm->lock);
> > +}
> > +
> > +int kvm_s390_try_set_tod_clock(struct kvm *kvm, const struct kvm_s390_vm_tod_clock *gtod)  
> 
> Why int instead of bool?

to be consistent with mutex_trylock, which also returns int

> 
> > +{
> > +	if (!mutex_trylock(&kvm->lock))
> > +		return 0;
> > +	__kvm_s390_set_tod_clock(kvm, gtod);
> >  	mutex_unlock(&kvm->lock);
> > +	return 1;
> >  }
> >   
> [...]