linux-kernel - Re: [PATCH v3] KVM: SEV: Mark nested locking of vcpu->lock

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Date:   Fri, 29 Apr 2022 11:12:36 -0600
From:   Peter Gonda <pgonda@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     John Sperbeck <jsperbeck@...gle.com>,
        kvm list <kvm@...r.kernel.org>,
        David Rientjes <rientjes@...gle.com>,
        Sean Christopherson <seanjc@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3] KVM: SEV: Mark nested locking of vcpu->lock

On Fri, Apr 29, 2022 at 9:59 AM Paolo Bonzini <pbonzini@...hat.com> wrote:
>
> On 4/29/22 17:51, Peter Gonda wrote:
> >> No, you don't need any of this.  You can rely on there being only one
> >> depmap, otherwise you wouldn't need the mock releases and acquires at
> >> all.  Also the unlocking order does not matter for deadlocks, only the
> >> locking order does.  You're overdoing it. :)
> >
> > Hmm I'm slightly confused here then. If I take your original suggestion of:
> >
> >          bool acquired = false;
> >          kvm_for_each_vcpu(...) {
> >                  if (acquired)
> >                          mutex_release(&vcpu->mutex.dep_map,
> > _THIS_IP_);  <-- Warning here
> >                  if (mutex_lock_killable_nested(&vcpu->mutex, role)
> >                          goto out_unlock;
> >                  acquired = true;
> >
> > """
> > [ 2810.088982] =====================================
> > [ 2810.093687] WARNING: bad unlock balance detected!
> > [ 2810.098388] 5.17.0-dbg-DEV #5 Tainted: G           O
> > [ 2810.103788] -------------------------------------
>
> Ah even if the contents of the dep_map are the same for all locks, it
> also uses the *pointer* to the dep_map to track (class, subclass) ->
> pointer and checks for a match.
>
> So yeah, prev_cpu is needed.  The unlock ordering OTOH is irrelevant so
> you don't need to visit the xarray backwards.

Sounds good. Instead of doing this prev_vcpu solution we could just
keep the 1st vcpu for source and target. I think this could work since
all the vcpu->mutex.dep_maps do not point to the same string.

Lock:
         bool acquired = false;
         kvm_for_each_vcpu(...) {
                 if (mutex_lock_killable_nested(&vcpu->mutex, role)
                     goto out_unlock;
                acquired = true;
                 if (acquired)
                      mutex_release(&vcpu->mutex, role)
         }

Unlock

         bool acquired = true;
         kvm_for_each_vcpu(...) {
               if (!acquired)
                     mutex_acquire(&vcpu->mutex, role)
               mutex_unlock(&vcpu->mutex);
               acquired = false;
         }

So when locking we release all but the first dep_maps. Then when
unlocking we acquire all but the first dep_maps. Thoughts?

>
> Paolo
>