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

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Date:   Fri, 29 Apr 2022 17:58:57 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Peter Gonda <pgonda@...gle.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 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.

Paolo