linux-kernel - Re: [PATCH 3/3] KVM: Add proper lockdep assertion in I/O bus unregister

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Date:   Thu, 15 Apr 2021 22:15:06 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Jim Mattson <jmattson@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Hao Sun <sunhao.th@...il.com>
Subject: Re: [PATCH 3/3] KVM: Add proper lockdep assertion in I/O bus
 unregister

On Thu, Apr 15, 2021, Jim Mattson wrote:
> On Mon, Apr 12, 2021 at 3:23 PM Sean Christopherson <seanjc@...gle.com> wrote:
> >
> > Convert a comment above kvm_io_bus_unregister_dev() into an actual
> > lockdep assertion, and opportunistically add curly braces to a multi-line
> > for-loop.
> >
> > Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> > ---
> >  virt/kvm/kvm_main.c | 6 ++++--
> >  1 file changed, 4 insertions(+), 2 deletions(-)
> >
> > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > index ab1fa6f92c82..ccc2ef1dbdda 100644
> > --- a/virt/kvm/kvm_main.c
> > +++ b/virt/kvm/kvm_main.c
> > @@ -4485,21 +4485,23 @@ int kvm_io_bus_register_dev(struct kvm *kvm, enum kvm_bus bus_idx, gpa_t addr,
> >         return 0;
> >  }
> >
> > -/* Caller must hold slots_lock. */
> >  int kvm_io_bus_unregister_dev(struct kvm *kvm, enum kvm_bus bus_idx,
> >                               struct kvm_io_device *dev)
> >  {
> >         int i, j;
> >         struct kvm_io_bus *new_bus, *bus;
> >
> > +       lockdep_assert_held(&kvm->slots_lock);
> > +
> >         bus = kvm_get_bus(kvm, bus_idx);
> >         if (!bus)
> >                 return 0;
> >
> > -       for (i = 0; i < bus->dev_count; i++)
> > +       for (i = 0; i < bus->dev_count; i++) {
> >                 if (bus->range[i].dev == dev) {
> >                         break;
> >                 }
> > +       }
> Per coding-style.rst, neither the for loop nor the if-block should have braces.
> 
> "Do not unnecessarily use braces where a single statement will do."

Doh, the if-statement should indeed not use braces.  I think I meant to clean
that up, and then saw something shiny...

But the for-loop... keep reading :-D

Also, use braces when a loop contains more than a single simple statement:

.. code-block:: c

        while (condition) {
                if (test)
                        do_something();
        }