[PATCH] drm/vc4: Fix false positive WARN() backtrace on refcount_inc() usage

[Eric Anholt]

Boris Brezillon <boris.brezillon at free-electrons.com> writes:

> With CONFIG_REFCOUNT_FULL enabled, refcount_inc() complains when it's
> passed a refcount object that has its counter set to 0. In this driver,
> this is a valid use case since we want to increment ->usecnt only when
> the BO object starts to be used by real HW components and this is
> definitely not the case when the BO is created.
>
> Fix the problem by using refcount_inc_not_zero() instead of
> refcount_inc() and fallback to refcount_set(1) when
> refcount_inc_not_zero() returns false. Note that this 2-steps operation
> is not racy here because the whole section is protected by a mutex
> which guarantees that the counter does not change between the
> refcount_inc_not_zero() and refcount_set() calls.

If we're not following the model, and protecting the refcount by a
mutex, shouldn't we just be using addition and subtraction instead of
refcount's atomics?
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