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

From: Eric Anholt <eric@anholt.net>
Cc: David Airlie <airlied@linux.ie>,
	Stefan Wahren <stefan.wahren@i2se.com>,
	dri-devel@lists.freedesktop.org,
	Boris Brezillon <boris.brezillon@free-electrons.com>
Subject: Re: [PATCH] drm/vc4: Fix false positive WARN() backtrace on refcount_inc() usage
Date: Wed, 22 Nov 2017 13:16:08 -0800	[thread overview]
Message-ID: <8760a2ko9z.fsf@anholt.net> (raw)
In-Reply-To: <20171122203928.28135-1-boris.brezillon@free-electrons.com>

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Boris Brezillon <boris.brezillon@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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