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

[Boris Brezillon]

On Wed, 22 Nov 2017 16:13:31 -0800
Eric Anholt <eric at anholt.net> wrote:

> Boris Brezillon <boris.brezillon at free-electrons.com> writes:
> 
> > On Wed, 22 Nov 2017 13:16:08 -0800
> > Eric Anholt <eric at anholt.net> wrote:
> >  
> >> 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?  
> >
> > Actually, this mutex is protecting the bo->madv value which has to be
> > checked when the counter reaches 0 (when decrementing) or 1 (when
> > incrementing). We just benefit from this protection here, but ideally
> > it would be better to have an refcount_inc_allow_zero() as suggested by
> > Daniel.  
> 
> Let me restate this to see if it makes sense: The refcount is always >=
> 0, this is is the only path that increases the refcount from 0 to 1, and
> it's (incidentally) protected by a mutex, so there's no race between the
> attempted increase from nonzero and the set from nonzero to 1.

Yep.

> 
> This seems fine to me as a bugfix.

The discussion is going on in the other thread, let's wait a bit
before taking a decision.