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

[Daniel Vetter]

On Thu, Nov 23, 2017 at 09:24:11AM +0100, Boris Brezillon wrote:
> 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.

Let's not block the bugfix on reaching perfection imo. I'd merge this as
the minimal fix, and then apply the pretty paint once we have a clear idea
for that.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch