[Development] atomic reference counting implementation

Mutz, Marc marc at kdab.com
Wed Aug 7 20:00:30 CEST 2019

Hi Philippe,

This was discussed in 
https://codereview.qt-project.org/c/qt/qtbase/+/66118. See, in 
particular, Olivier's comment.

TL;DR: ref() is documented to be ordered, so cannot be changed.

Consequently, I didn't merge it even with Thiago's +2.

To fix, we'd need to port all of the Qt classes away from the public 
classes (QSharedData, QAtomicInt) and implement ref-counting completely 
from scratch (well, copy QSharedData → QSharedDataV2, QAtomicInt → 
QAtomicIntV2, and do the change in V2). That, or we introduce silent 
breakages into user code by applying 66118, something our Chief Whip has 
just announced publicly should not happen:

> If we must break compatibility, a compile-time error is preferable
> over a silent breakage at runtime (as those are much harder to detect).

I think I'm on record saying such impl details as ref-counting for Qt 
implicitly shared classes should not be public API. This is a perfect 
example of why I believe that to be fundamentally true.


On 2019-08-07 19:36, Philippe wrote:
> I recently found that in Qt,  reference counting to guard a resource, 
> is using
> ref() / deref()
> But ref() is using memory_order_seq_cst
> while memory_order_relaxed, should be sufficient
> What is important is to guarantee that destruction is not subject to a
> race condition, to prevent double
> destruction. Hence deref() with memory_order_seq_cst is enough to 
> guarantee
> that.
> It does not matter how much the counter increase, but what is important
> is to control how it is decreased. Hence deref(with 
> memory_order_seq_cst)
> is just enough.
> I have verified the implementaiton of reference counting for shared_ptr
> in clang, and it does what I describe above
> (it even just use memory_order_acq_rel to decrement, and not
> memory_order_seq_cst)
> https://github.com/llvm-mirror/libcxx/blob/master/include/memory#L3344
> Is there a reason why Qt is not optimized in the same way? (since ref() 
> is
> used a lot, and atomic operations are a bit expensive).
> Is there a requirement at some stage that the reference counter must be
> ordered for increments?
> Philippe
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