[Development] atomic reference counting implementation
marc at kdab.com
Wed Aug 7 20:00:30 CEST 2019
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
> It does not matter how much the counter increase, but what is important
> is to control how it is decreased. Hence deref(with
> 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
> Is there a reason why Qt is not optimized in the same way? (since ref()
> 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?
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