From: Alexander Terekhov (terekhov_at_[hidden])
Date: 2003-05-22 15:00:01
Joseph Seigh wrote:
> Alexander Terekhov wrote:
> > Joseph Seigh wrote:
> > > Well, Detlefs calls it lock-free reference counting. But Boost
> > > would probably call what they're doing with thread-safe reference
> > > counting lock-free also, and they are not the same.
> > They don't call it "lock-free". Here's what they say:
> > http://std.dkuug.dk/JTC1/SC22/WG21/docs/papers/2003/n1450.html
> > (A Proposal to Add General Purpose Smart Pointers to...)
> > "....
> > The count structure C has the following members:
> > Virtual table pointer (C is polymorphic);
> > Use count;
> > Weak count;
> > Mutex (for multithreaded builds);
> > Original pointer passed to the constructor;
> > Deleter.
> > ....
> > The mutex presents another target for platform-specific
> > optimizations. On Windows, we have been able to use a simple
> > one-word spinlock instead of a 6 word CRITICAL_SECTION. The
> > portable architecture of the implementation that abstracts
> > the mutex concept in a separate class prevents further
> > optimizations, like reusing the use count word as the
> > spinlock, or using the Windows API InterlockedIncrement,
> > InterlockedDecrement, and InterlockedCompareExchange
> > primitives to attempt to eliminate the mutex altogether.
> > Nevertheless, we believe that such optimizations are
> > possible.
> > ...."
> The fact that they consider and have done simplistic spin locks
> speaks for itself.
> Also, they make a whole lot of distinctions
> that don't translate very well into a threaded environment.
> Their style of semantic definition with that pre and post internal
> data state stuff doesn't work in multi-threading.
I think it's fine. Please see the definitions of shared_ptr and
weak_ptr thread "safeties" (below).
> The one thing
> you usually cannot reliably observe in multi-threading is internal data
> state. You have to use other mechanisms to define semantics for
I don't think so.
> To say nothing of their multiple attempts to
> deal with data "ownership". It's why they have so many pointer
> types, all compromised in some degree or other for some small
> incremental benefit.
> > > The meaning
> > > of atomic w.r.t java pointers is or should be well understood, so
> > > atomic something would be indicated. Also, it wouldn't preclude
> > > a non lock-free implementation, though you would lose the bemefits
> > > of lock-free.
> > >
> > > Maybe atomic_shared_ptr.
> > Well, I'd love to have a policy-based framework that would
> > allow me to specify the *thread-safety policy*:
> > thread_safety::unsafe
> > "naked" count(s)
> > thread_safety::basic
> > pthread_refcount_t stuff (<http://tinyurl.com/cewa>)
> > thread_safety::strong
> > your "atomic" stuff
> > or something like that. Note that for the blocking stuff, one
> > would REALLY want to have some support for priority protocols
> > (priority protection or priority inheritance) to fight the
> > "unbounded priority inversion" problem in the "realtime" apps.
> I'm never quite sure what "thread-safe" really means. It seems
> to get used in different ways sometimes.
thread_safety::unsafe means that *copying* AND all mutation
operations on shared_ptr/weak_ptr copies (all pointing to the
same object; "typed-null" including) shall be *synchronized*;
otherwise, the behavior is undefined. Read-only stuff (copying
aside) can be done concurrently.
thread_safety::basic means that all "const" operations
(including copying) can be done concurrently, but mutations
shall be synchronized; otherwise, the behavior is undefined.
thread_safety::strong means "do-whatever-you-want-and-don't-
care-about-synchronization" (i.e. you can also mutate shared_ptr
and weak_ptr objects *concurrently* without any synchronization
because it's fully synchronized "internally").
> Atomic is easy to define.
> It just means that you will only ever read some previously stored
The C and C++ standards already mandate that! Here's what C99
says, for example:
"The lifetime of an object is the portion of program execution
during which storage is guaranteed to be reserved for it. An
object exists, has a constant address, and retains its last-
stored value throughout its lifetime."
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