<Forward Quoted> 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. 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 > multi-threading. 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. Atomic is easy to define. > It just means that you will only ever read some previously stored > value. > > Joe Seigh
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