Re: [boost] [proposal] Atomically Thread-SafeStatic InitializationFor Boost...

From: Chris Thomasson (cristom_at_[hidden])
Date: 2006-11-04 03:08:11

I found and killed a race-condition; the following three functions need to 
be changed:

atomic::sys::once_dec(...)

template<typename T>
atomic::sys::once_def_POD<T>::release()

Here is the fix:

// System API Def
namespace atomic {
namespace sys {

    [...]

  // Dec the refcount if its >= 0.
  // returns true for the last ref,
  // otherwise false
  bool once_dec(refs_t *_this) throw() {
    refs_t local, cmp, xchg;
    do {
      local = ::ac_mb_load_naked(_this);
      if (local < 0) { return false; }
      xchg = (local == 1) ? -1 : local - 1;
      cmp = local;
      local = ac_atomic_cas_release(_this, local, xchg);
    } while(cmp != local);
    std::printf("atomic::sys::once_dec(); - %i\n", local - 1);

    return (local == 1);
  }

   [...]

  // define is a POD that holds a once POD
  template<typename T>
  struct once_def_POD {
    [...]

    // Acquires a reference and calls ctor for first ref.
    // returns pointer to ref,
    // otherwise NULL
    type_t* acquire() {
      type_t *local = 0, *cmp;

      do {
        if (local) { ac_atomic_dec_release(&s_this.m_refs); }
        local = (type_t*)ac_mb_loadptr_depends(&s_this.m_state);
        if (! local) {
          // hashed_mutex::guard_t lock(&_this);
          local = (type_t*)ac_mb_loadptr_depends(&s_this.m_state);
          if (! once_inc(&s_this.m_refs)) { return 0; }
          if (! local) {
            try { local = new type_t; }
            catch(...) {
              (void)once_dec(&s_this.m_refs);
              throw;
            }
            dbg_allocs_inc();
            std::printf("\n(%p)once_def_POD::acquire(); - %p\n", 
(void*)this, (void*)local);
            ac_mb_storeptr_release(&s_this.m_state, local);
            return local;
          }
        } else if(! once_inc(&s_this.m_refs)) {
          return 0;
        }

        cmp = (type_t*)ac_mb_loadptr_depends(&s_this.m_state);
      } while(local != cmp);

      return local;
    }

    // Releases a reference and calls dtor if last ref.
    // returns nothing,
    // otherwise false
    void release() {
      type_t *local = (type_t*)ac_mb_loadptr_depends(&s_this.m_state);
      if (once_dec(&s_this.m_refs)) {
        ac_mb_storeptr_fence(&s_this.m_state, 0);
        ac_mb_store_fence(&s_this.m_refs, 0);
        if (local) {
          delete local;
          std::printf("(%p)once_def_POD::release(); - %p\n\n", (void*)this, 
(void*)local);
          dbg_allocs_dec();
        }
      }
    }

  };

  [...]

}} // namespace atomic::sys

I attached the complete fixed code; once-experimental.cpp. The 
race-condition allowed a thread to load a state pointer and inc a reference 
count to a static object that was possibly being dtor'd. I fixed this by 
zeroing the state pointer and invalidating the reference count when the 
count dropped to zero. I also had to make a thread double check the load of 
a state pointer after incrementing a valid reference count. Now I can't seem 
to find any other race-conditions after this... Can you?

;^)

IMHO, this particular algorithm may be worth spending a little time on... 
Humm...

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