+2012-09-06 Thiago Macieira <thiago.macieira@intel.com>
+
+ PR libstdc++/54172
+ * libsupc++/guard.cc (__cxa_guard_acquire): Exit the loop earlier if
+ we detect that another thread has had success. Don't compare_exchange
+ from a finished state back to a waiting state. Comment.
+
2012-09-05 François Dumont <fdumont@gcc.gnu.org>
PR libstdc++/54296
if (__gthread_active_p ())
{
int *gi = (int *) (void *) g;
- int expected(0);
const int guard_bit = _GLIBCXX_GUARD_BIT;
const int pending_bit = _GLIBCXX_GUARD_PENDING_BIT;
const int waiting_bit = _GLIBCXX_GUARD_WAITING_BIT;
while (1)
{
+ int expected(0);
if (__atomic_compare_exchange_n(gi, &expected, pending_bit, false,
__ATOMIC_ACQ_REL,
__ATOMIC_RELAXED))
// Already initialized.
return 0;
}
+
if (expected == pending_bit)
{
+ // Use acquire here.
int newv = expected | waiting_bit;
if (!__atomic_compare_exchange_n(gi, &expected, newv, false,
__ATOMIC_ACQ_REL,
- __ATOMIC_RELAXED))
- continue;
+ __ATOMIC_ACQUIRE))
+ {
+ if (expected == guard_bit)
+ {
+ // Make a thread that failed to set the
+ // waiting bit exit the function earlier,
+ // if it detects that another thread has
+ // successfully finished initialising.
+ return 0;
+ }
+ if (expected == 0)
+ continue;
+ }
expected = newv;
}