Comments
Patch
@@ -244,16 +244,16 @@ namespace __cxxabiv1
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))
+ __ATOMIC_ACQUIRE))
{
// This thread should do the initialization.
return 1;
@@ -264,13 +264,26 @@ namespace __cxxabiv1
// 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;
}
This is a merge of r191125 and r191191 from gcc-4_7-branch. It fixes a race in __cxa_guard_acquire which can cause duplicate initialization of static variables within functions. Okay for google/gcc-4_7? Thanks, Ollie Google ref b/7173106. * 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. Fix up the last argument of the first __atomic_compare_exchange_n. Comment. commit 40ec687ace62b4d4c64f72be2bdf4321f5213107 Author: Ollie Wild <aaw@google.com> Date: Wed Sep 19 14:52:53 2012 -0500 Merge r191125 and r191191 from gcc-4_7-branch. Google ref b/7173106. * 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. Fix up the last argument of the first __atomic_compare_exchange_n. Comment.