git: fba80a78193a - stable/13 - Merge libcxxrt commit f2e55091e2e878386c9f7974d4922bbdc4faed84

The branch stable/13 has been updated by dim:

URL: https://cgit.FreeBSD.org/src/commit/?id=fba80a78193a9b54ae184ff0bdf45186dae125d4

commit fba80a78193a9b54ae184ff0bdf45186dae125d4
Author:     Dimitry Andric <dim@FreeBSD.org>
AuthorDate: 2022-03-20 21:21:28 +0000
Commit:     Dimitry Andric <dim@FreeBSD.org>
CommitDate: 2022-03-23 20:03:59 +0000

    Merge libcxxrt commit f2e55091e2e878386c9f7974d4922bbdc4faed84
    
      Fix unlock in two-word version and add missing comment.
    
      Fixes #15
      Fixes #16
    
    This should fix the hangs in __cxa_guard_acquire() reported on i386 (and
    possibly other 32-bit platforms).
    
    Obtained from:  https://github.com/libcxxrt/libcxxrt/commit/f2e5509
    Fixes:          56aaed388b0a
    MFC after:      2 weeks
    
    (cherry picked from commit 7819a911ff5132b34996c8b09b2a024cd10fd4fb)
---
 contrib/libcxxrt/guard.cc | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/contrib/libcxxrt/guard.cc b/contrib/libcxxrt/guard.cc
index 515992563a10..cb58aa7da28e 100644
--- a/contrib/libcxxrt/guard.cc
+++ b/contrib/libcxxrt/guard.cc
@@ -69,7 +69,12 @@
  * progress).  The bit to use depends on the byte order of the target.
  *
  * On many 32-bit platforms, 64-bit atomics are unavailable (or slow) and so we
- * treat the two halves of the 64-bit word as independent values and
+ * treat the two halves of the 64-bit word as independent values and establish
+ * an ordering on them such that the guard word is never modified unless the
+ * lock word is in the locked state.  This means that we can do double-checked
+ * locking by loading the guard word and, if it is not initialised, trying to
+ * transition the lock word from the unlocked to locked state, and then
+ * manipulate the guard word.
  */
 namespace
 {
@@ -227,7 +232,7 @@ namespace
 					// If another thread did manage to initialise this, release
 					// the lock and notify the caller that initialisation is
 					// done.
-					lock_word.store(initialised, memory_order::release);
+					lock_word.store(0, memory_order::release);
 					return GuardState::InitDone;
 				}
 				return GuardState::InitLockSucceeded;