Fix semaphore destruction (bug 12674).

This commit fixes semaphore destruction by either using 64b atomic
operations (where available), or by using two separate fields when only
32b atomic operations are available.  In the latter case, we keep a
conservative estimate of whether there are any waiting threads in one
bit of the field that counts the number of available tokens, thus
allowing sem_post to atomically both add a token and determine whether
it needs to call futex_wake.

See:
https://sourceware.org/ml/libc-alpha/2014-12/msg00155.html

1 files changed, 16 insertions, 9 deletions

diff --git a/NEWS b/NEWS
index 9086b7ed17..db1c28261e 100644
--- a/NEWS
+++ b/NEWS
@@ -9,15 +9,22 @@ Version 2.21
 
 * The following bugs are resolved with this release:
 
-  6652, 10672, 12847, 12926, 13862, 14132, 14138, 14171, 14498, 15215,
-  15884, 16009, 16191, 16469, 16617, 16619, 16657, 16740, 16857, 17192,
-  17266, 17273, 17344, 17363, 17370, 17371, 17411, 17460, 17475, 17485,
-  17501, 17506, 17508, 17522, 17555, 17570, 17571, 17572, 17573, 17574,
-  17582, 17583, 17584, 17585, 17589, 17594, 17601, 17608, 17616, 17625,
-  17630, 17633, 17634, 17635, 17647, 17653, 17657, 17658, 17664, 17665,
-  17668, 17682, 17717, 17719, 17722, 17723, 17724, 17725, 17732, 17733,
-  17744, 17745, 17746, 17747, 17748, 17775, 17777, 17780, 17781, 17782,
-  17791, 17793, 17796, 17797, 17803, 17806, 17834, 17844, 17848
+  6652, 10672, 12674, 12847, 12926, 13862, 14132, 14138, 14171, 14498,
+  15215, 15884, 16009, 16191, 16469, 16617, 16619, 16657, 16740, 16857,
+  17192, 17266, 17273, 17344, 17363, 17370, 17371, 17411, 17460, 17475,
+  17485, 17501, 17506, 17508, 17522, 17555, 17570, 17571, 17572, 17573,
+  17574, 17582, 17583, 17584, 17585, 17589, 17594, 17601, 17608, 17616,
+  17625, 17630, 17633, 17634, 17635, 17647, 17653, 17657, 17658, 17664,
+  17665, 17668, 17682, 17717, 17719, 17722, 17723, 17724, 17725, 17732,
+  17733, 17744, 17745, 17746, 17747, 17748, 17775, 17777, 17780, 17781,
+  17782, 17791, 17793, 17796, 17797, 17803, 17806, 17834, 17844, 17848
+
+* A new semaphore algorithm has been implemented in generic C code for all
+  machines. Previous custom assembly implementations of semaphore were
+  difficult to reason about or ensure that they were safe. The new version
+  of semaphore supports machines with 64-bit or 32-bit atomic operations.
+  The new semaphore algorithm is used by sem_init, sem_open, sem_post,
+  sem_wait, sem_timedwait, sem_trywait, and sem_getvalue.
 
 * Port to Altera Nios II has been contributed by Mentor Graphics.