LinuxThreads read write lock patch.

[Kaz Kylheku <kaz at ashi dot footprints dot net>]

I've been doing some more testing which has covered recursive read locks in the
face of waiting writers. Looks good.  I made one minor change after reading
the damn spec more closely. The pthread_rwlock_tryrdlock function is required
to test for waiting writers and return EBUSY. This is even if the calling
thread has a read lock---whereas pthread_rwlock_rdlock must be recursive.
Go figure. Also, pthread_rwlock_rdlock is not required to give precedence
to waiting writers (it is unspecified what happens if writers are waiting)
whereas tryrdlock is required. Go figure again.  There is brain-damage at every
corner in the Single Unix Spec.

diff -u -r1.1.1.1.2.1 -r1.1.1.1.2.2
--- rwlock.c    2000/01/12 03:03:58     1.1.1.1.2.1
+++ rwlock.c    2000/01/12 07:34:51     1.1.1.1.2.2
@@ -276,7 +276,13 @@
 
   __pthread_lock (&rwlock->__rw_lock, self);
 
-  if (rwlock_can_rdlock(rwlock, have_lock_already))
+  /* 0 is passed to here instead of have_lock_already.
+     This is to meet Single Unix Spec requirements: 
+     if writers are waiting, pthread_rwlock_tryrdlock
+     does not acquire a read lock, even if the caller has
+     one or more read locks already. */
+
+  if (rwlock_can_rdlock(rwlock, 0))
     {
       ++rwlock->__rw_readers;
       retval = 0;