Changeset 6737 in vbox for trunk/src/VBox

Timestamp:

Feb 1, 2008 9:29:47 PM (17 years ago)

Author:

vboxsync

svn:sync-xref-src-repo-rev:

27819

Message:

save one ASMAtomicXchgS32(), added comments

File:

• trunk/src/VBox/Runtime/r3/linux/sems-linux.cpp (modified) (2 diffs)

trunk/src/VBox/Runtime/r3/linux/sems-linux.cpp

@@ -868 +868 @@
     if (RT_UNLIKELY(iOld != 0))
     {
-        iOld = ASMAtomicXchgS32(&pIntMutexSem->iState, 2);
-        while (iOld != 0)
-        {
+        for (;;)
+        {
+            iOld = ASMAtomicXchgS32(&pIntMutexSem->iState, 2);
+
+            /*
+             * Was the lock released in the meantime? This is unlikely (but possible)
+             */
+            if (RT_UNLIKELY(iOld == 0))
+                break;
+
             /*
              * Go to sleep.
@@ -901 +908 @@
                 return RTErrConvertFromErrno(rc);
             }
-
-            iOld = ASMAtomicXchgS32(&pIntMutexSem->iState, 2);
-        }
+        }
+
+        /*
+         * When leaving this loop, iState is set to 2. This means that we gained the
+         * Lock and there are _possibly_ some waiters. We don't know exactly as another
+         * Thread might entered this loop at nearly the same time. Therefore we will
+         * call futex_wakeup once too often (if _no_ other thread entered this loop).
+         * The key problem is the simple futex_wait test for x != y (iState != 2) in
+         * our case).
+         */
     }