Index: src/VBox/Runtime/r0drv/freebsd/semevent-r0drv-freebsd.c
===================================================================
--- src/VBox/Runtime/r0drv/freebsd/semevent-r0drv-freebsd.c	(revision 22051)
+++ src/VBox/Runtime/r0drv/freebsd/semevent-r0drv-freebsd.c	(working copy)
@@ -75,7 +75,7 @@
         pEventInt->cWaiters = 0;
         pEventInt->cWaking = 0;
         pEventInt->fSignaled = 0;
-        mtx_init(&pEventInt->Mtx, "IPRT Event Semaphore", NULL, 0);
+        mtx_init(&pEventInt->Mtx, "IPRT Event Semaphore", NULL, MTX_SPIN);
         *pEventSem = pEventInt;
         return VINF_SUCCESS;
     }
@@ -93,21 +93,21 @@
                     ("pEventInt=%p u32Magic=%#x\n", pEventInt, pEventInt->u32Magic),
                     VERR_INVALID_HANDLE);
 
-    mtx_lock(&pEventInt->Mtx);
+    mtx_lock_spin(&pEventInt->Mtx);
     ASMAtomicIncU32(&pEventInt->u32Magic); /* make the handle invalid */
     if (pEventInt->cWaiters > 0)
     {
         /* abort waiting thread, last man cleans up. */
         ASMAtomicXchgU32(&pEventInt->cWaking, pEventInt->cWaking + pEventInt->cWaiters);
         wakeup(pEventInt);
-        mtx_unlock(&pEventInt->Mtx);
+        mtx_unlock_spin(&pEventInt->Mtx);
     }
     else if (pEventInt->cWaking)
         /* the last waking thread is gonna do the cleanup */
-        mtx_unlock(&pEventInt->Mtx);
+        mtx_unlock_spin(&pEventInt->Mtx);
     else
     {
-        mtx_unlock(&pEventInt->Mtx);
+        mtx_unlock_spin(&pEventInt->Mtx);
         mtx_destroy(&pEventInt->Mtx);
         RTMemFree(pEventInt);
     }
@@ -124,7 +124,7 @@
                     ("pEventInt=%p u32Magic=%#x\n", pEventInt, pEventInt->u32Magic),
                     VERR_INVALID_HANDLE);
 
-    mtx_lock(&pEventInt->Mtx);
+    mtx_lock_spin(&pEventInt->Mtx);
 
     if (pEventInt->cWaiters > 0)
     {
@@ -135,7 +135,7 @@
     else
         ASMAtomicXchgU8(&pEventInt->fSignaled, true);
 
-    mtx_unlock(&pEventInt->Mtx);
+    mtx_unlock_spin(&pEventInt->Mtx);
     return VINF_SUCCESS;
 }
 
@@ -149,7 +149,7 @@
                     ("pEventInt=%p u32Magic=%#x\n", pEventInt, pEventInt->u32Magic),
                     VERR_INVALID_HANDLE);
 
-    mtx_lock(&pEventInt->Mtx);
+    mtx_lock_spin(&pEventInt->Mtx);
 
     if (pEventInt->fSignaled)
     {
@@ -172,17 +172,23 @@
 
         ASMAtomicIncU32(&pEventInt->cWaiters);
 
-        /** @todo r=bird: This doesn't handle cMillies == 0 correctly, it will assert
-         *        and or sleep for ever according to r47861. (That's probably an old bug
-         *        of my making.)
-         */
-        rc = mtx_sleep(pEventInt,          /* block id */
-                       &pEventInt->Mtx,    /* mtx */
-                       fInterruptible ? PZERO | PCATCH : PZERO,
-                       "iprtev",           /* max 6 chars */
-                       cMillies == RT_INDEFINITE_WAIT
-                       ? 0
-                       : tvtohz(&tv));
+        mtx_unlock_spin(&pEventInt->Mtx);
+/** @todo r=bird: This doesn't handle cMillies == 0 correctly, it will assert
+ *        and or sleep for ever according to r47861. (That's probably an old bug
+ *        of my making.)
+ *
+ *        And it really looks like it's racing signalling on MP systems. (It
+ *        *looks* like you leave the lock and then tries to go to sleep on a
+ *        block id, someone spinning on the lock attempt to wake us up before we
+ *        go to sleep. That's why the code was originally trying to use msleep
+ *        here.). */
+        rc = tsleep(pEventInt,          /* block id */
+                    fInterruptible ? PZERO | PCATCH : PZERO,
+                    "iprtev",           /* max 6 chars */
+                    cMillies == RT_INDEFINITE_WAIT
+                    ? 0
+                    : tvtohz(&tv));
+        mtx_lock_spin(&pEventInt->Mtx);
 
         switch (rc)
         {
@@ -200,7 +206,7 @@
                     {
                         /* The event was destroyed, as the last thread do the cleanup.
                            we don't actually know whether */
-                        mtx_unlock(&pEventInt->Mtx);
+                        mtx_unlock_spin(&pEventInt->Mtx);
                         mtx_destroy(&pEventInt->Mtx);
                         RTMemFree(pEventInt);
                         return rc;
@@ -230,7 +236,7 @@
         }
     }
 
-    mtx_unlock(&pEventInt->Mtx);
+    mtx_unlock_spin(&pEventInt->Mtx);
     return rc;
 }