diff --git a/src/ca/Makefile b/src/ca/Makefile
index 5598f02f5..3aef5d42b 100644
--- a/src/ca/Makefile
+++ b/src/ca/Makefile
@@ -69,7 +69,6 @@ LIBSRCS += comQueSend.cpp
 LIBSRCS += comBuf.cpp
 LIBSRCS += hostNameCache.cpp
 LIBSRCS += msgForMultiplyDefinedPV.cpp
-LIBSRCS += callbackMutex.cpp
 LIBSRCS += templateInstances.cpp
 
 LIBRARY=ca
diff --git a/src/ca/cac.cpp b/src/ca/cac.cpp
index c2b73166d..9e915d0f3 100644
--- a/src/ca/cac.cpp
+++ b/src/ca/cac.cpp
@@ -138,7 +138,6 @@ cac::cac ( cacNotify & notifyIn, bool enablePreemptiveCallbackIn ) :
     ipToAEngine ( "dnsQuery" ), 
     programBeginTime ( epicsTime::getCurrent() ),
     connTMO ( CA_CONN_VERIFY_PERIOD ),
-    cbMutex ( ! enablePreemptiveCallbackIn ),
     globalServiceList ( globalServiceListCAC.getReference () ),
     timerQueue ( epicsTimerQueueActive::allocate ( false, 
         lowestPriorityLevelAbove(epicsThreadGetPrioritySelf()) ) ),
@@ -1606,5 +1605,67 @@ void cac::pvMultiplyDefinedNotify ( msgForMultiplyDefinedPV & mfmdpv,
     this->mdpvFreeList.release ( & mfmdpv );
 }
 
+//
+// This is needed because in non-preemptive callback mode 
+// legacy applications that use file descriptor managers 
+// will register for ca receive thread activity and keep
+// calling ca_pend_event until all of the socket data has
+// been read. We must guarantee that other threads get a 
+// chance to run if there is data in any of the sockets.
+//
+void cac::waitUntilNoRecvThreadsPending ()
+{
+    if ( ! this->preemptiveCallbackEnabled ) {
+        {
+            const struct timeval delay = { 0, 0 };
+            fd_set mask;
+            FD_ZERO ( & mask );
+            int count = 0;
+            epicsGuard < cacMutex > guard ( this->mutex );
+            tsDLIter < tcpiiu > iter = this->serverList.firstIter ();
+            if ( this->pudpiiu ) {
+                this->pudpiiu->fdMaskSet ( mask );
+                count++;
+            }
+            while ( iter.valid() ) {
+                iter->fdMaskSet ( mask );
+                count++;
+                iter++;
+            }
+
+            int status = select ( count, & mask, 0, 0, & delay );
+            if ( status <= 0 ) {
+                return;
+            }
+            this->nRecvThreadsPending = 
+                static_cast < unsigned > ( count );
+        }
+
+        this->recvThreadActivityComplete.wait ( 0.1 );
+    }
+}
+
+void cac::signalRecvThreadActivity ()
+{
+    if ( ! this->preemptiveCallbackEnabled ) {
+        bool signalNeeded = false;
+        {
+            epicsGuard < cacMutex > guard ( this->mutex );
+            if ( this->nRecvThreadsPending <= 1 ) {
+                if ( this->nRecvThreadsPending == 1 ) {
+                    this->nRecvThreadsPending = 0;
+                    signalNeeded = true;
+                }
+            }
+            else {
+                this->nRecvThreadsPending--;
+            }
+        }
+        if ( signalNeeded ) {
+            this->recvThreadActivityComplete.signal ();
+        }
+    }
+}
+
 
 
diff --git a/src/ca/cac.h b/src/ca/cac.h
index 3b583fc48..d1becde71 100644
--- a/src/ca/cac.h
+++ b/src/ca/cac.h
@@ -73,17 +73,12 @@ extern epicsThreadPrivateId caClientCallbackThreadId;
 
 class callbackMutex {
 public:
-    callbackMutex ( bool threadsMayBeBlockingForRecvThreadsToFinish );
+    callbackMutex ();
     ~callbackMutex ();
     void lock ();
     void unlock ();
-    void waitUntilNoRecvThreadsPending ();
 private:
-    epicsMutex countMutex;
     epicsMutex primaryMutex; 
-    epicsEvent noRecvThreadsPending;
-    unsigned recvThreadsPendingCount;
-    bool threadsMayBeBlockingForRecvThreadsToFinish;
     callbackMutex ( callbackMutex & );
     callbackMutex & operator = ( callbackMutex & );
 };
@@ -198,6 +193,7 @@ public:
     void initiateAbortShutdown ( tcpiiu & );
     void disconnectNotify ( tcpiiu & );
     void uninstallIIU ( tcpiiu & ); 
+    void signalRecvThreadActivity ();
 
 private:
     localHostName hostNameCache;
@@ -244,6 +240,7 @@ private:
     callbackMutex cbMutex;
     mutable cacMutex mutex; 
     epicsEvent iiuUninstall;
+    epicsEvent recvThreadActivityComplete;
     epicsSingleton 
         < cacServiceList >::reference
             globalServiceList;
@@ -256,6 +253,7 @@ private:
     epicsThreadId initializingThreadsId;
     unsigned initializingThreadsPriority;
     unsigned maxRecvBytesTCP;
+    unsigned nRecvThreadsPending;
     bool preemptiveCallbackEnabled;
 
     void privateUninstallIIU ( epicsGuard < callbackMutex > &, tcpiiu &iiu ); 
@@ -406,11 +404,6 @@ inline bool cac::preemptiveCallbakIsEnabled () const
     return this->preemptiveCallbackEnabled;
 }
 
-inline void cac::waitUntilNoRecvThreadsPending ()
-{
-    this->cbMutex.waitUntilNoRecvThreadsPending ();
-}
-
 inline epicsGuard < callbackMutex > cac::callbackGuardFactory ()
 {
     // facilitate the return value optimization
@@ -432,5 +425,23 @@ inline void cacMutex::show ( unsigned level ) const
     this->mutex.show ( level );
 }
 
+inline callbackMutex::callbackMutex () 
+{
+}
+
+inline callbackMutex::~callbackMutex ()
+{
+}
+
+inline void callbackMutex::lock ()
+{
+    this->primaryMutex.lock ();
+}
+
+inline void callbackMutex::unlock ()
+{
+    this->primaryMutex.unlock ();
+}
+
 #endif // ifdef cach
 
diff --git a/src/ca/callbackMutex.cpp b/src/ca/callbackMutex.cpp
deleted file mode 100644
index c8d9d0071..000000000
--- a/src/ca/callbackMutex.cpp
+++ /dev/null
@@ -1,85 +0,0 @@
-/*************************************************************************\
-* Copyright (c) 2002 The University of Chicago, as Operator of Argonne
-*     National Laboratory.
-* Copyright (c) 2002 The Regents of the University of California, as
-*     Operator of Los Alamos National Laboratory.
-* EPICS BASE Versions 3.13.7
-* and higher are distributed subject to a Software License Agreement found
-* in file LICENSE that is included with this distribution. 
-\*************************************************************************/
-
-/*  
- *  $Id$
- *
- *                    L O S  A L A M O S
- *              Los Alamos National Laboratory
- *               Los Alamos, New Mexico 87545
- *
- *  Copyright, 1986, The Regents of the University of California.
- *
- *  Author: Jeff Hill
- */
-
-#define epicsAssertAuthor "Jeff Hill johill@lanl.gov"
-
-#include "cac.h"
-
-callbackMutex::callbackMutex ( bool threadsMayBeBlockingForRecvThreadsToFinishIn ) :
-    recvThreadsPendingCount ( 0u ), 
-    threadsMayBeBlockingForRecvThreadsToFinish ( threadsMayBeBlockingForRecvThreadsToFinishIn )
-{
-}
-
-callbackMutex::~callbackMutex ()
-{
-}
-
-void callbackMutex::lock ()
-{
-    if ( this->threadsMayBeBlockingForRecvThreadsToFinish ) {
-        if ( ! this->primaryMutex.tryLock () ) {
-            // the count must be incremented prior to blocking for the lock
-            {
-                epicsGuard < epicsMutex > autoMutex ( this->countMutex );
-                assert ( this->recvThreadsPendingCount < UINT_MAX );
-                this->recvThreadsPendingCount++;
-            }
-            
-            this->primaryMutex.lock ();
-            
-            bool signalRequired;
-            {
-                epicsGuard < epicsMutex > autoMutex ( this->countMutex );
-                assert ( this->recvThreadsPendingCount > 0 );
-                this->recvThreadsPendingCount--;
-                if ( this->recvThreadsPendingCount == 0u  ) {
-                    signalRequired = true;
-                }
-                else {
-                    signalRequired = false;
-                }
-            }
-            
-            if ( signalRequired ) {
-                this->noRecvThreadsPending.signal ();
-            }
-        }
-    }
-    else {
-        this->primaryMutex.lock ();
-    }
-}
-
-void callbackMutex::unlock ()
-{
-    this->primaryMutex.unlock ();
-}
-
-void callbackMutex::waitUntilNoRecvThreadsPending () 
-{
-    epicsGuard < epicsMutex > autoMutex ( this->countMutex );
-    while ( this->recvThreadsPendingCount > 0 ) {
-        epicsGuardRelease < epicsMutex > autoMutexRelease ( autoMutex );
-        this->noRecvThreadsPending.wait ();
-    }
-}