diff --git a/src/ca/cac.cpp b/src/ca/cac.cpp
index 79cdf83dd..0455c6e4f 100644
--- a/src/ca/cac.cpp
+++ b/src/ca/cac.cpp
@@ -117,12 +117,10 @@ cac::cac ( cacNotify &notifyIn, bool enablePreemptiveCallbackIn ) :
     threadsBlockingOnNotifyCompletion ( 0u ),
     maxRecvBytesTCP ( MAX_TCP ),
     recvProcessEnableRefCount ( 0u ),
-    recvProcessCompletionNBlockers ( 0u ),
     pndRecvCnt ( 0u ), 
     readSeq ( 0u ),
     enablePreemptiveCallback ( enablePreemptiveCallbackIn ),
     ioInProgress ( false ),
-    recvProcessInProgress ( false ),
     recvProcessThreadExitRequest ( false )
 {
 	long status;
@@ -211,6 +209,7 @@ cac::cac ( cacNotify &notifyIn, bool enablePreemptiveCallbackIn ) :
         freeListCleanup ( this->tcpLargeRecvBufFreeList );
         throwWithLocation ( caErrorCode ( ECA_ALLOCMEM ) );
     }
+    this->preemptiveCallbackLock.lock ();
 }
 
 cac::~cac ()
@@ -371,9 +370,8 @@ void cac::show ( unsigned level ) const
 {
     epicsAutoMutex autoMutex2 ( this->mutex );
 
-    ::printf ( "Channel Access Client Context at %p for user %s %s\n", 
-        static_cast <const void *> ( this ), this->pUserName,
-        this->recvProcessInProgress ? "busy" : "idle" );
+    ::printf ( "Channel Access Client Context at %p for user %s\n", 
+        static_cast <const void *> ( this ), this->pUserName );
     if ( level > 0u ) {
         tsDLIterConstBD < tcpiiu > piiu = this->iiuList.firstIter ();
         while ( piiu.valid() ) {
@@ -423,8 +421,6 @@ void cac::show ( unsigned level ) const
         this->ipToAEngine.show ( level - 3u );
         ::printf ( "recv process enable count %u\n", 
             this->recvProcessEnableRefCount );
-        ::printf ( "recv process blocking for completion count %u\n", 
-            this->recvProcessCompletionNBlockers );
         ::printf ( "\trecv process shutdown command boolean %u\n", 
             this->recvProcessThreadExitRequest );
         ::printf ( "\tthe current read sequence number is %u\n",
@@ -438,8 +434,6 @@ void cac::show ( unsigned level ) const
         this->recvProcessActivityEvent.show ( level - 3u );
         ::printf ( "Receive process exit event:\n" );
         this->recvProcessThreadExit.show ( level - 3u );
-        ::printf ( "Receive processing done event:\n" );
-        this->recvProcessCompletion.show ( level - 3u );
         ::printf ( "IO done event:\n");
         this->ioDone.show ( level - 3u );
         ::printf ( "mutex:\n" );
@@ -662,9 +656,15 @@ void cac::startRecvProcessThread ()
     {
         epicsAutoMutex epicsMutex ( this->mutex );
         if ( ! this->pRecvProcessThread ) {
+            unsigned priorityOfProcess;
+            epicsThreadBooleanStatus tbs  = epicsThreadLowestPriorityLevelAbove ( 
+                this->getInitializingThreadsPriority (), &priorityOfProcess );
+            if ( tbs != epicsThreadBooleanStatusSuccess ) {
+                priorityOfProcess = this->getInitializingThreadsPriority ();
+            }
+
             this->pRecvProcessThread = new epicsThread ( *this, "CAC-recv-process",
-                epicsThreadGetStackSize ( epicsThreadStackSmall ), 
-                this->getInitializingThreadsPriority () );
+                epicsThreadGetStackSize ( epicsThreadStackSmall ), priorityOfProcess );
             if ( ! this->pRecvProcessThread ) {
                 throw std::bad_alloc ();
             }
@@ -680,23 +680,14 @@ void cac::startRecvProcessThread ()
 // this is the recv process thread entry point
 void cac::run ()
 {
-printf ("recv process thread id is %x\n", epicsThreadGetIdSelf());
-    epicsAutoMutex autoMutex ( this->mutex );
     this->attachToClientCtx ();
     while ( ! this->recvProcessThreadExitRequest ) {
-        if ( this->recvProcessEnableRefCount ) {
-            this->recvProcessInProgress = true;
-            this->processRecvBacklog ();
-            this->recvProcessInProgress = false;
-        }
-        bool signalNeeded = this->recvProcessCompletionNBlockers > 0u;
         {
-            epicsAutoMutexRelease autoRelease ( this->mutex );
-            if ( signalNeeded ) {
-                this->recvProcessCompletion.signal ();
-            }
-            this->recvProcessActivityEvent.wait ();
+            epicsAutoMutex autoMutexPCB ( this->preemptiveCallbackLock );
+            epicsAutoMutex autoMutex ( this->mutex );
+            this->processRecvBacklog ();
         }
+        this->recvProcessActivityEvent.wait ();
     }
     this->recvProcessThreadExit.signal ();
 }
@@ -775,39 +766,27 @@ void cac::enableCallbackPreemption ()
     assert ( this->recvProcessEnableRefCount < UINT_MAX );
     this->recvProcessEnableRefCount++;
     if ( this->recvProcessEnableRefCount == 1u ) {
-        this->recvProcessActivityEvent.signal ();
+        this->preemptiveCallbackLock.unlock ();
     }
 }
 
 // lock must already be applied
 void cac::disableCallbackPreemption ()
 {
-    if ( ! this->recvProcessInProgress ) {
-        assert ( this->recvProcessEnableRefCount != 0u );
-        this->recvProcessEnableRefCount--;
-        return;
-    }
-    else {
-        this->recvProcessCompletionNBlockers++;
-    }
-    
-    while ( true ) {
-        {
-            epicsAutoMutexRelease autoMutexRelease ( this->mutex );
-            this->recvProcessCompletion.wait ();
-        }
-        
-        if ( ! this->recvProcessInProgress ) {
-            assert ( this->recvProcessEnableRefCount > 0u );
-            this->recvProcessEnableRefCount--;
-            assert ( this->recvProcessCompletionNBlockers > 0u );
-            this->recvProcessCompletionNBlockers--;
-            if ( this->recvProcessCompletionNBlockers > 0u ) {
-                this->recvProcessCompletion.signal ();
+    if ( this->recvProcessEnableRefCount == 1u ) {
+        if ( ! this->preemptiveCallbackLock.tryLock () ) {
+            {
+                epicsAutoMutexRelease autoMutexRelease ( this->mutex );
+                this->preemptiveCallbackLock.lock ();
+            }
+            // in case some thread enabled it while this->mutex was unlocked
+            if ( this->recvProcessEnableRefCount > 1u ) {
+                this->preemptiveCallbackLock.unlock ();
             }
-            return;
         }
     }
+    assert ( this->recvProcessEnableRefCount > 0u );
+    this->recvProcessEnableRefCount--;
 }
 
 void cac::repeaterSubscribeConfirmNotify ()
@@ -949,7 +928,6 @@ void cac::flushIfRequired ( nciu &chan )
         }
         if ( this->pudpiiu && blockPermit ) {
             if ( this->pudpiiu->isCurrentThread () ) {
-printf ("detected no-block cond and id is %x\n", epicsThreadGetIdSelf());
                 blockPermit = false;
             }
         }
@@ -1250,7 +1228,7 @@ void cac::connectAllIO ( nciu &chan )
             }
             pNetIO = next;
         }
-        chan.getPIIU()->flushRequest ();
+        chan.getPIIU()->requestRecvProcessPostponedFlush ();
         while ( baseNMIU *pIO = tmpList.get () ) {
             signalNeeded = this->blockForIOCallbackCompletion ( pIO->getID() );
             pIO->destroy ( *this );
@@ -1596,7 +1574,7 @@ bool cac::claimCIURespAction ( tcpiiu &iiu, const caHdrLargeArray &hdr, void *pM
         else {
             sidTmp = pChan->getSID ();
         }
-        pChan->connect ( hdr.m_dataType, hdr.m_count, sidTmp );
+        pChan->connect ( hdr.m_dataType, hdr.m_count, sidTmp, iiu.ca_v41_ok() );
         return true;
     }
     else {
diff --git a/src/ca/cac.h b/src/ca/cac.h
index 3d3b3d4bf..93bc5d150 100644
--- a/src/ca/cac.h
+++ b/src/ca/cac.h
@@ -160,9 +160,9 @@ private:
     epicsTime               programBeginTime;
     double                  connTMO;
     mutable epicsMutex      mutex; 
+    epicsMutex              preemptiveCallbackLock; 
     epicsEvent              notifyCompletionEvent;
     epicsEvent              recvProcessActivityEvent;
-    epicsEvent              recvProcessCompletion;
     epicsEvent              recvProcessThreadExit;
     epicsEvent              ioDone;
     epicsTimerQueueActive   *pTimerQueue;
@@ -180,12 +180,10 @@ private:
     unsigned                threadsBlockingOnNotifyCompletion;
     unsigned                maxRecvBytesTCP;
     unsigned                recvProcessEnableRefCount;
-    unsigned                recvProcessCompletionNBlockers;
     unsigned                pndRecvCnt;
     unsigned                readSeq;
     bool                    enablePreemptiveCallback;
     bool                    ioInProgress;
-    bool                    recvProcessInProgress;
     bool                    recvProcessThreadExitRequest;
 
     void flushRequestPrivate ();