Experimental Physics and
Industrial Control System

Andrew Johnson via Core-talk <core-talk at aps.anl.gov> · Mon, 25 May 2020 04:25:12 -0000

AFAIK the Github PR #73 fixed all the faults we could find in the
original code, and added tests. This PR was merged into 3.15.8 and
marked as Fix Released there; I just merged the 3.15 branch into 7.0 so
this fix has been incorporated and I'm adding the 7.0.3.2 milestone back
again.

Unfortunately the RTEMS implementation of osdMessageQueue.cpp seems to
be failing the new epicsMessageQueue tests, but that's a different issue
since it's a different implementation...

** Changed in: epics-base/7.0
       Status: In Progress => Fix Committed

** Changed in: epics-base/7.0
    Milestone: None => 7.0.3.2

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https://bugs.launchpad.net/bugs/1868486

Title:
  epicsMessageQueue lost messages

Status in EPICS Base:
  Fix Committed
Status in EPICS Base 3.15 series:
  Fix Released
Status in EPICS Base 7.0 series:
  Fix Committed

Bug description:
  https://epics.anl.gov/core-talk/2020/msg00396.php

  Mark Rivers observed epicsMessageQueue losing messages.

  https://epics.anl.gov/core-talk/2020/msg00408.php

  > I think I see the logic error in how the eventSent flag is handled,
  > specifically related to the fact that epicsEvent is a semaphore as
  > opposed to a condition variable.
  > 
  > This allows a "race" to occur if the first/only waiting receiver
  > times out, and epicsEventWaitWithTimeout() returns, while a sender
  > is in epicsMessageQueueSend() preparing to wake up a receiver.
  > 
  > This results in a situation where the sender has set the eventSent,
  > and indeed copied a message to the buffer of, a thread which has
  > decided to abort.
  > 
  > After timing out, the receiver sees the timeout and returns -1
  > even through eventSent has been set.  This can be trapped with:
  > 
  > > b osdMessageQueue.cpp:358 if status!=0 && threadNode.eventSent
  > 
  > So here is your lost message.
  > 
  > Now when epicsMessageQueueReceiveWithTimeout() is called again, no message
  > is waiting in the queue, so epicsEventWaitWithTimeout() is called.
  > Since the semaphore is already set, this returns immediately with status==0,
  > but this is a spurious wakeup and the eventSent flag is not set.
  > 
  > And here is the second "timeout".

  Line numbers circa 7.0.3.1

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Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System