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<== Date ==> <== Thread ==>

Subject: RE: Fixed: Segfaults in 3.14 branch since merging ca-over-tcp
From: "Jeff Hill" <[email protected]>
To: "'Ralph Lange'" <[email protected]>
Cc: "'EPICS Core Talk'" <[email protected]>
Date: Tue, 17 Aug 2010 18:05:16 -0600
Hi Ralph,

> I'm not sure about that race condition, though.

> But don't see where my fix would break the fixed behavior, as it is
> still using a countre, decrementing the iiuExistenceCount is still
> happening at the end of cac::destroyIIU(), and incrementing
> iiuExistenceCount (original fix) happens exactly when it is added to
> the
> list, so that (my fix) using circuitList.count() to set
> iiuExistenceCount yields the same result.

The cac::destroyIIU has roughly three sections of code. In the first section there is a lock protected remove of the per-tcp-circuit data structure from cac's list etc, in the second section the per-tcp-circuit destructor is run without holding the lock so we will not deadlock, and in the third section we are protected by the lock when returning the per-tcp-circuit's memory to the free list and decrementing the iiuExistenceCount.

So, of course the business at hand is that we need to prevent the cac destructor from finishing until each of the tcp circuits is completely shut down. We do not make the cac destructor wait for the list count to become zero because we have to wait for stuff that happens after the per-tcp-circuit is removed from the list. Hence the need for the iiuExistenceCount. 

Note that _multiple_ threads, each managing private TCP circuits, can all be shutting down simultaneously and that the lock is released briefly between removing the circuit from the list, and decrementing the iiuExistenceCount. Furthermore, at the same instant in time it is possible that a new TCP circuits might also be created. So setting the count to the number of items on the list can seriously scramble the proper state of the iiuExistenceCount even before the cac dtor runs. And of course if that happens then we can have a crash because the cac dtor might not wait for all of the threads managing the circuits to shutdown, or alternatively hang forever waiting for phantom circuits to shutdown.

Jeff

______________________________________________________
Jeffrey O. Hill           Email        [email protected]
LANL MS H820              Voice        505 665 1831
Los Alamos NM 87545 USA   FAX          505 665 5107

Message content: TSPA


> -----Original Message-----
> From: Ralph Lange [mailto:[email protected]]
> Sent: Tuesday, August 17, 2010 10:08 AM
> To: Jeff Hill
> Cc: EPICS Core Talk
> Subject: Re: Fixed: Segfaults in 3.14 branch since merging ca-over-tcp
> 
>   Hi Jeff,
> 
> thanks for the fix!
> 
> 
> I'm not sure about that race condition, though.
> 
> In my fix:
> Setting iiuExistenceCount to the circuitList.count() (line ~290)
> happens
> in the cac::~cac destructor while the lock is being held, immediately
> followed by calling unlinkAllChannels() on all circuits in that list.
> 
> The only place where the lock is given up is one block down (line
> ~307),
> when waiting for the tcp threads to exit.
> 
> What race condition are you seeing? I looked at the 11786 change and
> bug#541362. I perfectly see why a counter fixes the issue that appears
> when using a simple flag.
> But don't see where my fix would break the fixed behavior, as it is
> still using a countre, decrementing the iiuExistenceCount is still
> happening at the end of cac::destroyIIU(), and incrementing
> iiuExistenceCount (original fix) happens exactly when it is added to
> the
> list, so that (my fix) using circuitList.count() to set
> iiuExistenceCount yields the same result.
> 
> I'm perfectly fine with using the original (increment) fix, but I would
> like to understand where the difference is.
> 
> 
> I think I also see another new race condition introduced by ca-over-
> tcp,
> which probably breaks the shutdown behavior for clients using
> ca-over-tcp, and needs to be fixed.
> 
> When the lock is given up in cac::~cac() and the loop (line ~307) waits
> for termination of all TCP tasks, an incoming name-res reply from a
> still active TCP circuit (the UDP task has exited by that time in the
> cac::~cac destructor) could open a new circuit (tcpiiu.c #2188), which
> would then be added to the circuitList (and iiuExistenceCount be
> incremented). For that new circuit, that the loop would be waiting for,
> unlinkAllChannels() is not being called, so it does not have a reason
> to
> terminate its threads.
> 
> Maybe we still need a flag: to avoid creating new circuits while the
> cac:~cac destructor runs?!
> 
> What do you think?
> 
> 
> Thanks a lot,
> Ralph
> 
> 
> On 16.08.2010 19:46, Jeff Hill wrote:
> >> + this->iiuExistenceCount = this->circuitList.count();
> > Examining this change a bit closer I see that it will introduce a
> race condition when circuits are being created and destroyed at close
> to the same instant in time. See revision 11786 of cac.cpp which fixes
> mantis 334 if you are interested in what this code does. After running
> the regression tests, I pushed in a fix (which is removing above
> mentioned change and restoring the increment of iiuExistenceCount in
> cac::findOrCreateVirtCircuit.
> >
> > Jeff
> >
> >> -----Original Message-----
> >> From: Ralph Lange [mailto:[email protected]]
> >> Sent: Monday, August 16, 2010 10:03 AM
> >> To: Andrew Johnson
> >> Cc: Jeff Hill; EPICS Core Talk
> >> Subject: Fixed: Segfaults in 3.14 branch since merging ca-over-tcp
> >>
> >>    Hi Andrew,
> >>
> >> it looks as I have found the reason for the segfaults, which I think
> is
> >> a bug in CAC.
> >>
> >> In the cac::~cac() destructor, a private member
> >> "this->iiuExistenceCount" is used to determine if cac should wait
> for
> >> TCP threads to shutdown. While this counter gets correctly decreased
> in
> >> cac::destroyIIU(), it never gets increased or set (except
> initialized to
> >> 0 in the cpp constructor).
> >>
> >> So cac::~cac() was never waiting for the TCP threads to shutdown,
> and
> >> happily kept tearing down the timer/mutex infrastructure. If the
> threads
> >> were not fast enough, they were hitting on destroyed semaphores when
> >> cancelling their watchdog timers - that was causing the exceptions
> and
> >> segfaults.
> >>
> >> This one-line fix will take care of the problem:
> >>
> >>
> >> === modified file 'src/ca/cac.cpp'
> >> --- src/ca/cac.cpp      2010-04-15 21:06:16 +0000
> >> +++ src/ca/cac.cpp      2010-08-16 15:24:13 +0000
> >> @@ -287,6 +287,7 @@
> >>                //
> >>                // shutdown all tcp circuits
> >>                //
> >> +            this->iiuExistenceCount = this->circuitList.count();
> >>                tsDLIter<  tcpiiu>  iter = this-
> >circuitList.firstIter ();
> >>                while ( iter.valid() ) {
> >>                    // this causes a clean shutdown to occur
> >>
> >>
> >> Maybe it actually takes care of several shutdown related issues...
> >>
> >> Yay!
> >> Ralph



Replies:
Re: Fixed: Segfaults in 3.14 branch since merging ca-over-tcp Ralph Lange
References:
Fixed: Segfaults in 3.14 branch since merging ca-over-tcp Ralph Lange
RE: Fixed: Segfaults in 3.14 branch since merging ca-over-tcp Jeff Hill
Re: Fixed: Segfaults in 3.14 branch since merging ca-over-tcp Ralph Lange

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