[M3devel] purported condition variable problems on Win32?
Rodney M. Bates
rodney_bates at lcwb.coop
Thu Jul 21 19:04:40 CEST 2016
I think the first lost-wakeup bug can be fixed by merging the two lock sections
on c.lock or maybe moving just the DEC(c.tickets) up to the first. I would
need to write sample code, then review it to be confident. I have not done
that so far.
Maybe the Java folks knew this and got it right.
On 07/21/2016 01:43 AM, Jay K wrote:
> That question was meant for the other bug.
>
> Can this one be fixed by rechecking if tickets is positive before decrementing it?
> This seems like a big problem in Schmidt's code.
>
> I'm nervous about merging the locks, but maybe.
> In particular, I'm the multiple release/acquires in service of the condition variable, also releasing/acquiring the user variable.
> Though I realize that is actually necessarily part of what gets done.
> Schmidt clearly did not merge the locks, and the Java version does.
> The Java version also has its own simple and not-terrible critical section.
> The Java version also disappeared in newer releases and I didn't track down what happened to it.
>
> - Jay
>
>
> ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
> From: jay.krell at cornell.edu
> To: rodney.m.bates at acm.org
> CC: m3devel at elegosoft.com
> Subject: RE: [M3devel] purported condition variable problems on Win32?
> Date: Thu, 21 Jul 2016 06:35:41 +0000
>
> Am I understanding this?
> There are some waiters.
> There is a broadcast ("wake all"). Some waiters are woken.
> There are more waiters.
> There is a signal ("wake one").
> There is a chance that some of the original waiters will remain waiting while the later waiters are not.
> That is, the signaled threads kind of steal the wake intended by the broadcast.
>
>
> Is this unfairness or incorrectness?
> It feels like unfairness.
> It feels like condition variables are specified fairly weakly.
>
>
> - Jay
>
>
> > Date: Wed, 20 Jul 2016 11:43:27 -0500
> > From: rodney_bates at lcwb.coop
> > To: jay.krell at cornell.edu; rodney.m.bates at acm.org
> > CC: m3devel at elegosoft.com
> > Subject: Re: [M3devel] purported condition variable problems on Win32?
> >
> > In looking at ThreadWin32.m3 more, I think I see a significant bug.
> >
> > This gets confusing, since there are Modula-3 waits, signals,
> > etc. and similar concepts in Win32, but which are not the
> > same. I am going the try to put "M3-" and "Win-" prefixes on
> > things to make this clearer. I cartainly need to do this
> > for my own benefit, if for nobody else.
> >
> > Suppose two threads are both M3-waiting in Wait(m,c,...), further
> > Win-waiting on c.waitEvent in WaitForMultipleObjects at :301.
> > c.tickets = 0, and c.waiters = 2.
> >
> > Now, (M3-)Signal (c) happens. This does (Win-)SetEvent(c.waitEvent),
> > which will allow both the waiters to Win-wakeup sometime soon.
> > Before releasing c.lock, Signal increments c.tickets to 1 (supposedly
> > to eventually allow only one waiter to M3-wakeup) and increments
> > c.counter, setting things up so that the waitDone expression at :308
> > will be TRUE for the waiting threads, the next time they evaluate it.
> >
> > Each waiter in turn Win-wakes, gets c.lock, sets waitDone TRUE,
> > releases c.lock, and tries to acquire m. The problem arises
> > because the second waiter can get to these steps before the first
> > can acquire m, acquire c.lock, and DEC(c.tickets) at :322. The
> > second to reach :308 will find c.tickets # 0, as did the first,
> > and proceed.
> >
> > Each will eventually, do a M3-wake. If this were Posix condition
> > variables, this would be wrong, because Posix says only one
> > waiter proceeds. For M3, it's OK, if unnecessary, because M3
> > says one or more waiters proceed. However, each will, in the
> > process, get to :322, and DEC(c.tickets). Signal only provided
> > one ticket, but the waiters have taken two. c.tickets goes
> > negative.
> >
> > After this, when some thread can do another (M3-)Wait, and some
> > other does another (M3-)Signal, c.tickets increments only back to
> > zero, which means the new waiter will not M3-wake, even though it
> > should.
> >
> >
> > I am going to try to construct a test case that will force
> > this scenario.
> >
> > See more below:
> >
> >
> > On 07/18/2016 08:53 PM, Jay wrote:
> > > I wrote this version. Right, based on Schmidt. I also found similar code in Java. There are comments as to both of these in the code.
> > >
> > > Otherwise previous version I think had a very large lock.
> > >
> > > Changing the < to !=, is that really ok? I get that 32bit overflow is possible.
> > >
> > > We can change to a 64bit counter if < is required. That really can't rollover. I.e. Even a 64bit cycle counter can't rollover, and this would advance much slower.
> > >
> > > I guess you are saying it would be unfair, which is ok.
> > >
> > > There is pattern called "directed notification" or such that might be preferable? It has a certain significant inefficiency though, like creating an event per notify. I think it is what boost's condition variable are using.
> > >
> > > I'll check on the unused event.
> > >
> > > - Jay
> > >
> > >> On Jul 18, 2016, at 7:25 AM, "Rodney M. Bates" <rodney_bates at lcwb.coop> wrote:
> > >>
> > >>
> > >>
> > >>
> > >>
> > >> I spent some time looking over the signal part of ThreadWin32.m3. A few
> > >> comments:
> > >>
> > >> This looks like a very direct implementation of Schmidts' generation count
> > >> algorithm, with a number of important little details additionally taken care
> > >> of.
> > >>
> > >> On small difference is at line 308 (B, below) where Schmidt does
> > >> c.counter>count instead of #. Usually, this would make no difference,
> > >> since counter only increases, but on a long-running program, it could
> > >> overflow. If this happened,as it often does, as silent wrap-around to
> > >> FIRST(INTEGER), some threads could be trapped for an extremely long
> > >> time, waiting for counter come back around greater than their count. As
> > >> coded in ThreadWin32, they would be allowed to proceed normally.
> > >>
> > >> The only glitch would be that, if there were simultaneously waiting threads
> > >> separated by NUMBER(INTEGER) truly different generations (but the same value
> > >> of c.counter), the recent ones could unfairly proceed along with the
> > >> extremely old ones. That seems extremely less likely than the
> > >> mere existence of an overflow.
> > >>
> > >> Both type Condition and type Activation have a field waitEvent: HANDLE.
> > >> Condition.waitEvent is extensively used, but Activation.waitEvent is
> > >> only created at :537 and deleted at :554, but never used in any real
> > >> way. It could be deleted.
> > >>
> > >> As for this code:
> > >>
> > >> EnterCriticalSection(conditionLock);
> > >>
> > >> (* Capture the value of the counter before we start waiting.
> > >> * We will not stop waiting until the counter changes.
> > >> * That is, we will not stop waiting until a signal
> > >> * comes in after we start waiting.
> > >> *)
> > >>
> > >> count := c.counter;
> > >> INC(c.waiters);
> > >>
> > >> LeaveCriticalSection(conditionLock);
> > >> m.release(); (* can this be moved to before taking conditionLock? *)
> > >>
> > >> No, it is important to sample and save c.counter in the order threads wait.
> > >> Retaining m until this is done ensures this. Otherwise, some other thread
> > >> that waited later could have altered c.counter before this one gets conditionLock
> > >> and saves its copy of c.counter.
> > >>
> > >> I have made several comment changes to ThreadWin32.m3 that would have made
> > >> it easier to vet. I will commit these, but only comments. It is either
> > >> very difficult or impossible for me to test or even recompile this module,
> > >> and, especially as fragile as this kind of code is, I don't want to commit
> > >> any substantive changes untested and uncompiled.
> > >>
> > >> More below:
> > >>
> > >>> On 07/07/2016 04:24 AM, Jay K wrote:
> > >>> So...I do NOT understand all of this.
> > >>>
> > >>>
> > >>> However, comparing the three implementations
> > >>> and attempting to understand ours,
> > >>> I am struck by the following
> > >>>
> > >>> - Our broadcast seems ok, but
> > >>> - our signal seems to wake everyone,
> > >>> and then...they race a bit,
> > >>> one will decide it is last, and
> > >>> reset the event, but every prior waiter
> > >>> is still woken.
> > >>> Why not merge the following chunks:
> > >>
> > >> I don't think this will work. It is important that, once a thread has decided,
> > >> at A, that it can fairly proceed, it is guaranteed to be the next thread to proceed
> > >> (i.e, to return from [Alert]Wait), before it decrements c.tickets at C, and, thus,
> > >> if it is also the last of its generation to proceed, resets c.waitevent. This
> > >> is ensured by its having already reacquired m first, preventing any other thread
> > >> from returning from its Wait yet.
> >
> > I was wrong about this. There is already no fairness being enforced here. Even with
> > only one ticked, all waiters Win-wake at :301, the order they get c.lock at :307 is
> > arbitrary anyway. There is no benefit in trying to control the order they reacquire m.
> > c.counter has nothing to do with fairness. What it does is prevent Win-awakened waiters
> > that waited after the most recent Signal/Broadcast from M3-waking, a correctness issue.
> >
> > Merging, as you proposed, is probably close to the fix for this bug.
> >
> >
> > >>
> > >> And it can't attempt to acquire m while already holding conditionLock, because this would
> > >> invite deadlock, since, elsewhere, a thread can try to acquire conditionLock while
> > >> already holding m, at line 265.
> > >>
> > >>>
> > >>>
> > >>> WHILE (NOT alerted) AND (NOT waitDone) DO
> > >>> .
> > >>> .
> > >>> .
> > >>> 1
> > >>> EnterCriticalSection(conditionLock);
> > >>> waitDone := (c.tickets # 0 AND c.counter # count);
> > >> A:---------^
> > >> B:--------------------------------------------------^
> > >>> LeaveCriticalSection(conditionLock);
> > >>> END; (* WHILE *)
> > >>> IF waitDone THEN
> > >>> alerted := FALSE;
> > >>> END;
> > >>> m.acquire();
> > >>> 2
> > >>> EnterCriticalSection(conditionLock);
> > >>> DEC(c.waiters);
> > >>> IF waitDone THEN esp. here.
> > >>> DEC(c.tickets);
> > >>
> > >> C:---------------^
> > >>> lastWaiter := (c.tickets = 0);
> > >>> END;
> > >>> LeaveCriticalSection(conditionLock);
> > >>>
> > >>>
> > >>> That is, if we decrement tickets earlier
> > >>> within the first critical section,
> > >>> while we will still wake everyone,
> > >>> only one will decide waitDone and the rest will keep looping.
> > >>> A downside of this, perhaps, is that waking all for Broadcast
> > >>> might be a little slower.
> > >>> but more so, in both the "ptw32" (pthreads for win32) and Boost threads
> > >>> implementations, they seem to deal with this differently than us,
> > >>> and they each do about the same thing -- they use a counted semaphore.
> > >>> In the boost case, it appears they duplicate the semaphore for every
> > >>> notification generation, which seems expensive.
> > >>>
> > >>> Perhaps if they can guarantee some lifetimes, they don't need to duplicate it.
> > >>> Or they can do their own reference counting?
> > >>> jdk7 seems to looke like jdk6.
> > >>> The code is gone in jdk8 and I can't easily find the delete in history.
> > >>> The lock merging jdk does probably helps here too.
> > >>> In fact they merge #1 and #2 above as a result.
> > >>> But they still initially wake all threads for signal, not just broadcast.
> > >>> There is also the problem that all the event waits
> > >>> are followed by EnterCriticalSection (or jdk facsimile).
> > >>> - Jay
> > >>>
> > >>>
> > >>>
> > >>>
> > >>> -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------!
> > --!
> > >> ---
> > >>> From: jay.krell at cornell.edu
> > >>> To: m3devel at elegosoft.com
> > >>> Subject: RE: [M3devel] purported condition variable problems on Win32?
> > >>> Date: Tue, 5 Jul 2016 08:26:36 +0000
> > >>>
> > >>> Here is another implementation to consider:
> > >>>
> > >>> https://github.com/boostorg/thread/blob/develop/include/boost/thread/win32/condition_variable.hpp
> > >>>
> > >>> - Jay
> > >>>
> > >>>
> > >>> -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------!
> > --!
> > >> ---
> > >>> From: jay.krell at cornell.edu
> > >>> To: m3devel at elegosoft.com
> > >>> Date: Tue, 5 Jul 2016 08:19:23 +0000
> > >>> Subject: [M3devel] purported condition variable problems on Win32?
> > >>>
> > >>> https://sourceforge.net/p/pthreads4w/code/ci/master/tree/README.CV
> > >>>
> > >>> vs.
> > >>>
> > >>> http://www.cs.wustl.edu/~schmidt/win32-cv-1.html
> > >>> vs.
> > >>> https://modula3.elegosoft.com/cgi-bin/cvsweb.cgi/cm3/m3-libs/m3core/src/thread/WIN32/ThreadWin32.m3?rev=1.210.2.1;content-type=text%2Fplain
> > >>> I wrote this:
> > >>> PROCEDURE XWait(m: Mutex; c: Condition; act: Activation;
> > >>> alertable: BOOLEAN) RAISES {Alerted} =
> > >>> (* LL = m on entry and exit, but not for the duration
> > >>> * see C:\src\jdk-6u14-ea-src-b05-jrl-23_apr_2009\hotspot\agent\src\os\win32\Monitor.cpp
> > >>> * NOTE that they merge the user lock and the condition lock.
> > >>> * http://www.cs.wustl.edu/~schmidt/win32-cv-1.html
> > >>> * "3.3. The Generation Count Solution"
> > >>> *)
> > >>>
> > >>> Do we have the problems described in README.CV?
> > >>>
> > >>> I haven't looked through the ACE code to see
> > >>> to what extent they resemble solution 3.3, or if they
> > >>> changed as a result of this discussion -- which I admit I don't understand
> > >>> and haven't read closely.
> > >>>
> > >>>
> > >>> Spurious wakeups are ok, though should be minimized.
> > >>>
> > >>> I'd still rather not drop pre-Vista support but I realize it becomes more interesting as time advances.
> > >>>
> > >>> Thank you,
> > >>> - Jay
> > >>>
> > >>> _______________________________________________ M3devel mailing list M3devel at elegosoft.com https://m3lists.elegosoft.com/mailman/listinfo/m3devel
> > >>>
> > >>>
> > >>> _______________________________________________
> > >>> M3devel mailing list
> > >>> M3devel at elegosoft.com
> > >>> https://m3lists.elegosoft.com/mailman/listinfo/m3devel
> > >>
> > >> --
> > >> Rodney Bates
> > >> rodney.m.bates at acm.org
> > >>
> > >>
> > >
> >
> > --
> > Rodney Bates
> > rodney.m.bates at acm.org
--
Rodney Bates
rodney.m.bates at acm.org
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