[M3devel] purported condition variable problems on Win32?

Fri Jul 22 06:24:45 CEST 2016

Do our semantics allow merging the locks?
I.e. Must always use same lock with same condition and vice versa? That would indeed be a nice optimization, if the interface is limited that way. I'll look around later...

 - Jay

> On Jul 21, 2016, at 10:03 AM, "Rodney M. Bates" <rodney_bates at lcwb.coop> wrote:
> 
> 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