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<body class='hmmessage'><div dir='ltr'>I don't think Signal/Broadcast require caller be holding any lock, is a problem.<div><br></div><div> - Jay<br><br><br><br><div>> Date: Sat, 23 Jul 2016 16:01:43 -0500<br>> From: rodney_bates@lcwb.coop<br>> To: jay.krell@cornell.edu; rodney.m.bates@acm.org<br>> CC: m3devel@elegosoft.com<br>> Subject: Re: [M3devel] purported condition variable problems on Win32?<br>> <br>> <br>> <br>> On 07/22/2016 03:20 AM, Jay K wrote:<br>> > Here is very weak circumstantial evidence that the locks can be merged:<br>> ><br>> ><br>> ><br>> > https://msdn.microsoft.com/en-us/library/windows/desktop/ms682052(v=vs.85).aspx<br>> ><br>> > "It is often convenient to use more than one condition variable with the same lock.<br>> > For example, an implementation of a reader/writer lock might use a single critical<br>> > section but separate condition variables for readers and writers.<br>> > "<br>> ><br>> ><br>> > i.e. the mapping isn't one lock to one condition, but the convenient<br>> > construct is multiple conditions to one lock, not multiple locks to one condition.<br>> ><br>> ><br>> > http://linux.die.net/man/3/pthread_cond_wait<br>> ><br>> > "<br>> > The effect of using more than one mutex for concurrent pthread_cond_timedwait() or<br>> > pthread_cond_wait() operations on the same condition variable is undefined; that is,<br>> > a condition variable becomes bound to a unique mutex when a thread waits on the condition<br>> > variable, and this (dynamic) binding shall end when the wait returns.<br>> > "<br>> ><br>> <br>> Yes, these are the usual rules about the relationship between mutexs and condition variables.<br>> There is sometimes a definite need for >1 condition variable associated with a single<br>> mutex. The other way around might be definable, but I think would create a big mess.<br>> <br>> Our Thread interface documents only that Wait(m,c) must have m locked at the time of<br>> call (and will return with it locked too, but not necessarily for the duration.<br>> <br>> Chapter 4 of SPwM3, "An Introduction to Programming with Threads" informally defines<br>> a _monitor_ as "some data, a mutex, and zero or more condition variables" and says<br>> further "A particular condition variable is always used in conjunction with the same<br>> mutex and its data". (SPwM3, 4.3.2, p93)<br>> <br>> It looks to me like the formal specification of Wait, Signal, and Broadcast in<br>> SRwM3, 5.3.3, p124 would allow a condition to simultaneously contain waiters<br>> that passed in different mutexes. Each thread remembers locally what mutex it<br>> wants to reacquire when it wakes up.<br>> <br>> I'm not sure we really want to commit to that. Client code's doing it would no doubt<br>> create even trickier code than this kind of stuff already is. Off hand, I have a<br>> hard time thinking of a reasonable use-case.<br>> <br>> If we decide to go with the informal statement, the implementations really should<br>> enforce it by storing the associated mutex in the condition variable, and verifying<br>> that additional waiters supply the same one.<br>> <br>> Probably, in most implementations, you could get away with moving the association<br>> of a condition variable from one mutex to another when no threads were waiting on<br>> the condition. Buy\t the only possible benefit I can think of would be memory savings,<br>> not needing as many condition variables, since the different mutex associations would<br>> be, well, mutually exclusive.<br>> <br>> <br>> <br>> <br>> > i.e. while the lock used with a condition variable can change through time,<br>> > there can be only one at a time.<br>> ><br>> > And then, presumably win32 condition variables and posix condition variables<br>> > and Modula-3 Thread.Condition are all kind of similar.<br>> ><br>> > Hm, ok, a problem with merging the locks, is it leaves no lock for Signal/Broadcast.<br>> > Perhaps they should use atomic/Interlocked.<br>> ><br>> > - Jay<br>> ><br>> > ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------!<br>> ---<br>> > From: jay.krell@cornell.edu<br>> > To: rodney.m.bates@acm.org<br>> > CC: m3devel@elegosoft.com<br>> > Subject: RE: [M3devel] purported condition variable problems on Win32?<br>> > Date: Thu, 21 Jul 2016 06:43:16 +0000<br>> ><br>> > That question was meant for the other bug.<br>> ><br>> > Can this one be fixed by rechecking if tickets is positive before decrementing it?<br>> > This seems like a big problem in Schmidt's code.<br>> ><br>> > I'm nervous about merging the locks, but maybe.<br>> > In particular, I'm the multiple release/acquires in service of the condition variable, also releasing/acquiring the user variable.<br>> > Though I realize that is actually necessarily part of what gets done.<br>> > Schmidt clearly did not merge the locks, and the Java version does.<br>> > The Java version also has its own simple and not-terrible critical section.<br>> > The Java version also disappeared in newer releases and I didn't track down what happened to it.<br>> ><br>> > - Jay<br>> ><br>> ><br>> > ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------!<br>> ---<br>> > From: jay.krell@cornell.edu<br>> > To: rodney.m.bates@acm.org<br>> > CC: m3devel@elegosoft.com<br>> > Subject: RE: [M3devel] purported condition variable problems on Win32?<br>> > Date: Thu, 21 Jul 2016 06:35:41 +0000<br>> ><br>> > Am I understanding this?<br>> > There are some waiters.<br>> > There is a broadcast ("wake all"). Some waiters are woken.<br>> > There are more waiters.<br>> > There is a signal ("wake one").<br>> > There is a chance that some of the original waiters will remain waiting while the later waiters are not.<br>> > That is, the signaled threads kind of steal the wake intended by the broadcast.<br>> ><br>> ><br>> > Is this unfairness or incorrectness?<br>> > It feels like unfairness.<br>> > It feels like condition variables are specified fairly weakly.<br>> ><br>> ><br>> > - Jay<br>> ><br>> ><br>> > > Date: Wed, 20 Jul 2016 11:43:27 -0500<br>> > > From: rodney_bates@lcwb.coop<br>> > > To: jay.krell@cornell.edu; rodney.m.bates@acm.org<br>> > > CC: m3devel@elegosoft.com<br>> > > Subject: Re: [M3devel] purported condition variable problems on Win32?<br>> > ><br>> > > In looking at ThreadWin32.m3 more, I think I see a significant bug.<br>> > ><br>> > > This gets confusing, since there are Modula-3 waits, signals,<br>> > > etc. and similar concepts in Win32, but which are not the<br>> > > same. I am going the try to put "M3-" and "Win-" prefixes on<br>> > > things to make this clearer. I cartainly need to do this<br>> > > for my own benefit, if for nobody else.<br>> > ><br>> > > Suppose two threads are both M3-waiting in Wait(m,c,...), further<br>> > > Win-waiting on c.waitEvent in WaitForMultipleObjects at :301.<br>> > > c.tickets = 0, and c.waiters = 2.<br>> > ><br>> > > Now, (M3-)Signal (c) happens. This does (Win-)SetEvent(c.waitEvent),<br>> > > which will allow both the waiters to Win-wakeup sometime soon.<br>> > > Before releasing c.lock, Signal increments c.tickets to 1 (supposedly<br>> > > to eventually allow only one waiter to M3-wakeup) and increments<br>> > > c.counter, setting things up so that the waitDone expression at :308<br>> > > will be TRUE for the waiting threads, the next time they evaluate it.<br>> > ><br>> > > Each waiter in turn Win-wakes, gets c.lock, sets waitDone TRUE,<br>> > > releases c.lock, and tries to acquire m. The problem arises<br>> > > because the second waiter can get to these steps before the first<br>> > > can acquire m, acquire c.lock, and DEC(c.tickets) at :322. The<br>> > > second to reach :308 will find c.tickets # 0, as did the first,<br>> > > and proceed.<br>> > ><br>> > > Each will eventually, do a M3-wake. If this were Posix condition<br>> > > variables, this would be wrong, because Posix says only one<br>> > > waiter proceeds. For M3, it's OK, if unnecessary, because M3<br>> > > says one or more waiters proceed. However, each will, in the<br>> > > process, get to :322, and DEC(c.tickets). Signal only provided<br>> > > one ticket, but the waiters have taken two. c.tickets goes<br>> > > negative.<br>> > ><br>> > > After this, when some thread can do another (M3-)Wait, and some<br>> > > other does another (M3-)Signal, c.tickets increments only back to<br>> > > zero, which means the new waiter will not M3-wake, even though it<br>> > > should.<br>> > ><br>> > ><br>> > > I am going to try to construct a test case that will force<br>> > > this scenario.<br>> > ><br>> > > See more below:<br>> > ><br>> > ><br>> > > On 07/18/2016 08:53 PM, Jay wrote:<br>> > > > 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.<br>> > > ><br>> > > > Otherwise previous version I think had a very large lock.<br>> > > ><br>> > > > Changing the < to !=, is that really ok? I get that 32bit overflow is possible.<br>> > > ><br>> > > > 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.<br>> > > ><br>> > > > I guess you are saying it would be unfair, which is ok.<br>> > > ><br>> > > > 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.<br>> > > ><br>> > > > I'll check on the unused event.<br>> > > ><br>> > > > - Jay<br>> > > ><br>> > > >> On Jul 18, 2016, at 7:25 AM, "Rodney M. Bates" <rodney_bates@lcwb.coop> wrote:<br>> > > >><br>> > > >><br>> > > >><br>> > > >><br>> > > >><br>> > > >> I spent some time looking over the signal part of ThreadWin32.m3. A few<br>> > > >> comments:<br>> > > >><br>> > > >> This looks like a very direct implementation of Schmidts' generation count<br>> > > >> algorithm, with a number of important little details additionally taken care<br>> > > >> of.<br>> > > >><br>> > > >> On small difference is at line 308 (B, below) where Schmidt does<br>> > > >> c.counter>count instead of #. Usually, this would make no difference,<br>> > > >> since counter only increases, but on a long-running program, it could<br>> > > >> overflow. If this happened,as it often does, as silent wrap-around to<br>> > > >> FIRST(INTEGER), some threads could be trapped for an extremely long<br>> > > >> time, waiting for counter come back around greater than their count. As<br>> > > >> coded in ThreadWin32, they would be allowed to proceed normally.<br>> > > >><br>> > > >> The only glitch would be that, if there were simultaneously waiting threads<br>> > > >> separated by NUMBER(INTEGER) truly different generations (but the same value<br>> > > >> of c.counter), the recent ones could unfairly proceed along with the<br>> > > >> extremely old ones. That seems extremely less likely than the<br>> > > >> mere existence of an overflow.<br>> > > >><br>> > > >> Both type Condition and type Activation have a field waitEvent: HANDLE.<br>> > > >> Condition.waitEvent is extensively used, but Activation.waitEvent is<br>> > > >> only created at :537 and deleted at :554, but never used in any real<br>> > > >> way. It could be deleted.<br>> > > >><br>> > > >> As for this code:<br>> > > >><br>> > > >> EnterCriticalSection(conditionLock);<br>> > > >><br>> > > >> (* Capture the value of the counter before we start waiting.<br>> > > >> * We will not stop waiting until the counter changes.<br>> > > >> * That is, we will not stop waiting until a signal<br>> > > >> * comes in after we start waiting.<br>> > > >> *)<br>> > > >><br>> > > >> count := c.counter;<br>> > > >> INC(c.waiters);<br>> > > >><br>> > > >> LeaveCriticalSection(conditionLock);<br>> > > >> m.release(); (* can this be moved to before taking conditionLock? *)<br>> > > >><br>> > > >> No, it is important to sample and save c.counter in the order threads wait.<br>> > > >> Retaining m until this is done ensures this. Otherwise, some other thread<br>> > > >> that waited later could have altered c.counter before this one gets conditionLock<br>> > > >> and saves its copy of c.counter.<br>> > > >><br>> > > >> I have made several comment changes to ThreadWin32.m3 that would have made<br>> > > >> it easier to vet. I will commit these, but only comments. It is either<br>> > > >> very difficult or impossible for me to test or even recompile this module,<br>> > > >> and, especially as fragile as this kind of code is, I don't want to commit<br>> > > >> any substantive changes untested and uncompiled.<br>> > > >><br>> > > >> More below:<br>> > > >><br>> > > >>> On 07/07/2016 04:24 AM, Jay K wrote:<br>> > > >>> So...I do NOT understand all of this.<br>> > > >>><br>> > > >>><br>> > > >>> However, comparing the three implementations<br>> > > >>> and attempting to understand ours,<br>> > > >>> I am struck by the following<br>> > > >>><br>> > > >>> - Our broadcast seems ok, but<br>> > > >>> - our signal seems to wake everyone,<br>> > > >>> and then...they race a bit,<br>> > > >>> one will decide it is last, and<br>> > > >>> reset the event, but every prior waiter<br>> > > >>> is still woken.<br>> > > >>> Why not merge the following chunks:<br>> > > >><br>> > > >> I don't think this will work. It is important that, once a thread has decided,<br>> > > >> at A, that it can fairly proceed, it is guaranteed to be the next thread to proceed<br>> > > >> (i.e, to return from [Alert]Wait), before it decrements c.tickets at C, and, thus,<br>> > > >> if it is also the last of its generation to proceed, resets c.waitevent. This<br>> > > >> is ensured by its having already reacquired m first, preventing any other thread<br>> > > >> from returning from its Wait yet.<br>> > ><br>> > > I was wrong about this. There is already no fairness being enforced here. Even with<br>> > > only one ticked, all waiters Win-wake at :301, the order they get c.lock at :307 is<br>> > > arbitrary anyway. There is no benefit in trying to control the order they reacquire m.<br>> > > c.counter has nothing to do with fairness. What it does is prevent Win-awakened waiters<br>> > > that waited after the most recent Signal/Broadcast from M3-waking, a correctness issue.<br>> > ><br>> > > Merging, as you proposed, is probably close to the fix for this bug.<br>> > ><br>> > ><br>> > > >><br>> > > >> And it can't attempt to acquire m while already holding conditionLock, because this would<br>> > > >> invite deadlock, since, elsewhere, a thread can try to acquire conditionLock while<br>> > > >> already holding m, at line 265.<br>> > > >><br>> > > >>><br>> > > >>><br>> > > >>> WHILE (NOT alerted) AND (NOT waitDone) DO<br>> > > >>> .<br>> > > >>> .<br>> > > >>> .<br>> > > >>> 1<br>> > > >>> EnterCriticalSection(conditionLock);<br>> > > >>> waitDone := (c.tickets # 0 AND c.counter # count);<br>> > > >> A:---------^<br>> > > >> B:--------------------------------------------------^<br>> > > >>> LeaveCriticalSection(conditionLock);<br>> > > >>> END; (* WHILE *)<br>> > > >>> IF waitDone THEN<br>> > > >>> alerted := FALSE;<br>> > > >>> END;<br>> > > >>> m.acquire();<br>> > > >>> 2<br>> > > >>> EnterCriticalSection(conditionLock);<br>> > > >>> DEC(c.waiters);<br>> > > >>> IF waitDone THEN esp. here.<br>> > > >>> DEC(c.tickets);<br>> > > >><br>> > > >> C:---------------^<br>> > > >>> lastWaiter := (c.tickets = 0);<br>> > > >>> END;<br>> > > >>> LeaveCriticalSection(conditionLock);<br>> > > >>><br>> > > >>><br>> > > >>> That is, if we decrement tickets earlier<br>> > > >>> within the first critical section,<br>> > > >>> while we will still wake everyone,<br>> > > >>> only one will decide waitDone and the rest will keep looping.<br>> > > >>> A downside of this, perhaps, is that waking all for Broadcast<br>> > > >>> might be a little slower.<br>> > > >>> but more so, in both the "ptw32" (pthreads for win32) and Boost threads<br>> > > >>> implementations, they seem to deal with this differently than us,<br>> > > >>> and they each do about the same thing -- they use a counted semaphore.<br>> > > >>> In the boost case, it appears they duplicate the semaphore for every<br>> > > >>> notification generation, which seems expensive.<br>> > > >>><br>> > > >>> Perhaps if they can guarantee some lifetimes, they don't need to duplicate it.<br>> > > >>> Or they can do their own reference counting?<br>> > > >>> jdk7 seems to looke like jdk6.<br>> > > >>> The code is gone in jdk8 and I can't easily find the delete in history.<br>> > > >>> The lock merging jdk does probably helps here too.<br>> > > >>> In fact they merge #1 and #2 above as a result.<br>> > > >>> But they still initially wake all threads for signal, not just broadcast.<br>> > > >>> There is also the problem that all the event waits<br>> > > >>> are followed by EnterCriticalSection (or jdk facsimile).<br>> > > >>> - Jay<br>> > > >>><br>> > > >>><br>> > > >>><br>> > > >>><br>> > > >>> --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------!<br>> -----!<br>> > > --!<br>> > > >> ---<br>> > > >>> From: jay.krell@cornell.edu<br>> > > >>> To: m3devel@elegosoft.com<br>> > > >>> Subject: RE: [M3devel] purported condition variable problems on Win32?<br>> > > >>> Date: Tue, 5 Jul 2016 08:26:36 +0000<br>> > > >>><br>> > > >>> Here is another implementation to consider:<br>> > > >>><br>> > > >>> https://github.com/boostorg/thread/blob/develop/include/boost/thread/win32/condition_variable.hpp<br>> > > >>><br>> > > >>> - Jay<br>> > > >>><br>> > > >>><br>> > > >>> --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------!<br>> -----!<br>> > > --!<br>> > > >> ---<br>> > > >>> From: jay.krell@cornell.edu<br>> > > >>> To: m3devel@elegosoft.com<br>> > > >>> Date: Tue, 5 Jul 2016 08:19:23 +0000<br>> > > >>> Subject: [M3devel] purported condition variable problems on Win32?<br>> > > >>><br>> > > >>> https://sourceforge.net/p/pthreads4w/code/ci/master/tree/README.CV<br>> > > >>><br>> > > >>> vs.<br>> > > >>><br>> > > >>> http://www.cs.wustl.edu/~schmidt/win32-cv-1.html<br>> > > >>> vs.<br>> > > >>> 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<br>> > > >>> I wrote this:<br>> > > >>> PROCEDURE XWait(m: Mutex; c: Condition; act: Activation;<br>> > > >>> alertable: BOOLEAN) RAISES {Alerted} =<br>> > > >>> (* LL = m on entry and exit, but not for the duration<br>> > > >>> * see C:\src\jdk-6u14-ea-src-b05-jrl-23_apr_2009\hotspot\agent\src\os\win32\Monitor.cpp<br>> > > >>> * NOTE that they merge the user lock and the condition lock.<br>> > > >>> * http://www.cs.wustl.edu/~schmidt/win32-cv-1.html<br>> > > >>> * "3.3. The Generation Count Solution"<br>> > > >>> *)<br>> > > >>><br>> > > >>> Do we have the problems described in README.CV?<br>> > > >>><br>> > > >>> I haven't looked through the ACE code to see<br>> > > >>> to what extent they resemble solution 3.3, or if they<br>> > > >>> changed as a result of this discussion -- which I admit I don't understand<br>> > > >>> and haven't read closely.<br>> > > >>><br>> > > >>><br>> > > >>> Spurious wakeups are ok, though should be minimized.<br>> > > >>><br>> > > >>> I'd still rather not drop pre-Vista support but I realize it becomes more interesting as time advances.<br>> > > >>><br>> > > >>> Thank you,<br>> > > >>> - Jay<br>> > > >>><br>> > > >>> _______________________________________________ M3devel mailing list M3devel@elegosoft.com https://m3lists.elegosoft.com/mailman/listinfo/m3devel<br>> > > >>><br>> > > >>><br>> > > >>> _______________________________________________<br>> > > >>> M3devel mailing list<br>> > > >>> M3devel@elegosoft.com<br>> > > >>> https://m3lists.elegosoft.com/mailman/listinfo/m3devel<br>> > > >><br>> > > >> --<br>> > > >> Rodney Bates<br>> > > >> rodney.m.bates@acm.org<br>> > > >><br>> > > >><br>> > > ><br>> > ><br>> > > --<br>> > > Rodney Bates<br>> > > rodney.m.bates@acm.org<br>> <br>> -- <br>> Rodney Bates<br>> rodney.m.bates@acm.org<br></div></div> </div></body>
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