[M3devel] purported condition variable problems on Win32?
Jay
jay.krell at cornell.edu
Tue Jul 19 03:53:24 CEST 2016
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.
>
> 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
>>
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>
> --
> Rodney Bates
> rodney.m.bates at acm.org
>
>
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