July 28, 2007
tryLock does not seem to work as I expected.  I thought it returns if it was successful or not.  What could possibly cause the code below to fail?  I'm using linux.

myMutex.tryLock();
assert(myMutex.tryLock() == false); // fails
July 29, 2007
Jason House wrote:
> tryLock does not seem to work as I expected.  I thought it returns if it was successful or not.  What could possibly cause the code below to fail?  I'm using linux.
> 
> myMutex.tryLock();
> assert(myMutex.tryLock() == false); // fails

Locks in Tango are recursive, so you're guaranteed to succeed if you already hold the lock.


Sean
July 29, 2007
Sean Kelly wrote:
> Jason House wrote:
>> tryLock does not seem to work as I expected.  I thought it returns if it was successful or not.  What could possibly cause the code below to fail?  I'm using linux.
>>
>> myMutex.tryLock();
>> assert(myMutex.tryLock() == false); // fails
> 
> Locks in Tango are recursive, so you're guaranteed to succeed if you already hold the lock.
> 
> 
> Sean

Does that mean that the mutex locking and unlocking calls are thread aware?  A given thread can repeatedly lock a mutex but must then unlock it the same number of times before the mutex is released for another thread to access it?

I was trying to be creative with some queue structures.  I wanted to add a blocking version of pop.  If nothing is in the queue, wait for that to change...  I was thinking that if I had a mutex that was automatically locked when the queue was empty, then I could implement blocking by quickly grabbing, then releasing a mutex.  For an empty queue, the lock would sit and wait.

It sounds like I can't use tango's Mutex class for this purpose because the push's and pop's are done by different threads.  In fact, push calls are done by lots of threads.  Is there anything in tango.core.sync that I should be using for this purpose?
July 29, 2007
Jason House wrote:
> Sean Kelly wrote:
>> Jason House wrote:
>>> tryLock does not seem to work as I expected.  I thought it returns if it was successful or not.  What could possibly cause the code below to fail?  I'm using linux.
>>>
>>> myMutex.tryLock();
>>> assert(myMutex.tryLock() == false); // fails
>>
>> Locks in Tango are recursive, so you're guaranteed to succeed if you already hold the lock.
>>
>>
>> Sean
> 
> Does that mean that the mutex locking and unlocking calls are thread aware?  A given thread can repeatedly lock a mutex but must then unlock it the same number of times before the mutex is released for another thread to access it?
> 
> I was trying to be creative with some queue structures.  I wanted to add a blocking version of pop.  If nothing is in the queue, wait for that to change...  I was thinking that if I had a mutex that was automatically locked when the queue was empty, then I could implement blocking by quickly grabbing, then releasing a mutex.  For an empty queue, the lock would sit and wait.
> 
> It sounds like I can't use tango's Mutex class for this purpose because the push's and pop's are done by different threads.  In fact, push calls are done by lots of threads.  Is there anything in tango.core.sync that I should be using for this purpose?

Aside from D, many system APIs for mutexes require that locking and unlocking be done by the same thread.  Some will allow it to work across threads, and on some it will tend to work across threads until you turn on debugging options which will alert you to the potential problem.

If D aims to be somewhat portable, it likely won't want to support designs which lock a mutex in one thread and then unlock it from another.

-- James
July 29, 2007
Jason House wrote:
> Sean Kelly wrote:
>> Jason House wrote:
>>> tryLock does not seem to work as I expected.  I thought it returns if it was successful or not.  What could possibly cause the code below to fail?  I'm using linux.
>>>
>>> myMutex.tryLock();
>>> assert(myMutex.tryLock() == false); // fails
>>
>> Locks in Tango are recursive, so you're guaranteed to succeed if you already hold the lock.
> 
> Does that mean that the mutex locking and unlocking calls are thread aware?  A given thread can repeatedly lock a mutex but must then unlock it the same number of times before the mutex is released for another thread to access it?

Yes.  And for what it's worth, this is largely an artifact of the OS implementations.  Windows mutexes can only be recursive, though the feature must be enabled on some POSIX implementations.  So Tango uses recursive mutexes largely for consistency, though I personally find them to be far more useful anyway (D 'synchronized' blocks are recursive as well).

> I was trying to be creative with some queue structures.  I wanted to add a blocking version of pop.  If nothing is in the queue, wait for that to change...  I was thinking that if I had a mutex that was automatically locked when the queue was empty, then I could implement blocking by quickly grabbing, then releasing a mutex.  For an empty queue, the lock would sit and wait.
> 
> It sounds like I can't use tango's Mutex class for this purpose because the push's and pop's are done by different threads.  In fact, push calls are done by lots of threads.  Is there anything in tango.core.sync that I should be using for this purpose?

You should look at Condition.  Use is something like this:

auto myMutex = new Mutex;
auto myCondition = new Condition( myMutex );

Thread A (producer):

synchronized( myMutex ) {
    myQueue.push( data );
    myCond.notify();
}

Thread B (consumer):

synchronized( myMutex ) {
    while( myQueue.isEmpty )
        myCond.wait();
}

In essence, Conditions are associated with a specific mutex, which is atomically unlocked when wait() is called.  Thus, when thread B waits it allows thread A to enter the protected region to add more data to the queue.  When wait unblocks it atomically acquires the mutex again, by which time thread A will have exited the protected region (earlier implementations actually blocked thread A and simply transferred control--this was indicated by 'signal' rather than 'notify').


Sean
July 29, 2007
Sean Kelly wrote:
> You should look at Condition.  Use is something like this:
> 
> auto myMutex = new Mutex;
> auto myCondition = new Condition( myMutex );
> 
> Thread A (producer):
> 
> synchronized( myMutex ) {
>     myQueue.push( data );
>     myCond.notify();
> }
> 
> Thread B (consumer):
> 
> synchronized( myMutex ) {
>     while( myQueue.isEmpty )
>         myCond.wait();
> }


Correct me if I'm wrong, but the Mutex operation of tango does not do strange interaction with the synchronized keyword?  Wouldn't Thread B's "synchrnoized( myMutex )" cause Thread A to be unable to enter its "synchronized( myMutex )"?


> 
> In essence, Conditions are associated with a specific mutex, which is atomically unlocked when wait() is called.  Thus, when thread B waits it allows thread A to enter the protected region to add more data to the queue.  When wait unblocks it atomically acquires the mutex again, by which time thread A will have exited the protected region (earlier implementations actually blocked thread A and simply transferred control--this was indicated by 'signal' rather than 'notify').
> 
> 
> Sean
July 30, 2007
Jason House wrote:
> Sean Kelly wrote:
>> You should look at Condition.  Use is something like this:
>>
>> auto myMutex = new Mutex;
>> auto myCondition = new Condition( myMutex );
>>
>> Thread A (producer):
>>
>> synchronized( myMutex ) {
>>     myQueue.push( data );
>>     myCond.notify();
>> }
>>
>> Thread B (consumer):
>>
>> synchronized( myMutex ) {
>>     while( myQueue.isEmpty )
>>         myCond.wait();
>> }
> 
> 
> Correct me if I'm wrong, but the Mutex operation of tango does not do strange interaction with the synchronized keyword?  Wouldn't Thread B's "synchrnoized( myMutex )" cause Thread A to be unable to enter its "synchronized( myMutex )"?

Normally, yes.  But myCond.wait() unlocks the mutex, allowing Thread A to enter.  When wait returns, the lock is re-acquired.

>> In essence, Conditions are associated with a specific mutex, which is atomically unlocked when wait() is called.  Thus, when thread B waits it allows thread A to enter the protected region to add more data to the queue.  When wait unblocks it atomically acquires the mutex again, by which time thread A will have exited the protected region (earlier implementations actually blocked thread A and simply transferred control--this was indicated by 'signal' rather than 'notify').


Sean
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