Re: [Threads-devel] Request Help On boost::lock_error

Subject: Re: [Threads-devel] Request Help On boost::lock_error
From: Ashutosh Warikoo (awarikoo_at_[hidden])
Date: 2010-09-15 00:20:52

Hello Vicente

>Why do you use a shared mutex if you lock it exclusively in firstFunction 
and secondFunction? 
The myLock is defined as an upgrade_lock in the class definition. 
Executing a lock() operation on a upgrade_lock actually takes upgrade 
ownership & not exclusive ownership. So I am not locking it exclusively in 
the first function only locking it "upgradely" (sic). Here is the 
definition of the lock() function.
locks.hpp
00846         void lock()
00847         {
00848             if(owns_lock())
00849             {
00850                 boost::throw_exception(boost::lock_error());
00851             }
00852             m->lock_upgrade();
00853             is_locked=true;
00854         }

>upgrade_to_unique_lock has a sens only if the mutext has been 
lock_shared.
upgrade_to_unique_lock has no relation with shared_lock. However it does 
use the shared_mutex as core entity. If a lock has to be upgraded it has 
to be of type upgrade_lock & not of shared_lock which is what I need. 
Before entering a transaction I have no idea if I would need to update the 
variable. In addition upgrade_to_unique_lock takes input a upgrade_lock & 
not a shared_lock.

>Shouldn't you user lock_shared()/unlock_shared() in firstFunction?
If you notice, the lock is a upgrade_lock & not a shared_lock. So I don't 
have the functions lock_shared()/unlock_shared(); as its not a 
shared_lock.

Anyway I am posting the code again for brevity
#include <iostream>
#include <boost/thread.hpp>
class boostThreadLocksTest
{
        public:
                boost::shared_mutex myMutex;
                boost::upgrade_lock<boost::shared_mutex> myLock;
                static int firstFunction(boostThreadLocksTest 
*pBoostThreadLocksTest);
                static int secondFunction(boostThreadLocksTest 
*pBoostThreadLocksTest);
 boostThreadLocksTest():myMutex(),myLock(myMutex,boost::defer_lock_t()){};
};
int boostThreadLocksTest::firstFunction(boostThreadLocksTest 
*pBoostThreadLocksTest)
{
        std::cout<<"Before Locking "<<boost::this_thread::get_id()<<" 
"<<__PRETTY_FUNCTION__<<std::endl;
        pBoostThreadLocksTest->myLock.lock();
        std::cout<<"After Locking "<<boost::this_thread::get_id()<<" 
"<<__PRETTY_FUNCTION__<<std::endl;
        pBoostThreadLocksTest->secondFunction(pBoostThreadLocksTest);
        std::cout<<"Returned From Call "<<boost::this_thread::get_id()<<" 
"<<__PRETTY_FUNCTION__<<std::endl;
        pBoostThreadLocksTest->myLock.unlock();
        std::cout<<"After Unlocking "<<boost::this_thread::get_id()<<" 
"<<__PRETTY_FUNCTION__<<std::endl;
        return(0);
}
int boostThreadLocksTest::secondFunction(boostThreadLocksTest 
*pBoostThreadLocksTest)
{
        std::cout<<"Before Exclusive Locking 
"<<boost::this_thread::get_id()<<" "<<__PRETTY_FUNCTION__<<std::endl;
        boost::upgrade_to_unique_lock<boost::shared_mutex> 
localUniqueLock(pBoostThreadLocksTest->myLock);
        std::cout<<"After Exclusive Locking 
"<<boost::this_thread::get_id()<<" "<<__PRETTY_FUNCTION__<<std::endl;
        return(0);
}
int main()
{
        boostThreadLocksTest myObject;
        boost::thread_group myThreadGroup;
 
myThreadGroup.create_thread(boost::bind(boostThreadLocksTest::firstFunction,&myObject));
 
myThreadGroup.create_thread(boost::bind(boostThreadLocksTest::firstFunction,&myObject));
 
myThreadGroup.create_thread(boost::bind(boostThreadLocksTest::firstFunction,&myObject));
        myThreadGroup.join_all();
}

Thanks
bornlibra23

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