Re: [Boost-users] mutexes : Is this thread-safe ?

From: Kevin Heifner (heifner_k_at_[hidden])
Date: 2006-02-17 11:21:14

Tom Widmer wrote:
> Dhanvi Kapila wrote:
>> class BoostMutexLock
>> {
>>    public :
>>         void lock() {
>>                lk = new boost::mutex::scoped_lock(global_mutex);
>>        }
>>         void unlock() {
>>               delete lk;
>>         }
>>   BoostMutexLock() : lk( 0 ) { }
>>   ~BoostMutexLock() { }
>>
>> private:
>>       boost::mutex global_mutex;
>>       boost::mutex::scoped_lock* lk;
>> };
>> ------------------------------------------------------

[snip]

> It will be threadsafe though, yes, since the scoped lock constructor 
> will have to lock global_mutex before assigning to lk. If two threads on 
> two CPUs run lock at the same time, they will create separate, 
> independent scoped_lock objects, and one will get the lock and unlock it 
> before the other one gets to assign lk, so there's no problem. It's 
> inefficient though due to the memory allocation - why do you want it?

I don't think it is threadsafe on every platform.  It it my 
understanding that the following line:
   lk = new boost::mutex::scoped_lock(global_mutex);
can be split by the compiler to be:
   lk = malloc(sizeof(boost::mutex::scoped_lock)); // alloc and 
assign
   // another thread could come in here, causing
   // leak of the first malloc and a later double
   // delete
   lk->boost::mutex::scoped_lock(global_mutex); // call constructor

See, for example:
http://www.cs.umd.edu/~pugh/java/memoryModel/DoubleCheckedLocking.html
for more info.

KevinH
-- 
Kevin Heifner  heifner @ ociweb.com  http://heifner.blogspot.com
           Object Computing, Inc. (OCI) www.ociweb.com