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Re: Locks, cache coherency, @synchronized, double-checked locking, and lots and lots of doubts
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Re: Locks, cache coherency, @synchronized, double-checked locking, and lots and lots of doubts

On Nov 29, 2006, at 12:46 AM, leenoori wrote:
So I accept that that point too, but I am struggling to understand it, and as a result I am doubting that even the slow (always-lock) pattern will work:

+ (void)onceOnlyInit
{
    static BOOL initialized = NO;
    @synchronized (thing)
    {
        if (!initialized)
        {
            PerformInitialization();
            initialized = YES;
        }
    }
}

This code is thread-safe. @synchronized uses pthread mutexes internally. pthread mutexes include any necessary barriers. Think of it this way:

+ (void)onceOnlyInit
{
    static BOOL initialized = NO;
    {
        pthread_mutex_lock(thing_lock);
        exception_safety_magic();
        if (!initialized)
        {
            PerformInitialization();
            initialized = YES;
        }
        pthread_mutex_unlock(thing_lock);
    }
}


Jens Ayton wrote:

It is my understanding that the compiler handles this by implicitly marking variables used both inside and outside a @synchronized() block as volatile. (In fact, I think it makes *all* variables in any function/method containing a @synchronized() block volatile, but don't quote me on this.)

To a first approximation, `volatile` never fixes thread-safety problems. @try and @synchronized use `volatile` to make sure variables have correct values if an exception is thrown.


--
Greg Parker     email@hidden     Runtime Wrangler

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 >Locks, cache coherency, @synchronized, double-checked locking,	and lots and lots of doubts (From: leenoori <email@hidden>)























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