Re: Using OSMemoryBarrier() with KVO
Re: Using OSMemoryBarrier() with KVO
- Subject: Re: Using OSMemoryBarrier() with KVO
- From: Ken Thomases <email@hidden>
- Date: Tue, 22 Apr 2008 21:41:27 -0500
On Apr 22, 2008, at 6:37 AM, Paul Thomas wrote:
I'm trying to implement a basic promise variable and I'm not sure if
OSMemoryBarrier() is enough to make it correct. If anyone is
familiar with the memory model, I'd appreciate a quick look-over.
It's basically like this:
{
BOOL done;
id result;
NSError* error;
NSException* exception;
}
Users ask for KVO notifications on 'done' and don't attempt to
access any other ivars until done == YES. Then whatever thread or
NSOperation or whatever that is responsible for delivering the
result calls:
-(void)completeWithResult:(id)res
{
result = res;
[self willChangeValueForKey:@"done"];
done = YES;
OSMemoryBarrier();
[self didChangeValueForKey:@"done"];
}
Is this enough? Or will I need to use a full blown lock?
What is it that you fear going wrong, such that you think even
OSMemoryBarrier is necessary?
KVO notifications are delivered on the thread where will/
didChangeValueForKey: are called. There's no multi-threading issue
inherent in the above code, even without OSMemoryBarrier.
-Ken
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