On Dec 8, 2012, at 10:06 AM, Steve Sisak <sgs-li...@codewell.com> wrote:
> At 8:35 AM -0800 12/8/12, Kyle Sluder wrote: >> On Dec 7, 2012, at 8:38 PM, Marco S Hyman <m...@snafu.org> wrote: >> >>> On Dec 7, 2012, at 8:18 PM, Steve Sisak <sgs-li...@codewell.com> wrote: >>> >>>> I'm interested if there are an any issued I'm missing in the Obj-C, >>>> @synchronized(self), instance variable case. >>> >>> >>> Your pattern can fail if this line >>> _someDictionary = temp; >>> isn't atomic. >> >> The real issue with double-checked locking is whether the compiler promises >> to generate the proper memory barriers such that other threads are >> guaranteed to see the assignment to _someDictionary *after* the object has >> been constructed. C makes no such guarantee; other threads might see a >> non-nil value for _someDictionary before the first thread is done >> constructing the object. > > I'm fairly sure that @synchronized, being a compiler built-in, rather than a > function, makes that guarantee -- specifically the the values of instance > variables can change across entry and exit of an @synchronized block. If you actually understood the problem with double-checked locking, you'd understand that the problem is with the early return that happens before the @synchronized block. Please reread the Wikipedia article Ken linked to; the Java example exhibits *exactly* the same flaw as your Objective-C code. > > Further, if writes were not complete at the end of the block, the construct > would be essentially useless for its intended purpose. Again, the problem exists *before* entering the @synchronized block. > > In any case, removing the outer check makes the code correct regardless of > compiler guarantees. Yes, which is why your "optimization" is a classic anti-pattern. --Kyle Sluder _______________________________________________ Cocoa-dev mailing list (Cocoa-dev@lists.apple.com) Please do not post admin requests or moderator comments to the list. Contact the moderators at cocoa-dev-admins(at)lists.apple.com Help/Unsubscribe/Update your Subscription: https://lists.apple.com/mailman/options/cocoa-dev/archive%40mail-archive.com This email sent to arch...@mail-archive.com
