Re: Threads and synchronization

FROM : John Stiles
DATE : Thu Aug 10 17:36:33 2006

Cem Karan wrote:
>> 1. Start a periodic timer in your main thread and use a shared
>> variable to store progress information. You protect your shared
>> variables using @synchronized or NSLock objects if necessary (if it's
>> just a 32 bit integer storing progress that's not necessary).

>
> Please use locks or @synchronized on everything that's shared, unless
> you are utterly certain that there is no way that corruption of the
> shared data can ever affect you.  I've seen some very, very strange
> problems happen because someone decided that not locking was OK 'just
> this once' which can be easily avoided by locking/synchronizing.
>
> And before you say that it's OK because it is just a 32 bit integer,
> yes, I HAVE seen weirdness that way too; I'll admit, it was on a
> misaligned data access (part of a packed struct), but still, it CAN
> bite you!
>

It's possible even with aligned stuff. Every time you do "x += y", the
generated code reads the value of x out of RAM into a register, modifies
it, and then writes it back. There is a brief window of opportunity
where some other thread could overwrite its value while you have it in a
register, and then your thread will clobber that write.

This issue may be more complicated on Intel where there are
"direct-to-memory" operations, but the basic principle is still
applicable; any time you have the value in a register and you're
performing operations on it, you are vulnerable.