Objective-C Objects and Locking

Objective-C Objects and Locking

[John Richard Moser]

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I'm writing up my own objective-c framework, something to use in place
of OpenStep/Cocoa/etc.  So, I'm in complete control of what happens with
my base object, SVObject (which ironically inherits from Object :).

I'm attempting to implement a swizzling subsystem which allows any
SVObject inheriting object to dynamicly handle swizzle requests safely
and effectively.  Because I want to work in a threaded environment, I
need to lock properly.

What I'm thinking is to use a mutex lock around all members that may not
run during a swizzle and deflect in-swizzle calls to the new swizzle
master.  The current logic I'm pondering is:

- -(void) someMemberThatIsAffected {
~  if ([my_swizzle_lock tryReadLock]) { /*Can we lock?*/
~    /*
~     * We couldn't lock, so we wait for the swizzle lock to become
~     * unlocked, then send the message to whatever swizzled us.
~     */
~    [my_swizzle_lock readLock];
~    [my_swizzle_master someMemberThatIsAffected];
~  }
~  else { /*Or we just complete*/
~    ...
~  }
~  [my_swizzle_lock unlock];
}

A swizzle would -writeLock my_swizzle_lock, so this would detect the lock.

Here's the issue I have:  Looking at this logic, if the entire swizzle
is done between the actual message passing call and the -tryReadLock,
which is quite plausible in an SMP situation, this will fail to detect.
~ Also, the new object may want to pass messages down to this object, so
I can't just feed all messages upwards.

I don't want to abandon the idea of supplying simplistic swizzling;
however, I would have to detect in-progress swizzles reliably.  I can't
think of anything to atomicize, and don't know how to make chunks of
code atomic anyway.

Any thoughts?
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