问题
There doesn't appear to be synchronization between establishing/removing callbacks (e.g. kauth_unlisten_scope) and the callbacks themselves (in the xnu codebase, yes, I know, it's dated). This puts the burden of tracking/draining callbacks and synchronizing with calls on the extension itself. But this is problematic as well in that there is a window in noting that a thread has exited the callback AND actually returning out of the extension code.
Is there any pattern that gives a correct avoidance of this race? Or, is there any documentation from Apple that indicates they've synchronized this correctly?
回答1:
As far as I'm aware there's no 100% reliable way of preventing kauth callback deregistration race conditions; the API is just badly designed. Apple themselves implement/recommend a simple atomic counter based mechanism, which you can see in the Kauth-O-Rama example. Search for gActivationCount in the KauthORama.c source file. There's still a small chance that a thread is running code before the increment or after the decrement in the callback, but I have never seen a crash caused by this.
来源：https://stackoverflow.com/questions/38084755/race-callbacks-and-removing-callbacks-during-unload-of-kext-in-osx