Is the kernel 100% reentrant?

No. It's getting there though. As of this date (12-Sep-96), some of the drivers are still not reentrant. What does this mean? It means that threads making various system calls will block other threads until the call is completed. Will it crash? No. When will the fine-threading (not treating system calls as a single operation) work? That's currently in the works (the 2.1.xx goal is to get the kernel 100% reentrant).

Please note that this does not mean: avoid at all costs. It means that as you work on your threaded code, you will not have all the performance as you might with a fully reentrant kernel. Be patient; we're getting there.