Our world is built on modular, composable software. We routinely expect that we don't need to know about the internal implementation details of a library or plug-in to be able to use it correctly.
The problem is that locks, and most other kinds of synchronization we use today, are not modular or composable. That is, given two separately authored modules, each of which is provably correct but uses a lock or similar synchronization internally, we generally can't combine them and know that the result is still correct and will not deadlock. There are only two ways to combine such a pair of lock-using modules safely:
- Option 1. Each module must know about the complete internal implementation of any function it calls in the other module (generally impossible for code you don't control).
- Option 2. Each module must be careful not to call into the other module while the calling code is inside a critical section (while holding a lock, for example).
Let's examine why Option 2 is generally the only viable choice, and what consequences it has for how we write concurrent code. For convenience, I'm going to cast the problem in terms of locks, but the general case arises whenever:
- The caller is inside one critical section.
- The callee directly or indirectly tries to enter another critical section, or performs another blocking call.
- Some other thread could try to enter the two critical sections in the opposite order.
Quick Recap: Deadlock
A deadlock (or deadly embrace) can happen anytime two different threads can try to acquire the same two locks (or more generally, acquire exclusive use of the resources protected by the same two synchronization objects) in opposite orders. Therefore, anytime you write code where a thread holds one lock L1 and tries to acquire another lock L2, that code is liable to be one arm of a potential deadly embraceunless you can eliminate the possibility that some other thread might try to acquire the locks in the other order.
We can use techniques such as lock hierarchies to guarantee that locks are taken in order. Unfortunately, those techniques do not compose either. For example, you can use a lock hierarchy and guarantee that the code you control follows the discipline, but you can't guarantee that other people's code will know anything about your lock levels, much less follow them correctly.