Avoiding deadlocks

The recommended order for locking very important objects is getting a handler or an update lock first, if required, followed by read locks or write locks. This is not the only ordering that's possible, but this is the recommended order.

The following example demonstrates a common pitfall:

vip_t::handlerlock(vip).install(notifier, *vip_t::readlock(vip));

// elsewhere...


This innocent code results in undefined behavior, and subtle deadlocks. This is because the compiler can generate code which acquires the read lock before or after the handler lock, sometimes depending on the compilation options; or even a different order for different occurences of the same code sequence, depending on other code in the same function or method. The compiler is allowed to evaluate the argument to install(), which acquires a read lock, before or after the compiler instantiates the temporary handler lock object.

The end result: one thread gets a read lock on the very important object. At the same time, another thread gets an update lock. Then, the first thread tries to get a handler lock, which gets blocked by the update lock held by the second thread. The second thread invokes update(), which tries to get a write lock, which gets blocked by the read lock held by the first thread. Deadlock.

Always instantiate very important object locks individually, in their individual sequence points, so that the order of lock acquisition is consistent, and is not implementation-defined. Avoid temporary lock objects, which are subject to reordering within their sequence point span.