Race condition?

Race condition?

In other words, the (overwhelmingly) most frequent users of the data structure never need to lock it. Concurrent writers do need to lock, but since writing happens so rarely, collisions requiring one writer to wait are vanishingly rare. Where you can guarantee there can be no writer collisions (e.g. there's only ever one writer, or they're already co-operating), writers don't need to lock.

The more subtle danger is when there is a dependency relationship between the data copied, and other data elsewhere in the kernel (or in user space!). You just have to be very careful never to let that happen.