[$] The seqcount latch lock type

The kernel contains a wide variety of locking primitives; it can be hard tostay on top of all of them. So even veteran kernel developers might beforgiven for being unaware of the "seqcount latch" lock type or its use.While this lock type has existed in the kernel for several years, it isonly being formalized with a proper type declaration in 5.10. So thisseems like a good time to look at what these locks are and howthey work.