local_t Documentation update

typedef struct { atomic_long_t a; } local_t;

* Rules to follow when using local atomic operations

- Variables touched by local ops must be per cpu variables.

- _Only_ the CPU owner of these variables must write to them.

- This CPU can use local ops from any context (process, irq, softirq, nmi, ...)

  to update its local_t variables.

- Preemption (or interrupts) must be disabled when using local ops in

  process context to   make sure the process won't be migrated to a

  different CPU between getting the per-cpu variable and doing the

  actual local op.

- When using local ops in interrupt context, no special care must be

  taken on a mainline kernel, since they will run on the local CPU with

  preemption already disabled. I suggest, however, to explicitly

  disable preemption anyway to make sure it will still work correctly on

  -rt kernels.

- Reading the local cpu variable will provide the current copy of the

  variable.

- Reads of these variables can be done from any CPU, because updates to

  "long", aligned, variables are always atomic. Since no memory

  synchronization is done by the writer CPU, an outdated copy of the

  variable can be read when reading some _other_ cpu's variables.

* Rules to follow when using local atomic operations

- Variables touched by local ops must be per cpu variables.