rcu/tree: Make rcu_do_batch count how many callbacks were executed

The rcu_do_batch() function extracts the ready-to-invoke callbacks from the rcu_segcblist located in the ->cblist field of the current CPU's rcu_data structure. These callbacks are first moved to a local (unsegmented) rcu_cblist. The rcu_do_batch() function then uses this rcu_cblist's ->len field to count how many CBs it has invoked, but it does so by counting that field down from zero. Finally, this function negates the value in this ->len field (resulting in a positive number) and subtracts the result from the ->len field of the current CPU's ->cblist field. Except that it is sometimes necessary for rcu_do_batch() to stop invoking callbacks mid-stream, despite there being more ready to invoke, for example, if a high-priority task wakes up. In this case the remaining not-yet-invoked callbacks are requeued back onto the CPU's ->cblist, but remain in the ready-to-invoke segment of that list. As above, the negative of the local rcu_cblist's ->len field is still subtracted from the ->len field of the current CPU's ->cblist field. The design of counting down from 0 is confusing and error-prone, plus use of a positive count will make it easier to provide a uniform and consistent API to deal with the per-segment counts that are added later in this series. For example, rcu_segcblist_extract_done_cbs() can unconditionally populate the resulting unsegmented list's ->len field during extraction. This commit therefore explicitly counts how many callbacks were executed in rcu_do_batch() itself, counting up from zero, and then uses that to update the per-CPU segcb list's ->len field, without relying on the downcounting of rcl->len from zero.

JFJoel Fernandes (Google) committed 6 years ago