rcu: Expedite deboost in case of deferred quiescent state

rcu: Expedite deboost in case of deferred quiescent state

Historically, a task that has been subjected to RCU priority boosting is
deboosted at rcu_read_unlock() time.  However, with the advent of deferred
quiescent states, if the outermost rcu_read_unlock() was invoked with
either bottom halves, interrupts, or preemption disabled, the deboosting
will be delayed for some time.  During this time, a low-priority process
might be incorrectly running at a high real-time priority level.

Fortunately, rcu_read_unlock_special() already provides mechanisms for
forcing a minimal deferral of quiescent states, at least for kernels
built with CONFIG_IRQ_WORK=y.  These mechanisms are currently used
when expedited grace periods are pending that might be blocked by the
current task.  This commit therefore causes those mechanisms to also be
used in cases where the current task has been or might soon be subjected
to RCU priority boosting.  Note that this applies to all kernels built
with CONFIG_RCU_BOOST=y, regardless of whether or not they are also
built with CONFIG_PREEMPT_RT=y.

This approach assumes that kernels build for use with aggressive real-time
applications are built with CONFIG_IRQ_WORK=y.  It is likely to be far
simpler to enable CONFIG_IRQ_WORK=y than to implement a fast-deboosting
scheme that works correctly in its absence.

While in the area, alphabetize the rcu_preempt_deferred_qs_handler()
function's local variables.

Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Scott Wood <swood@redhat.com>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>