rcu: add documentation saying which RCU flavor to choose

See the comment headers in the source code (or the docbook generated

from them) for more information.

However, given that there are no fewer than four families of RCU APIs

in the Linux kernel, how do you choose which one to use?  The following

list can be helpful:

a.	Will readers need to block?  If so, you need SRCU.

b.	What about the -rt patchset?  If readers would need to block

	in an non-rt kernel, you need SRCU.  If readers would block

	in a -rt kernel, but not in a non-rt kernel, SRCU is not

	necessary.

c.	Do you need to treat NMI handlers, hardirq handlers,

	and code segments with preemption disabled (whether

	via preempt_disable(), local_irq_save(), local_bh_disable(),

	or some other mechanism) as if they were explicit RCU readers?

	If so, you need RCU-sched.

d.	Do you need RCU grace periods to complete even in the face

	of softirq monopolization of one or more of the CPUs?  For

	example, is your code subject to network-based denial-of-service

	attacks?  If so, you need RCU-bh.

e.	Is your workload too update-intensive for normal use of

	RCU, but inappropriate for other synchronization mechanisms?

	If so, consider SLAB_DESTROY_BY_RCU.  But please be careful!

f.	Otherwise, use RCU.

Of course, this all assumes that you have determined that RCU is in fact

the right tool for your job.

8.  ANSWERS TO QUICK QUIZZES