rcu: Make the TASKS_RCU Kconfig option be selected

Currently, any kernel built with CONFIG_PREEMPTION=y also gets
CONFIG_TASKS_RCU=y, which is not helpful to people trying to build
preemptible kernels of minimal size.

Because CONFIG_TASKS_RCU=y is needed only in kernels doing tracing of
one form or another, this commit moves from TASKS_RCU deciding when it
should be enabled to the tracing Kconfig options explicitly selecting it.
This allows building preemptible kernels without TASKS_RCU, if desired.

This commit also updates the SRCU-N and TREE09 rcutorture scenarios
in order to avoid Kconfig errors that would otherwise result from
CONFIG_TASKS_RCU being selected without its CONFIG_RCU_EXPERT dependency
being met.

[ paulmck: Apply BPF_SYSCALL feedback from Andrii Nakryiko. ]

Reported-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Tested-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Tested-by: Zhouyi Zhou <zhouzhouyi@gmail.com>
Cc: Andrii Nakryiko <andrii@kernel.org>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Acked-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

arch/Kconfig

@@ -35,6 +35,7 @@ config KPROBES
 	depends on MODULES
 	depends on HAVE_KPROBES
 	select KALLSYMS
+	select TASKS_RCU if PREEMPTION
 	help
 	  Kprobes allows you to trap at almost any kernel address and
 	  execute a callback function.  register_kprobe() establishes

kernel/bpf/Kconfig

@@ -27,6 +27,7 @@ config BPF_SYSCALL
 	bool "Enable bpf() system call"
 	select BPF
 	select IRQ_WORK
+	select TASKS_RCU if PREEMPTION
 	select TASKS_TRACE_RCU
 	select BINARY_PRINTF
 	select NET_SOCK_MSG if NET

kernel/rcu/Kconfig

@@ -78,7 +78,8 @@ config TASKS_RCU_GENERIC
 	  task-based RCU implementations.  Not for manual selection.
 
 config TASKS_RCU
-	def_bool PREEMPTION
+	def_bool 0
+	select IRQ_WORK
 	help
 	  This option enables a task-based RCU implementation that uses
 	  only voluntary context switch (not preemption!), idle, and

kernel/trace/Kconfig

@@ -144,6 +144,7 @@ config TRACING
 	select BINARY_PRINTF
 	select EVENT_TRACING
 	select TRACE_CLOCK
+	select TASKS_RCU if PREEMPTION
 
 config GENERIC_TRACER
 	bool

tools/testing/selftests/rcutorture/configs/rcu/SRCU-N

@@ -6,3 +6,5 @@ CONFIG_PREEMPT_NONE=y
 CONFIG_PREEMPT_VOLUNTARY=n
 CONFIG_PREEMPT=n
 #CHECK#CONFIG_RCU_EXPERT=n
+CONFIG_KPROBES=n
+CONFIG_FTRACE=n

tools/testing/selftests/rcutorture/configs/rcu/TREE09

@@ -13,3 +13,5 @@ CONFIG_DEBUG_LOCK_ALLOC=n
 CONFIG_RCU_BOOST=n
 CONFIG_DEBUG_OBJECTS_RCU_HEAD=n
 #CHECK#CONFIG_RCU_EXPERT=n
+CONFIG_KPROBES=n
+CONFIG_FTRACE=n