[PATCH RFC v4 13/13] x86, arm64: Build Tasks RCU on Tasks Trace readers in trampolines

@@ -2756,6 +2756,30 @@ synchronize_rcu(), and rcu_barrier(), respectively. In
 three APIs are therefore implemented by separate functions that check
 for voluntary context switches.
 
+Architectures that select ``CONFIG_HAVE_RCU_TRAMPOLINE_READERS`` keep the
+same three APIs but implement the grace period differently
+(``CONFIG_TASKS_RCU_TRAMPOLINE_READERS``).  There, every trampoline whose
+lifetime Tasks RCU guards enters a Tasks Trace RCU read-side critical
+section (rcu_read_lock_trace() or its assembly equivalent) before calling
+out and leaves it before returning, so a task anywhere inside such a
+call-out is an ordinary Tasks Trace reader whether or not it is
+preempted.  The few trampoline instructions outside that reader can only
+be occupied by a task that was interrupted there, so the grace period
+additionally waits for each CPU to pass through a context switch, and the
+irq-exit preemption path, the only switch that can catch a task inside
+such text (rcu_tasks_trampoline_text()), briefly makes such a task a
+holdout until it is next seen elsewhere.  On such kernels an involuntary
+context switch outside trampoline text *is* a Tasks-RCU quiescent state,
+a Tasks RCU grace period no longer depends on how long any task runs
+without sleeping, cond_resched_tasks_rcu_qs() is unnecessary, and the
+obligation moves to the trampolines: anything that relies on
+synchronize_rcu_tasks() to protect code a task may be preempted in must
+take the Tasks Trace reader (see register_ftrace_direct()), or, where
+that is impossible because the code is ordinary text with no trampoline
+of its own, make it known to rcu_tasks_trampoline_text() and wait out
+tasks already parked there with rcu_tasks_wait_irq_preempted(), as the
+kprobe jump optimizer does.
+
 Tasks Rude RCU
 ~~~~~~~~~~~~~~
 

@@ -252,7 +252,12 @@ over a rather long period of time, but improvements are always welcome!
 	a.	If the updater uses synchronize_rcu_tasks() or
 		call_rcu_tasks(), then the readers must refrain from
 		executing voluntary context switches, that is, from
-		blocking.
+		blocking.  On architectures that select
+		CONFIG_HAVE_RCU_TRAMPOLINE_READERS a reader must in
+		addition be a Tasks Trace RCU reader (that is what the
+		trampolines there do around their call-outs) or be text
+		that rcu_tasks_trampoline_text() recognises; an arbitrary
+		stretch of preemptible kernel code is not protected.
 
 	b.	If the updater uses call_rcu_tasks_trace()
 		or synchronize_rcu_tasks_trace(), then the

@@ -218,6 +218,7 @@ config ARM64
 	select HAVE_PERF_REGS
 	select HAVE_PERF_USER_STACK_DUMP
 	select HAVE_PREEMPT_DYNAMIC_KEY
+	select HAVE_RCU_TRAMPOLINE_READERS if DYNAMIC_FTRACE_WITH_ARGS
 	select HAVE_REGS_AND_STACK_ACCESS_API
 	select HAVE_RELIABLE_STACKTRACE
 	select HAVE_POSIX_CPU_TIMERS_TASK_WORK

@@ -288,6 +288,7 @@ config X86
 	select MMU_GATHER_RCU_TABLE_FREE
 	select MMU_GATHER_MERGE_VMAS
 	select HAVE_POSIX_CPU_TIMERS_TASK_WORK
+	select HAVE_RCU_TRAMPOLINE_READERS	if X86_64 && SMP && DYNAMIC_FTRACE
 	select HAVE_REGS_AND_STACK_ACCESS_API
 	select HAVE_RELIABLE_STACKTRACE		if UNWINDER_ORC || STACK_VALIDATION
 	select HAVE_FUNCTION_ARG_ACCESS_API

@@ -95,8 +95,10 @@ config FORCE_TASKS_RCU
 	help
 	  This option force-enables a task-based RCU implementation
 	  that uses only voluntary context switch (not preemption!),
-	  idle, and user-mode execution as quiescent states.  Not for
-	  manual selection in most cases.
+	  idle, and user-mode execution as quiescent states, or, on
+	  HAVE_RCU_TRAMPOLINE_READERS architectures, the variant built on
+	  Tasks Trace RCU readers in trampolines.  Not for manual
+	  selection in most cases.
 
 config NEED_TASKS_RCU
 	bool