Re: [PATCH RFC v3 13/13] x86, arm64: Build Tasks RCU on Tasks Trace readers in trampolines

diff --git a/Documentation/RCU/Design/Requirements/Requirements.rst b/Documentation/RCU/Design/Requirements/Requirements.rst
index 8101fe6229d57..34b81512cc5f3 100644
--- a/Documentation/RCU/Design/Requirements/Requirements.rst
+++ b/Documentation/RCU/Design/Requirements/Requirements.rst

@@ -2756,6 +2756,26 @@ 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()).

diff --git a/Documentation/RCU/checklist.rst b/Documentation/RCU/checklist.rst
index 4b30f701225fd..7082686cbd66d 100644
--- a/Documentation/RCU/checklist.rst
+++ b/Documentation/RCU/checklist.rst

@@ -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); an
+		arbitrary stretch of preemptible kernel code is not
+		protected.