rcu-tasks: Fix IPI failure handling in trc_wait_for_one_reader

The trc_wait_for_one_reader() function is called at multiple stages
of trace rcu-tasks GP function, rcu_tasks_wait_gp():

- First, it is called as part of per task function -
  rcu_tasks_trace_pertask(), for all non-idle tasks. As part of per task
  processing, this function add the task in the holdout list and if the
  task is currently running on a CPU, it sends IPI to the task's CPU.
  The IPI handler takes action depending on whether task is in trace
  rcu-tasks read side critical section or not:

  - a. If the task is in trace rcu-tasks read side critical section
       (t->trc_reader_nesting != 0), the IPI handler sets the task's
       ->trc_reader_special.b.need_qs, so that this task notifies exit
       from its outermost read side critical section (by decrementing
       trc_n_readers_need_end) to the GP handling function.
       trc_wait_for_one_reader() also increments trc_n_readers_need_end,
       so that the trace rcu-tasks GP handler function waits for this
       task's read side exit notification. The IPI handler also sets
       t->trc_reader_checked to true, and no further IPIs are sent for
       this task, for this trace rcu-tasks grace period and this
       task can be removed from holdout list.

  - b. If the task is in the process of exiting its trace rcu-tasks
       read side critical section, (t->trc_reader_nesting < 0), defer
       this task's processing to future calls to trc_wait_for_one_reader().

  - c. If task is not in rcu-task read side critical section,
       t->trc_reader_nesting == 0, ->trc_reader_checked is set for this
       task, so that this task is removed from holdout list.

- Second, trc_wait_for_one_reader() is called as part of post scan, in
  function rcu_tasks_trace_postscan(), for all idle tasks.

- Third, in function check_all_holdout_tasks_trace(), this function is
  called for each task in the holdout list, but only if there isn't
  a pending IPI for the task (->trc_ipi_to_cpu == -1). This function
  removed the task from holdout list, if IPI handler has completed the
  required work, to ensure that the current trace rcu-tasks grace period
  either waits for this task, or this task is not in a trace rcu-tasks
  read side critical section.

Now, considering the scenario where smp_call_function_single() fails in
first case, inside rcu_tasks_trace_pertask(). In this case,
->trc_ipi_to_cpu is set to the current CPU for that task. This will
result in trc_wait_for_one_reader() getting skipped in third case,
inside check_all_holdout_tasks_trace(), for this task. This further
results in ->trc_reader_checked never getting set for this task,
and the task not getting removed from holdout list. This can cause
the current trace rcu-tasks grace period to stall.

Fix the above problem, by resetting ->trc_ipi_to_cpu to -1, on
smp_call_function_single() failure, so that future IPI calls can
be send for this task.

Note that all three of the trc_wait_for_one_reader() function's
callers (rcu_tasks_trace_pertask(), rcu_tasks_trace_postscan(),
check_all_holdout_tasks_trace()) hold cpu_read_lock().  This means
that smp_call_function_single() cannot race with CPU hotplug, and thus
should never fail.  Therefore, also add a warning in order to report
any such failure in case smp_call_function_single() grows some other
reason for failure.
Signed-off-by: Neeraj Upadhyay <neeraju@codeaurora.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/rcu/tasks.h

@@ -1012,9 +1012,11 @@ static void trc_wait_for_one_reader(struct task_struct *t,
 		if (smp_call_function_single(cpu, trc_read_check_handler, t, 0)) {
 			// Just in case there is some other reason for
 			// failure than the target CPU being offline.
+			WARN_ONCE(1, "%s():  smp_call_function_single() failed for CPU: %d\n",
+				  __func__, cpu);
 			rcu_tasks_trace.n_ipis_fails++;
 			per_cpu(trc_ipi_to_cpu, cpu) = false;
-			t->trc_ipi_to_cpu = cpu;
+			t->trc_ipi_to_cpu = -1;
 		}
 	}
 }