[PATCH] rcu-tasks: Correct IPI failure handling in trc_wait_for_one_reader

[Neeraj Upadhyay]

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(), for each task in
  the holdout list, this function is called, 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. As all three callers (rcu_tasks_trace_pertask(),
rcu_tasks_trace_postscan(), check_all_holdout_tasks_trace())
of trc_wait_for_one_reader() run with CPU read lock held,
smp_call_function_single() cannot race with CPU hotplug. So,
also add a warning, to report and identify any such failure.

Signed-off-by: Neeraj Upadhyay <neeraju@xxxxxxxxxxxxxx>
---

Ran rcutorture TRACE01 tree for 10 hrs with below commandline, didn't
observe warning:

Command line: debug_boot_weak_hash panic=-1 selinux=0 initcall_debug debug
console=ttyS0 rcutree.rcu_fanout_leaf=4 nohz_full=1-N rcutorture.onoff_interval=1000
rcutorture.onoff_holdoff=30 rcutorture.n_barrier_cbs=4 rcutorture.stat_interval=15
rcutorture.shutdown_secs=36000 rcutorture.test_no_idle_hz=1 rcutorture.verbose=1
rcutorture.torture_type=tasks-tracing

 kernel/rcu/tasks.h | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index e6f0def..c110fd0 100644
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -1014,9 +1014,11 @@ 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;
 		}
 	}
 }
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