From: "Paul E. McKenney" <paulmck@kernel.org>
To: rcu@vger.kernel.org
Cc: linux-kernel@vger.kernel.org, kernel-team@fb.com,
mingo@kernel.org, jiangshanlai@gmail.com,
akpm@linux-foundation.org, mathieu.desnoyers@efficios.com,
josh@joshtriplett.org, tglx@linutronix.de, peterz@infradead.org,
rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com,
fweisbec@gmail.com, oleg@redhat.com, joel@joelfernandes.org,
Neeraj Upadhyay <neeraju@codeaurora.org>,
"Paul E . McKenney" <paulmck@kernel.org>
Subject: [PATCH rcu 12/13] rcu-tasks: Fix IPI failure handling in trc_wait_for_one_reader
Date: Wed, 15 Sep 2021 16:46:03 -0700 [thread overview]
Message-ID: <20210915234604.3907802-12-paulmck@kernel.org> (raw)
In-Reply-To: <20210915234538.GA3907674@paulmck-ThinkPad-P17-Gen-1>
From: Neeraj Upadhyay <neeraju@codeaurora.org>
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 | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)
diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index 47a29e411217..0c10c8407dca 100644
--- a/kernel/rcu/tasks.h
+++ b/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;
}
}
}
--
2.31.1.189.g2e36527f23
next prev parent reply other threads:[~2021-09-15 23:46 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-09-15 23:45 [PATCH rcu 0/13] Tasks RCU updates for v5.16 Paul E. McKenney
2021-09-15 23:45 ` [PATCH rcu 01/13] rcu-tasks: Wait for trc_read_check_handler() IPIs Paul E. McKenney
2021-09-15 23:45 ` [PATCH rcu 02/13] rcu-tasks: Simplify trc_read_check_handler() atomic operations Paul E. McKenney
2021-09-15 23:45 ` [PATCH rcu 03/13] rcu-tasks: Add trc_inspect_reader() checks for exiting critical section Paul E. McKenney
2021-09-15 23:45 ` [PATCH rcu 04/13] rcu-tasks: Remove second argument of rcu_read_unlock_trace_special() Paul E. McKenney
2021-09-15 23:45 ` [PATCH rcu 05/13] rcu-tasks: Fix s/instruction/instructions/ typo in comment Paul E. McKenney
2021-09-15 23:45 ` [PATCH rcu 06/13] rcu-tasks: Move RTGS_WAIT_CBS to beginning of rcu_tasks_kthread() loop Paul E. McKenney
2021-09-15 23:45 ` [PATCH rcu 07/13] rcu-tasks: Fix s/rcu_add_holdout/trc_add_holdout/ typo in comment Paul E. McKenney
2021-09-15 23:45 ` [PATCH rcu 08/13] rcu-tasks: Correct firstreport usage in check_all_holdout_tasks_trace Paul E. McKenney
2021-09-15 23:46 ` [PATCH rcu 09/13] rcu-tasks: Correct comparisons for CPU numbers in show_stalled_task_trace Paul E. McKenney
2021-09-15 23:46 ` [PATCH rcu 10/13] rcu-tasks: Clarify read side section info for rcu_tasks_rude GP primitives Paul E. McKenney
2021-09-15 23:46 ` [PATCH rcu 11/13] rcu-tasks: Fix read-side primitives comment for call_rcu_tasks_trace Paul E. McKenney
2021-09-15 23:46 ` Paul E. McKenney [this message]
2021-09-15 23:46 ` [PATCH rcu 13/13] rcu-tasks: Update comments to cond_resched_tasks_rcu_qs() Paul E. McKenney
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