5.15-stable review patch. If anyone has any objections, please let me know.
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From: Neeraj Upadhyay neeraju@codeaurora.org
commit 46aa886c483f57ef13cd5ea0a85e70b93eb1d381 upstream.
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 Cc: Joel Fernandes joel@joelfernandes.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- kernel/rcu/tasks.h | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-)
--- a/kernel/rcu/tasks.h +++ b/kernel/rcu/tasks.h @@ -1041,9 +1041,11 @@ static void trc_wait_for_one_reader(stru 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; } } }