Re: [PATCH v3 16/18] perf sched: Fixes for thread safety analysis

[Adrian Hunter <adrian.hunter@intel.com>]

On 26/08/22 21:26, Namhyung Kim wrote:
> On Fri, Aug 26, 2022 at 10:48 AM Ian Rogers <irogers@google.com> wrote:
>>
>> On Fri, Aug 26, 2022 at 10:41 AM Adrian Hunter <adrian.hunter@intel.com> wrote:
>>>
>>> On 26/08/22 19:06, Ian Rogers wrote:
>>>> On Fri, Aug 26, 2022 at 5:12 AM Adrian Hunter <adrian.hunter@intel.com> wrote:
>>>>>
>>>>> On 24/08/22 18:38, Ian Rogers wrote:
>>>>>> Add annotations to describe lock behavior. Add unlocks so that mutexes
>>>>>> aren't conditionally held on exit from perf_sched__replay. Add an exit
>>>>>> variable so that thread_func can terminate, rather than leaving the
>>>>>> threads blocked on mutexes.
>>>>>>
>>>>>> Signed-off-by: Ian Rogers <irogers@google.com>
>>>>>> ---
>>>>>>  tools/perf/builtin-sched.c | 46 ++++++++++++++++++++++++--------------
>>>>>>  1 file changed, 29 insertions(+), 17 deletions(-)
>>>>>>
>>>>>> diff --git a/tools/perf/builtin-sched.c b/tools/perf/builtin-sched.c
>>>>>> index 7e4006d6b8bc..b483ff0d432e 100644
>>>>>> --- a/tools/perf/builtin-sched.c
>>>>>> +++ b/tools/perf/builtin-sched.c
>>>>>> @@ -246,6 +246,7 @@ struct perf_sched {
>>>>>>       const char      *time_str;
>>>>>>       struct perf_time_interval ptime;
>>>>>>       struct perf_time_interval hist_time;
>>>>>> +     volatile bool   thread_funcs_exit;
>>>>>>  };
>>>>>>
>>>>>>  /* per thread run time data */
>>>>>> @@ -633,31 +634,34 @@ static void *thread_func(void *ctx)
>>>>>>       prctl(PR_SET_NAME, comm2);
>>>>>>       if (fd < 0)
>>>>>>               return NULL;
>>>>>> -again:
>>>>>> -     ret = sem_post(&this_task->ready_for_work);
>>>>>> -     BUG_ON(ret);
>>>>>> -     mutex_lock(&sched->start_work_mutex);
>>>>>> -     mutex_unlock(&sched->start_work_mutex);
>>>>>>
>>>>>> -     cpu_usage_0 = get_cpu_usage_nsec_self(fd);
>>>>>> +     while (!sched->thread_funcs_exit) {
>>>>>> +             ret = sem_post(&this_task->ready_for_work);
>>>>>> +             BUG_ON(ret);
>>>>>> +             mutex_lock(&sched->start_work_mutex);
>>>>>> +             mutex_unlock(&sched->start_work_mutex);
>>>>>>
>>>>>> -     for (i = 0; i < this_task->nr_events; i++) {
>>>>>> -             this_task->curr_event = i;
>>>>>> -             perf_sched__process_event(sched, this_task->atoms[i]);
>>>>>> -     }
>>>>>> +             cpu_usage_0 = get_cpu_usage_nsec_self(fd);
>>>>>>
>>>>>> -     cpu_usage_1 = get_cpu_usage_nsec_self(fd);
>>>>>> -     this_task->cpu_usage = cpu_usage_1 - cpu_usage_0;
>>>>>> -     ret = sem_post(&this_task->work_done_sem);
>>>>>> -     BUG_ON(ret);
>>>>>> +             for (i = 0; i < this_task->nr_events; i++) {
>>>>>> +                     this_task->curr_event = i;
>>>>>> +                     perf_sched__process_event(sched, this_task->atoms[i]);
>>>>>> +             }
>>>>>>
>>>>>> -     mutex_lock(&sched->work_done_wait_mutex);
>>>>>> -     mutex_unlock(&sched->work_done_wait_mutex);
>>>>>> +             cpu_usage_1 = get_cpu_usage_nsec_self(fd);
>>>>>> +             this_task->cpu_usage = cpu_usage_1 - cpu_usage_0;
>>>>>> +             ret = sem_post(&this_task->work_done_sem);
>>>>>> +             BUG_ON(ret);
>>>>>>
>>>>>> -     goto again;
>>>>>> +             mutex_lock(&sched->work_done_wait_mutex);
>>>>>> +             mutex_unlock(&sched->work_done_wait_mutex);
>>>>>> +     }
>>>>>> +     return NULL;
>>>>>>  }
>>>>>>
>>>>>>  static void create_tasks(struct perf_sched *sched)
>>>>>> +     EXCLUSIVE_LOCK_FUNCTION(sched->start_work_mutex)
>>>>>> +     EXCLUSIVE_LOCK_FUNCTION(sched->work_done_wait_mutex)
>>>>>>  {
>>>>>>       struct task_desc *task;
>>>>>>       pthread_attr_t attr;
>>>>>> @@ -687,6 +691,8 @@ static void create_tasks(struct perf_sched *sched)
>>>>>>  }
>>>>>>
>>>>>>  static void wait_for_tasks(struct perf_sched *sched)
>>>>>> +     EXCLUSIVE_LOCKS_REQUIRED(sched->work_done_wait_mutex)
>>>>>> +     EXCLUSIVE_LOCKS_REQUIRED(sched->start_work_mutex)
>>>>>>  {
>>>>>>       u64 cpu_usage_0, cpu_usage_1;
>>>>>>       struct task_desc *task;
>>>>>> @@ -738,6 +744,8 @@ static void wait_for_tasks(struct perf_sched *sched)
>>>>>>  }
>>>>>>
>>>>>>  static void run_one_test(struct perf_sched *sched)
>>>>>> +     EXCLUSIVE_LOCKS_REQUIRED(sched->work_done_wait_mutex)
>>>>>> +     EXCLUSIVE_LOCKS_REQUIRED(sched->start_work_mutex)
>>>>>>  {
>>>>>>       u64 T0, T1, delta, avg_delta, fluct;
>>>>>>
>>>>>> @@ -3309,11 +3317,15 @@ static int perf_sched__replay(struct perf_sched *sched)
>>>>>>       print_task_traces(sched);
>>>>>>       add_cross_task_wakeups(sched);
>>>>>>
>>>>>> +     sched->thread_funcs_exit = false;
>>>>>>       create_tasks(sched);
>>>>>>       printf("------------------------------------------------------------\n");
>>>>>>       for (i = 0; i < sched->replay_repeat; i++)
>>>>>>               run_one_test(sched);
>>>>>>
>>>>>> +     sched->thread_funcs_exit = true;
>>>>>> +     mutex_unlock(&sched->start_work_mutex);
>>>>>> +     mutex_unlock(&sched->work_done_wait_mutex);
>>>>>
>>>>> I think you still need to wait for the threads to exit before
>>>>> destroying the mutexes.
>>>>
>>>> This is a pre-existing issue and beyond the scope of this patch set.
>>>
>>> You added the mutex_destroy functions in patch 8, so it is still
>>> fallout from that.
>>
>> In the previous code the threads were blocked on mutexes that were
>> stack allocated and the stack memory went away. You are correct to say
>> that to those locks I added an init and destroy call. The lifetime of
>> the mutex was wrong previously and remains wrong in this change.
> 
> I think you fixed the lifetime issue with sched->thread_funcs_exit here.
> All you need to do is calling pthread_join() after the mutex_unlock, no?

Like this maybe:

diff --git a/tools/perf/builtin-sched.c b/tools/perf/builtin-sched.c
index b483ff0d432e..8090c1218855 100644
--- a/tools/perf/builtin-sched.c
+++ b/tools/perf/builtin-sched.c
@@ -3326,6 +3326,13 @@ static int perf_sched__replay(struct perf_sched *sched)
 	sched->thread_funcs_exit = true;
 	mutex_unlock(&sched->start_work_mutex);
 	mutex_unlock(&sched->work_done_wait_mutex);
+	/* Get rid of threads so they won't be upset by mutex destruction */
+	for (i = 0; i < sched->nr_tasks; i++) {
+		int err = pthread_join(sched->tasks[i]->thread, NULL);
+
+		if (err)
+			pr_err("pthread_join() failed for task nr %lu, error %d\n", i, err);
+	}
 	return 0;
 }