thread_group_cputime: simplify, document the "alive" check

thread_group_cputime() looks as if it is rcu-safe, but in fact this was wrong until ea6d290c which pins task->signal to task_struct. It checks ->sighand != NULL under rcu, but this can't help if ->signal can go away. Fortunately the caller either holds ->siglock, or it is fastpath_timer_check() which uses current and checks exit_state == 0.

- Since ea6d290c commit tsk->signal is stable, we can read it first and avoid the initialization from INIT_CPUTIME.

- Even if tsk->signal is always valid, we still have to check it is safe to use next_thread() under rcu_read_lock(). Currently the code checks ->sighand != NULL, change it to use pid_alive() which is commonly used to ensure the task wasn't unhashed before we take rcu_read_lock().

Quoted 5 lines · Oleg Nesterov

thread_group_cputime() looks as if it is rcu-safe, but in fact this was wrong until ea6d290c which pins task->signal to task_struct. It checks ->sighand != NULL under rcu, but this can't help if ->signal can go away. Fortunately the caller either holds ->siglock, or it is fastpath_timer_check() which uses current and checks exit_state == 0.

Hmm, I thought we avoided calling thread_group_cputime() from fastpatch_timer_check(), but seems it is still possible when we call run_posix_cpu_timers() on two different cpus simultaneously ...

Quoted 8 lines

- Since ea6d290c commit tsk->signal is stable, we can read it first and avoid the initialization from INIT_CPUTIME.

- Even if tsk->signal is always valid, we still have to check it is safe to use next_thread() under rcu_read_lock(). Currently the code checks ->sighand != NULL, change it to use pid_alive() which is commonly used to ensure the task wasn't unhashed before we take rcu_read_lock().

I'm not sure how important are values of almost dead task, but perhaps would be better to return times form all threads using as base sig->curr_target in loop.

Quoted 11 lines · Stanislaw Gruszka

Quoted 5 lines · Oleg Nesterov

thread_group_cputime() looks as if it is rcu-safe, but in fact this was wrong until ea6d290c which pins task->signal to task_struct. It checks ->sighand != NULL under rcu, but this can't help if ->signal can go away. Fortunately the caller either holds ->siglock, or it is fastpath_timer_check() which uses current and checks exit_state == 0.

Hmm, I thought we avoided calling thread_group_cputime() from fastpatch_timer_check(), but seems it is still possible when we call run_posix_cpu_timers() on two different cpus simultaneously ...

No, we can't. thread_group_cputimer() does test-and-set ->running under cputimer->lock.

But when I sent these patches, I realized we have another race here (with or without these patches). I am already doing the fix.

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- Since ea6d290c commit tsk->signal is stable, we can read it first and avoid the initialization from INIT_CPUTIME.

- Even if tsk->signal is always valid, we still have to check it is safe to use next_thread() under rcu_read_lock(). Currently the code checks ->sighand != NULL, change it to use pid_alive() which is commonly used to ensure the task wasn't unhashed before we take rcu_read_lock().

Quoted 3 lines

I'm not sure how important are values of almost dead task, but perhaps would be better to return times form all threads using as base sig->curr_target in loop.

Could you clarify?

Quoted 13 lines · Oleg Nesterov

Quoted 11 lines · Stanislaw Gruszka

Quoted 5 lines · Oleg Nesterov

thread_group_cputime() looks as if it is rcu-safe, but in fact this was wrong until ea6d290c which pins task->signal to task_struct. It checks ->sighand != NULL under rcu, but this can't help if ->signal can go away. Fortunately the caller either holds ->siglock, or it is fastpath_timer_check() which uses current and checks exit_state == 0.

Hmm, I thought we avoided calling thread_group_cputime() from fastpatch_timer_check(), but seems it is still possible when we call run_posix_cpu_timers() on two different cpus simultaneously ...

Quoted 5 lines

No, we can't. thread_group_cputimer() does test-and-set ->running under cputimer->lock.

But when I sent these patches, I realized we have another race here (with or without these patches). I am already doing the fix.

Don't know what you catch, I was thinking about:

cpu0 cpu1

fastpath_timer_check():

if (sig->cputimer.running) {

  struct task_cputime group_sample;
							stop_process_timers():	
        						
							spin_lock_irqsave(&cputimer->lock, flags);
        						cputimer->running = 0;
        						spin_unlock_irqrestore(&cputimer->lock, flags);

  thread_group_cputimer(tsk, &group_sample);

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- Since ea6d290c commit tsk->signal is stable, we can read it first and avoid the initialization from INIT_CPUTIME.

- Even if tsk->signal is always valid, we still have to check it is safe to use next_thread() under rcu_read_lock(). Currently the code checks ->sighand != NULL, change it to use pid_alive() which is commonly used to ensure the task wasn't unhashed before we take rcu_read_lock().

Quoted 5 lines

Quoted 3 lines

I'm not sure how important are values of almost dead task, but perhaps would be better to return times form all threads using as base sig->curr_target in loop.

Could you clarify?

Avoid pid_alive check and loop starting from sig->curr_target:

       t = tsk = sig->curr_target;
       do {
                times->utime = cputime_add(times->utime, t->utime);
                times->stime = cputime_add(times->stime, t->stime);
                times->sum_exec_runtime += t->se.sum_exec_runtime;
       } while_each_thread(tsk, t);

I don't know what are rules regarding accessing sig->curr_target, but if this is done under sighand->siglock we should be safe. Question if if we always have lock taken, we tried to assure that in the past, but if we really do?