Re: [PATCH RFC] time: drop do_sys_times spinlock

Oleg Nesterov
2014-08-13T17:24:56Z

On 08/12, Rik van Riel wrote:
>
> Any other ideas?

To simplify, lets suppose that we only need sum_exec_runtime.

Perhaps we can do something like this

u64 thread_group_sched_runtime(void)
{
struct task_struct *tsk = current;
spinlock_t *siglock = &tsk->sighand->siglock; /* stable */
struct task_struct *t;
u64 x1, x2;

retry:
x1 = tsk->signal->sum_sched_runtime;
rmb();
spin_unlock_wait(siglock);
rmb();

x2 = 0;
rcu_read_lock();
for_each_thread(tsk, t)
x2 += task_sched_runtime(t);
rcu_read_unlock();

rmb();
spin_unlock_wait(siglock);
rmb();

if (x1 != tsk->signal->sum_sched_runtime)
goto retry;

return x1 + x2;
}

?

We do not care if for_each_thread() misses the new thread, we can pretend
thread_group_sched_runtime() was called before clone.

We do not care if a thread with sum_sched_runtime == 0 exits, obviously.

Otherwise "x1 != tsk->signal->sum_sched_runtime" should tell us that we
raced with __exit_signal().

Oleg.