Re: [PATCH RFC] time,signal: protect resource use statistics with seqlock

Oleg Nesterov
2014-08-14T17:51:17Z

On 08/14, Oleg Nesterov wrote:
>
> OK, lets forget about alternative approach for now. We can reconsider
> it later. At least I have to admit that seqlock is more straighforward.

Yes.

But just for record, the "lockless" version doesn't look that bad to me,

void thread_group_cputime(struct task_struct *tsk, struct task_cputime *times)
{
struct signal_struct *sig = tsk->signal;
bool lockless, is_dead;
struct task_struct *t;
unsigned long flags;
u64 exec;

lockless = true;
is_dead = !lock_task_sighand(p, &flags);
retry:
times->utime = sig->utime;
times->stime = sig->stime;
times->sum_exec_runtime = exec = sig->sum_sched_runtime;
if (is_dead)
return;

if (lockless)
unlock_task_sighand(p, &flags);

rcu_read_lock();
for_each_thread(tsk, t) {
cputime_t utime, stime;
task_cputime(t, &utime, &stime);
times->utime += utime;
times->stime += stime;
times->sum_exec_runtime += task_sched_runtime(t);
}
rcu_read_unlock();

if (lockless) {
lockless = false;
is_dead = !lock_task_sighand(p, &flags);
if (is_dead || exec != sig->sum_sched_runtime)
goto retry;
}
unlock_task_sighand(p, &flags);
}

The obvious problem is that we should shift lock_task_sighand() from the
callers to thread_group_cputime() first, or add thread_group_cputime_lockless()
and change the current users one by one.

And of course, stats_lock is more generic.

Oleg.