Re: [PATCH -mm 2/4] cputimers: make sure thread_group_cputime() can't count the same thread twice lockless

[Stanislaw Gruszka <sgruszka@redhat.com>]

On Mon, Mar 29, 2010 at 08:13:29PM +0200, Oleg Nesterov wrote:
> - change __exit_signal() to do __unhash_process() before we accumulate
>   the counters in ->signal
> 
> - add a couple of barriers into thread_group_cputime() and __exit_signal()
>   to make sure thread_group_cputime() can never account the same thread
>   twice if it races with exit.
>
>   If any thread T was already accounted in ->signal, next_thread() or
>   pid_alive() must see the result of __unhash_process(T).

Memory barriers prevents to account times twice, but as we write
sig->{u,s}time and sig->sum_sched_runtime on one cpu and read them
on another cpu, without a lock, this patch make theoretically possible
that some accumulated values of struct task_cputime may include exited
task values and some not. In example times->utime will include values
from 10 threads, and times->{stime,sum_exec_runtime} values form 9
threads, because local cpu updates sig->utime but not two other values.
This can make scaling in thread_group_times() not correct. I'm not sure
how big drawback is that.

Stanislaw