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

Oleg Nesterov
2014-08-14T19:39:37Z

On 08/14, Rik van Riel wrote:
>
> On 08/14/2014 02:15 PM, Oleg Nesterov wrote:
> > On 08/14, Rik van Riel wrote:
> >>
> >> On 08/14/2014 12:12 PM, Oleg Nesterov wrote:
> >>>
> >>> Or you can expand the scope of write_seqlock/write_sequnlock, so that
> >>> __unhash_process in called from inside the critical section. This looks
> >>> simpler at first glance.
> >>
> >> The problem with that is that wait_task_zombie() calls
> >> thread_group_cputime_adjusted() in that if() branch, and
> >> that code ends up taking the seqlock for read...
> >
> > Not sure I understand... This modifies parent->signal->c* counters,
> > and obviously the exiting thread is not the member of parent's thread
> > group, so thread_group_cputime_adjusted(parent) can never account the
> > exiting child twice simply because it won't see it?
>
> You are right, the tree of processes only goes one way,
> so there should be no deadlock in taking psig->stats_lock
> and having thread_group_cputime_adjusted take sig->stats_lock
> for read within that section.
>
> However, it might need some lockdep annotation to keep
> lockdep from thinking we might the same lock recursively :)

But wait_task_zombie() can (and should) call
thread_group_cputime_adjusted(zombie_child) outside of parent's ->siglock
or ->stats_lock so this this should be safe.

Oleg.