Re: [PATCH v2 2/3] time,signal: protect resource use statistics with seqlock

[Mike Galbraith <umgwanakikbuti@gmail.com>]

On Sat, 2014-08-16 at 19:50 +0200, Oleg Nesterov wrote: 
> On 08/16, Rik van Riel wrote:
> >
> > +	do {
> > +		seq = nextseq;
> > +		read_seqbegin_or_lock(&sig->stats_lock, &seq);
> > +		times->utime = sig->utime;
> > +		times->stime = sig->stime;
> > +		times->sum_exec_runtime = sig->sum_sched_runtime;
> > +
> > +		for_each_thread(tsk, t) {
> > +			task_cputime(t, &utime, &stime);
> > +			times->utime += utime;
> > +			times->stime += stime;
> > +			times->sum_exec_runtime += task_sched_runtime(t);
> > +		}
> > +		/* If lockless access failed, take the lock. */
> > +		nextseq = 1;
> 
> Yes, thanks, this answers my concerns.
> 
> Cough... can't resist, and I still think that we should take rcu_read_lock()
> only around for_each_thread() and the patch expands the critical section for
> no reason. But this is minor, I won't insist.

Hm.  Should traversal not also disable preemption to preserve the error
bound Peter mentioned?

-Mike