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

[Rik van Riel]

On 08/18/2014 12:44 AM, Mike Galbraith wrote:
> 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?

The second traversal takes the spinlock, which automatically
disables preemption.

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