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

[Oleg Nesterov <oleg@redhat.com>]

On 08/15, riel@redhat.com wrote:
>
> --- a/kernel/sched/cputime.c
> +++ b/kernel/sched/cputime.c
> @@ -288,18 +288,31 @@ void thread_group_cputime(struct task_struct *tsk, struct task_cputime *times)
>  	struct signal_struct *sig = tsk->signal;
>  	cputime_t utime, stime;
>  	struct task_struct *t;
> -
> -	times->utime = sig->utime;
> -	times->stime = sig->stime;
> -	times->sum_exec_runtime = sig->sum_sched_runtime;
> +	unsigned int seq, nextseq;
>
>  	rcu_read_lock();
> -	for_each_thread(tsk, t) {
> -		task_cputime(t, &utime, &stime);
> -		times->utime += utime;
> -		times->stime += stime;
> -		times->sum_exec_runtime += task_sched_runtime(t);
> -	}
> +	/* Attempt a lockless read on the first round. */
> +	nextseq = 0;
> +	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 a writer is currently active, seq will be odd, and
> +		 * read_seqbegin_or_lock will take the lock.
> +		 */
> +		nextseq = raw_read_seqcount(&sig->stats_lock.seqcount);
> +	} while (need_seqretry(&sig->stats_lock, seq));
> +	done_seqretry(&sig->stats_lock, seq);
>  	rcu_read_unlock();
>  }

Rik, I do not understand why did you silently ignore my comments about
this change twice ;)

Please see

	http://marc.info/?l=linux-kernel&m=140802271907396
	http://marc.info/?l=linux-kernel&m=140811486607850

I still do not think that the read_seqbegin_or_lock logic is correct,
in a sense that unless I missed something it does not guarantee the
forward progress.

Oleg.