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

From: Rik van Riel
Date: Mon Aug 18 2014 - 10:04:14 EST


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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