Messages in this thread | | | Date | Tue, 12 Aug 2014 18:27:41 -0400 | From | Rik van Riel <> | Subject | Re: [PATCH RFC] time: drop do_sys_times spinlock |
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On 08/12/2014 03:12 PM, Oleg Nesterov wrote:
> Afaics, the problem is that a single thread can observe the > decreasing (say) sum_exec_runtime if it calls do_sys_times() twice > without the lock. > > This is because it can account the exiting sub-thread twice if it > races with __exit_signal() which increments sig->sum_sched_runtime, > but this exiting thread can still be visible to > thread_group_cputime(). > > IOW, it is not actually about decreasing, the problem is that the > lockless thread_group_cputime() can return the wrong result, and > the next ys_times() can show the right value.
You are right, changing the test case to call times() many times in a row in each thread can result in the wrong value being returned.
Not entirely sure what I can do there...
Replacing the spinlock with a seqlock, and taking it for write in most places is pretty gross, and may lead to other issues like reader livelock when there is a lot of write activity.
Having a seqlock just for the stats? Not sure the calls to times() are a big enough issue for most workloads to justify that...
Any other ideas?
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