Messages in this thread | | | Date | Tue, 12 Aug 2014 21:12:18 +0200 | From | Oleg Nesterov <> | Subject | Re: [PATCH RFC] time: drop do_sys_times spinlock |
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On 08/12, Rik van Riel wrote: > > Back in 2009, Spencer Candland pointed out there is a race with > do_sys_times, where multiple threads calling do_sys_times can > sometimes get decreasing results. > > https://lkml.org/lkml/2009/11/3/522 > > As a result of that discussion, some of the code in do_sys_times > was moved under a spinlock. > > However, that does not seem to actually make the race go away on > larger systems. One obvious remaining race is that after one thread > is about to return from do_sys_times, it is preempted by another > thread, which also runs do_sys_times, and stores a larger value in > the shared variable than what the first thread got. > > This race is on the kernel/userspace boundary, and not fixable > with spinlocks.
Not sure I understand...
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.
> Back in 2009, in changeset 2b5fe6de5 Oleg Nesterov already found > that it should be safe to remove the spinlock.
Yes, it is safe but only in a sense that for_each_thread() is fine lockless. So this change was reverted.
Oleg.
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