Messages in this thread | | | Date | Fri, 12 Dec 2014 11:58:50 -0800 (PST) | From | David Lang <> | Subject | Re: frequent lockups in 3.18rc4 |
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On Fri, 12 Dec 2014, Linus Torvalds wrote:
> I'm also not sure if the bug ever happens with preemption disabled. > Sasha, was that you who reported that you cannot reproduce it without > preemption? It strikes me that there's a race condition in > __cond_resched() wrt preemption, for example: we do > > __preempt_count_add(PREEMPT_ACTIVE); > __schedule(); > __preempt_count_sub(PREEMPT_ACTIVE); > > and in between the __schedule() and __preempt_count_sub(), if an > interrupt comes in and wakes up some important process, it won't > reschedule (because preemption is active), but then we enable > preemption again and don't check whether we should reschedule (again), > and we just go on our merry ways. > > Now, I don't see how that could really matter for a long time - > returning to user space will check need_resched, and sleeping will > obviously force a reschedule anyway, so these kinds of races should at > most delay things by just a tiny amount,
If the machine has NOHZ and has a cpu bound userspace task, it could take quite a while before userspace would trigger a reschedule (at least if I've understood the comments on this thread properly)
David Lang
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