Messages in this thread | | | Subject | Re: [PATCH 1/3] sched: add sched_task_call() | From | Vojtech Pavlik <> | Date | Thu, 19 Feb 2015 18:48:15 +0100 |
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On February 19, 2015 6:32:55 PM CET, Josh Poimboeuf <jpoimboe@redhat.com> wrote:
>> Yes. I'm saying that rather than guaranteeing they don't enter the >> kernel (by having them spin) you can flip them in case they try to do >> that instead. That solves the race condition just as well. > >Ok, gotcha. > >We'd still need a safe way to check if they're in user space though. > >How about with a TIF_IN_USERSPACE thread flag? It could be cleared/set >right at the border. Then for running tasks it's as simple as: > >if (test_tsk_thread_flag(task, TIF_IN_USERSPACE)) > klp_switch_task_universe(task);
The s390x arch used to have a TIF_SYSCALL, which was doing exactly that (well, negated). I think it would work well, but it isn't entirely for free: two instructions per syscall and an extra TIF bit, which are (near) exhausted on some archs.
-- Vojtech Pavlik Director SuSE Labs
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