linux-kernel - Re: [PATCH 1/3] sched: add sched_task

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Date:	Thu, 19 Feb 2015 18:48:15 +0100
From:	Vojtech Pavlik <vojtech@...e.cz>
To:	Josh Poimboeuf <jpoimboe@...hat.com>,
	Vojtech Pavlik <vojtech@...e.com>
CC:	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...hat.com>, Jiri Kosina <jkosina@...e.cz>,
	Seth Jennings <sjenning@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] sched: add sched_task_call()

On February 19, 2015 6:32:55 PM CET, Josh Poimboeuf <jpoimboe@...hat.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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