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Date:   Thu, 29 Nov 2018 10:46:41 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Andy Lutomirski <luto@amacapital.net>
Cc:     Nadav Amit <namit@vmware.com>, Ingo Molnar <mingo@redhat.com>,
        Andrew Lutomirski <luto@kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.org>,
        Borislav Petkov <bp@alien8.de>,
        "Woodhouse, David" <dwmw@amazon.co.uk>
Subject: Re: [RFC PATCH 1/5] x86: introduce preemption disable prefix
Message-ID: <20181129094641.GD2131@hirez.programming.kicks-ass.net>
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On Fri, Oct 19, 2018 at 07:29:45AM -0700, Andy Lutomirski wrote:
> > On Oct 19, 2018, at 1:33 AM, Peter Zijlstra <peterz@infradead.org> wrote:
> > 
> >> On Fri, Oct 19, 2018 at 01:08:23AM +0000, Nadav Amit wrote:
> >> Consider for example do_int3(), and see my inlined comments:
> >> 
> >> dotraplinkage void notrace do_int3(struct pt_regs *regs, long error_code)
> >> {
> >>    ...
> >>    ist_enter(regs);        // => preempt_disable()
> >>    cond_local_irq_enable(regs);    // => assume it enables IRQs
> >> 
> >>    ...
> >>    // resched irq can be delivered here. It will not caused rescheduling
> >>    // since preemption is disabled
> >> 
> >>    cond_local_irq_disable(regs);    // => assume it disables IRQs
> >>    ist_exit(regs);            // => preempt_enable_no_resched()
> >> }
> >> 
> >> At this point resched will not happen for unbounded length of time (unless
> >> there is another point when exiting the trap handler that checks if
> >> preemption should take place).
> >> 
> >> Another example is __BPF_PROG_RUN_ARRAY(), which also uses
> >> preempt_enable_no_resched().
> >> 
> >> Am I missing something?
> > 
> > Would not the interrupt return then check for TIF_NEED_RESCHED and call
> > schedule() ?
> 
> The paranoid exit path doesn’t check TIF_NEED_RESCHED because it’s
> fundamentally atomic — it’s running on a percpu stack and it can’t
> schedule. In theory we could do some evil stack switching, but we
> don’t.
> 
> How does NMI handle this?  If an NMI that hit interruptible kernel
> code overflows a perf counter, how does the wake up work?

NMIs should never set NEED_RESCHED. What the perf does it self-IPI
(irq_work) and do the wakeup from there.