linux-kernel - Re: [RFC PATCH 1/5] x86: introduce preemption disable prefix

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Date:   Fri, 19 Oct 2018 10:33:25 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Nadav Amit <namit@...are.com>
Cc:     Andy Lutomirski <luto@...capital.net>,
        Ingo Molnar <mingo@...hat.com>,
        Andrew Lutomirski <luto@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        "Woodhouse, David" <dwmw@...zon.co.uk>
Subject: Re: [RFC PATCH 1/5] x86: introduce preemption disable prefix

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() ?

I think (and this certainly wants a comment) is that the ist_exit()
thing hard relies on the interrupt-return path doing the reschedule.