linux-kernel - Re: [RFC PATCH] irqchip/gic-v3: Make gic_handle

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Date:   Fri, 29 Mar 2019 23:35:59 +0800
From:   Zenghui Yu <yuzenghui@...wei.com>
To:     Steven Rostedt <rostedt@...dmis.org>,
        Marc Zyngier <marc.zyngier@....com>
CC:     <julien.thierry@....com>, <mingo@...hat.com>,
        <catalin.marinas@....com>, <will.deacon@....com>,
        <mark.rutland@....com>, <linux@...linux.org.uk>,
        <james.morse@....com>, <suzuki.poulose@....com>,
        <wanghaibin.wang@...wei.com>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH] irqchip/gic-v3: Make gic_handle_irq() notrace

Hi Steven,

On 2019/3/29 22:53, Steven Rostedt wrote:
> On Fri, 29 Mar 2019 13:58:40 +0000
> Marc Zyngier <marc.zyngier@....com> wrote:
> 
>> On the other hand, if you can generate pseudo-NMIs, you could end-up
>> tracing gic_handle_irq whilst being inside the tracing code with
>> interrupts being notionally disabled (and that could be pretty bad).
> 
> Actually, that should still be safe. The tracing code is expected to
> trace NMIs.
> 
> Now the entry of an NMI can be an issue because of the way tracing is
> enabled. But this would also cause function tracing to crash, which was
> not stated. Does function tracing have the same issue, or is this just
> with function_graph tracing?

This is just with function_graph tracing. Function tracing works fine.

> 
> This is because a breakpoint is added to all the places that are going
> to be traced so that the "nops" at the beginning of function calls are
> going to be converted to calls to the tracer. Now that means a
> breakpoint is being added at the beginning of gic_handle_irq(). I don't
> know how this pseudo NMI works, but we have notrace set for do_NMI
> because breakpoints at the entry (before it can fix things) causes
> issues. But this still doesn't make sense because the gic_handle_irq()
> call doesn't fix things up either, so other functions that are traced
> by gic_handle_irq() should blow up too, which appears (by the patch)
> not to be the case.

Thanks for your explanation. I will read the code around further, and
get back to you if there's any discovery.


thanks,

zenghui

> 
> -- Steve
> 
> .
>