linux-kernel - Re: [RFC 1/5] tty/sysrq: Make sysrq handler NMI aware

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Date:   Fri, 14 Aug 2020 16:34:00 +0200
From:   peterz@...radead.org
To:     Sumit Garg <sumit.garg@...aro.org>
Cc:     Doug Anderson <dianders@...omium.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Daniel Thompson <daniel.thompson@...aro.org>,
        linux-serial@...r.kernel.org, kgdb-bugreport@...ts.sourceforge.net,
        Jiri Slaby <jslaby@...e.com>,
        Russell King - ARM Linux <linux@...linux.org.uk>,
        Jason Wessel <jason.wessel@...driver.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [RFC 1/5] tty/sysrq: Make sysrq handler NMI aware

On Fri, Aug 14, 2020 at 12:54:35PM +0530, Sumit Garg wrote:
> On Thu, 13 Aug 2020 at 05:30, Doug Anderson <dianders@...omium.org> wrote:
> > On Tue, Jul 21, 2020 at 5:10 AM Sumit Garg <sumit.garg@...aro.org> wrote:
> > Wishful thinking, but (as far as I can tell) irq_work_queue() only
> > queues work on the CPU running the NMI.  I don't have lots of NMI
> > experience, but any chance there is a variant that will queue work on
> > any CPU?  Then sysrq handlers that aren't NMI aware will be more
> > likely to work.
> >
> 
> Unfortunately, queuing work on other CPUs isn't safe in NMI context,
> see this warning [1]. The comment mentions:
> 
> /* Arch remote IPI send/receive backend aren't NMI safe */
> 
> Peter,
> 
> Can you throw some light here as to why it isn't considered NMI-safe
> to send remote IPI in NMI context? Is it an arch specific limitation?

Yeah, remote irq_work uses __smp_call_single_queue() /
send_call_function_single_ipi() which isn't safe to be used from NMI
context in general.

arch_irq_work_raise() is very carefully implemented on a number of
platforms to be able to (self) IPI from NMI context.