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From:   Sumit Garg <sumit.garg@linaro.org>
Date:   Mon, 7 Mar 2022 17:02:02 +0530
Message-ID: <CAFA6WYM77XTttu4H35PL7tkZxtBA8XaX23QW1UTBcESBE4V0fw@mail.gmail.com>
Subject: Re: [PATCH v2 1/2] tty/sysrq: Make sysrq handler NMI aware
To:     Doug Anderson <dianders@chromium.org>
Cc:     Hitomi Hasegawa <hasegawa-hitomi@fujitsu.com>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        SoC Team <soc@kernel.org>, linux-serial@vger.kernel.org,
        Arnd Bergmann <arnd@arndb.de>, Olof Johansson <olof@lixom.net>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Jiri Slaby <jirislaby@kernel.org>,
        Jason Wessel <jason.wessel@windriver.com>,
        Daniel Thompson <daniel.thompson@linaro.org>,
        LKML <linux-kernel@vger.kernel.org>,
        kgdb-bugreport@lists.sourceforge.net,
        Peter Zijlstra <peterz@infradead.org>
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Precedence: bulk

Hi Doug,

On Fri, 4 Mar 2022 at 23:36, Doug Anderson <dianders@chromium.org> wrote:
>
> Hi,
>
> On Thu, Mar 3, 2022 at 10:45 PM Hitomi Hasegawa
> <hasegawa-hitomi@fujitsu.com> wrote:
> >
> >  void __handle_sysrq(int key, bool check_mask)
> >  {
> >         const struct sysrq_key_op *op_p;
> > @@ -573,6 +606,10 @@ void __handle_sysrq(int key, bool check_mask)
> >         int orig_suppress_printk;
> >         int i;
> >
> > +       /* Skip sysrq handling if one already in progress */
> > +       if (sysrq_nmi_key != -1)
> > +               return;
>
> Should this give a warning?
>
> Also, can you remind me why this is safe if two CPUs both call
> handle_sysrq() at the same time? Can't both of them make it past this?
> That doesn't seem so great.
>
>
> > @@ -596,7 +633,13 @@ void __handle_sysrq(int key, bool check_mask)
> >                 if (!check_mask || sysrq_on_mask(op_p->enable_mask)) {
> >                         pr_info("%s\n", op_p->action_msg);
> >                         console_loglevel = orig_log_level;
> > -                       op_p->handler(key);
> > +
> > +                       if (in_nmi() && !op_p->nmi_safe) {
> > +                               sysrq_nmi_key = key;
> > +                               irq_work_queue(&sysrq_irq_work);
>
> It looks like irq_work_queue() returns false if it fails to queue.
> Maybe it's worth checking and setting "sysrq_nmi_key" back to -1 if it
> fails?

Thanks for your comments. I hope v4 here [1] addresses all of them.
Please have a look again.

[1] https://lkml.org/lkml/2022/3/7/1059

-Sumit