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From:   Arnd Bergmann <arnd@arndb.de>
Date:   Fri, 8 Jul 2022 10:52:25 +0200
Message-ID: <CAK8P3a3DDEkr0D=dLiXN+Xek_OO-HX5xWaQpzdX1GQxTYWnHLw@mail.gmail.com>
Subject: Re: [RFC PATCH 2/2] arm64: support HAVE_IRQ_EXIT_ON_IRQ_STACK
To:     Qi Zheng <zhengqi.arch@bytedance.com>
Cc:     Arnd Bergmann <arnd@arndb.de>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Fri, Jul 8, 2022 at 5:13 AM Qi Zheng <zhengqi.arch@bytedance.com> wrote:
> On 2022/7/8 04:55, Arnd Bergmann wrote:
> > On Thu, Jul 7, 2022 at 5:00 PM Qi Zheng <zhengqi.arch@bytedance.com> wrote:
> >> On 2022/7/7 22:41, Arnd Bergmann wrote:
> >>> On Thu, Jul 7, 2022 at 3:38 PM Qi Zheng <zhengqi.arch@bytedance.com> wrote:
> >>>> On 2022/7/7 20:49, Arnd Bergmann wrote:
> >>>
> >>> -asmlinkage void noinstr el1h_64_fiq_handler(struct pt_regs *regs)
> >>> +asmlinkage void noinstr el1h_64_irq_handler(struct pt_regs *regs)
> >>> +{
> >>> +       if (on_thread_stack())
> >>> +               call_on_irq_stack(regs, el1_irq);
> >>
> >> IMO, this can't work. Because el1_interrupt() will invoke
> >> arm64_preempt_schedule_irq(), which will cause scheduling on the
> >> IRQ stack.
> >
> > Ah, too bad. I spent some more time looking for a simpler approach,
> > but couldn't find one I'm happy with. One idea might be to have
> > callback functions for each combinations of irq/fiq with irq/pnmi
> > to avoid the nested callback pointers. Not sure if that helps.
>
> Maybe nested callback pointers are not always a wild beast. ;)
> This method does not change much, and we can also conveniently stuff
> all kinds of things in do_handler() that we want to run on the IRQ
> stack in addition to the handler().

Right, your approach is probably the one that changes the existing
code the least. I see that x86 handles this by having call_on_irq_stack()
in an inline asm, but this in turn complicates the asm implementation,
which is also worth keeping simple.

         Arnd