linux-kernel - Re: [Linuxarm] Re: [PATCH for next v1 0/2] gpio: few clean up patches to replace spin_lock_irqsave with spin

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Message-ID: <YCP0JeEUcoPp9B/H@smile.fi.intel.com>
Date:   Wed, 10 Feb 2021 16:56:37 +0200
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     "Song Bao Hua (Barry Song)" <song.bao.hua@...ilicon.com>
Cc:     luojiaxing <luojiaxing@...wei.com>,
        Linus Walleij <linus.walleij@...aro.org>,
        Grygorii Strashko <grygorii.strashko@...com>,
        Santosh Shilimkar <ssantosh@...nel.org>,
        Kevin Hilman <khilman@...nel.org>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "linuxarm@...neuler.org" <linuxarm@...neuler.org>
Subject: Re: [Linuxarm]  Re: [PATCH for next v1 0/2] gpio: few clean up
 patches to replace spin_lock_irqsave with spin_lock

On Wed, Feb 10, 2021 at 11:50:45AM +0000, Song Bao Hua (Barry Song) wrote:
> > -----Original Message-----
> > From: Andy Shevchenko [mailto:andy.shevchenko@...il.com]
> > Sent: Wednesday, February 10, 2021 11:51 PM
> > On Wed, Feb 10, 2021 at 5:43 AM luojiaxing <luojiaxing@...wei.com> wrote:
> > > On 2021/2/9 17:42, Andy Shevchenko wrote:

...

> > > Between IRQ handler A and IRQ handle A, it's no need for a SLIS.
> > 
> > Right, but it's not the case in the patches you provided.
> 
> The code still holds spin_lock. So if two cpus call same IRQ handler,
> spin_lock makes them spin; and if interrupts are threaded, spin_lock
> makes two threads run the same handler one by one.

If you run on an SMP system and it happens that spin_lock_irqsave() just
immediately after spin_unlock(), you will get into the troubles. Am I mistaken?

I think this entire activity is a carefully crafted mine field for the future
syzcaller and fuzzers alike. I don't believe there are no side effects in a long
term on all possible systems and configurations (including forced threaded IRQ
handlers).

I would love to see a better explanation in the commit message of such patches
which makes it clear that there are *no* side effects.

For time being, NAK to the all patches of this kind.

-- 
With Best Regards,
Andy Shevchenko