linux-kernel - Re: [PATCH] gpio: gpiolib-cdev: Fix potential &lr->wait.lock deadlock issue

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Message-ID: <ZJrOuLXmnxuRlbLL@sol>
Date:   Tue, 27 Jun 2023 19:57:44 +0800
From:   Kent Gibson <warthog618@...il.com>
To:     Bartosz Golaszewski <brgl@...ev.pl>
Cc:     YE Chengfeng <cyeaa@...nect.ust.hk>,
        "linus.walleij@...aro.org" <linus.walleij@...aro.org>,
        "andy@...nel.org" <andy@...nel.org>,
        "linux-gpio@...r.kernel.org" <linux-gpio@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] gpio: gpiolib-cdev: Fix potential &lr->wait.lock
 deadlock issue

On Tue, Jun 27, 2023 at 01:47:16PM +0200, Bartosz Golaszewski wrote:
> On Tue, Jun 27, 2023 at 3:43 AM Kent Gibson <warthog618@...il.com> wrote:
> >
> > On Mon, Jun 26, 2023 at 05:50:47PM +0200, Bartosz Golaszewski wrote:
> > > On Mon, Jun 26, 2023 at 9:23 AM Kent Gibson <warthog618@...il.com> wrote:
> > > >
> > > >
> >
> > While we are on the subject of spin_locks, why does
> > gpio_desc_to_lineinfo() use spin_lock_irqsave()?
> > I assume the _irq is necessary as the desc could be updated at interrupt
> > level, but AFAICT gpio_desc_to_lineinfo() is only ever called from process
> > context, so why not just spin_lock_irq()?
> >
> > Cheers,
> > Kent.
> 
> Didn't we use an atomic notifier before for some reason? Then it got
> changed to blocking but the lock stayed like this? It does look like
> spin_lock_irq() would be fine here. On the other hand - if something
> isn't broken... :)
> 

Yeah, it was atomic before blocking, but that doesn't explain the need
for the save - interrupts are always enabled in that function.
Not a big difference either way, and irqsave is always safe, so no
problem with leaving it as is - I just thought it odd when I noticed it,
while spin locks and context were front of mind.

Cheers,
Kent.