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

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Date:   Tue, 27 Jun 2023 13:47:16 +0200
From:   Bartosz Golaszewski <brgl@...ev.pl>
To:     Kent Gibson <warthog618@...il.com>
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 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:
> > >
> > >
> > > spin_lock_bh() should be sufficient, given that edge_irq_thread() is run
> > > in a softirq?  That is faster and would allow the hard irq handlers to
> > > still run, and timestamp the event, but inhibit the edge_irq_thread()
> > > from being called on that CPU until the lock is released.
> > > (hmmm, gpio_desc_to_lineinfo() also uses spin_lock_irqsave() but it is
> > > never called from hard irq context, so there is a good chance I'm missing
> > > something here??)
> > > More on spin_lock choice below.
> >
> > Again: this is incorrect - edge_irq_thread() doesn't execute in
> > softirq context which can be verified by calling in_softirq() from it.
> >
>
> Ok, that matches what I had initially thought.  Wading through the kernel
> doc got me thinking the secondary handler was run as a softirq.
> But it is a threaded irq used here, so the thread handler runs in a
> kernel thread, as does the debounce_work_func() and hte thread handler
> process_hw_ts_thread().
> That's a relief.
>
> 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... :)

Bart