linux-kernel - Re: [PATCH v2 7/9] gpio: 104-idi-48: make use of raw

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Message-ID: <20170322161116.GI10423@jcartwri.amer.corp.natinst.com>
Date:   Wed, 22 Mar 2017 11:11:16 -0500
From:   Julia Cartwright <julia@...com>
To:     William Breathitt Gray <vilhelm.gray@...il.com>
CC:     Linus Walleij <linus.walleij@...aro.org>,
        Alexandre Courbot <gnurou@...il.com>,
        <linux-kernel@...r.kernel.org>, <linux-rt-users@...r.kernel.org>,
        <linux-gpio@...r.kernel.org>
Subject: Re: [PATCH v2 7/9] gpio: 104-idi-48: make use of raw_spinlock
 variants

On Wed, Mar 22, 2017 at 08:44:14AM -0400, William Breathitt Gray wrote:
> On Tue, Mar 21, 2017 at 05:43:07PM -0500, Julia Cartwright wrote:
> >The 104-idi-48 gpio driver currently implements an irq_chip for handling
> >interrupts; due to how irq_chip handling is done, it's necessary for the
> >irq_chip methods to be invoked from hardirq context, even on a a
> >real-time kernel.  Because the spinlock_t type becomes a "sleeping"
> >spinlock w/ RT kernels, it is not suitable to be used with irq_chips.
> >
> >A quick audit of the operations under the lock reveal that they do only
> >minimal, bounded work, and are therefore safe to do under a raw spinlock.
> >
> >Signed-off-by: Julia Cartwright <julia@...com>
> 
> Hi Julia,
> 
> This driver also uses a second spinlock_t, called ack_lock, to prevent
> reentrance into the idi_48_irq_handler function. Should ack_lock also be
> implemented as a raw_spinlock_t?

I saw this lock, and I don't even understand it's purpose.

However, I think I convinced myself that it's harmless.  Why?  It's only
ever acquired in a handler registered with request_irq(), which, on RT,
is invoked in a context which can sleep.

Thanks for taking a closer look!

   Julia