linux-kernel - Re: [PATCH v8 21/22] counter: 104-quad-8: Replace mutex with spinlock

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Date:   Tue, 16 Feb 2021 11:16:03 +0900
From:   William Breathitt Gray <vilhelm.gray@...il.com>
To:     Jonathan Cameron <jic23@...nel.org>
Cc:     kernel@...gutronix.de, linux-stm32@...md-mailman.stormreply.com,
        a.fatoum@...gutronix.de, kamel.bouhara@...tlin.com,
        gwendal@...omium.org, alexandre.belloni@...tlin.com,
        david@...hnology.com, linux-iio@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        syednwaris@...il.com, patrick.havelange@...ensium.com,
        fabrice.gasnier@...com, mcoquelin.stm32@...il.com,
        alexandre.torgue@...com, o.rempel@...gutronix.de
Subject: Re: [PATCH v8 21/22] counter: 104-quad-8: Replace mutex with spinlock

On Sun, Feb 14, 2021 at 06:19:46PM +0000, Jonathan Cameron wrote:
> On Fri, 12 Feb 2021 21:13:45 +0900
> William Breathitt Gray <vilhelm.gray@...il.com> wrote:
> 
> > This patch replaces the mutex I/O lock with a spinlock. This is in
> > preparation for a subsequent patch adding IRQ support for 104-QUAD-8
> > devices; we can't sleep in an interrupt context, so we'll need to use a
> > spinlock instead.
> > 
> > Cc: Syed Nayyar Waris <syednwaris@...il.com>
> > Signed-off-by: William Breathitt Gray <vilhelm.gray@...il.com>
> 
> Why do these need to be raw_spin_locks?
> Normally only need to do that if in code related to interrupt chips etc,
> not their use.
> 
> Jonathan

This lock can be taken in an interrupt context in a subsequent patch:
counter_push_event() called by quad8_irq_handler() can end up calling
the Counter component callbacks which take this lock. We can't use a
mutex nor a regular spinlock because those can sleep on RT setups [1]
which would result in a deadlock due to the interrupt context here -- so
therefore we're left with using raw_spin_lock.

[1] https://lwn.net/Articles/367219/

William Breathitt Gray

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