linux-kernel - Re: [PATCH v2] staging: iio: adc: ad7816: add mutex to serialize SPI/GPIO operations

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Message-ID: <aLWYJKSDjNajmAlh@stanley.mountain>
Date: Mon, 1 Sep 2025 15:57:08 +0300
From: Dan Carpenter <dan.carpenter@...aro.org>
To: Andy Shevchenko <andriy.shevchenko@...el.com>
Cc: Mohammad Amin Hosseini <moahmmad.hosseinii@...il.com>,
	linux-iio@...r.kernel.org, linux-staging@...ts.linux.dev,
	linux-kernel@...r.kernel.org, gregkh@...uxfoundation.org,
	jic23@...nel.org, lars@...afoo.de, Michael.Hennerich@...log.com,
	dlechner@...libre.com, nuno.sa@...log.com, andy@...nel.org
Subject: Re: [PATCH v2] staging: iio: adc: ad7816: add mutex to serialize
 SPI/GPIO operations

On Mon, Sep 01, 2025 at 03:40:13PM +0300, Andy Shevchenko wrote:
> On Mon, Sep 01, 2025 at 01:23:40PM +0300, Dan Carpenter wrote:
> > On Mon, Sep 01, 2025 at 10:24:45AM +0330, Mohammad Amin Hosseini wrote:
> > > From: mohammad amin hosseini <moahmmad.hosseinii@...il.com>
> > > 
> > > The ad7816 driver was accessing SPI and GPIO lines without
> > > synchronization, which could lead to race conditions when accessed
> > > concurrently from multiple contexts. This might result in corrupted
> > > readings or inconsistent GPIO states.
> > > 
> > > Introduce an io_lock mutex in the driver structure to serialize:
> > > - SPI transactions in ad7816_spi_read() and ad7816_spi_write()
> > > - GPIO pin toggling sequences
> > > - Updates to device state via sysfs store functions (mode, channel, oti)
> > > 
> > > The mutex ensures proper mutual exclusion and prevents race
> > > conditions under concurrent access.
> 
> ...
> 
> > > +	mutex_lock(&chip->io_lock);
> > >  	chip->channel_id = data;
> > > +	mutex_unlock(&chip->io_lock);
> 
> > > +	mutex_lock(&chip->io_lock);
> > >  	chip->oti_data[chip->channel_id] = data;
> > > +	mutex_unlock(&chip->io_lock);
> 
> > I'm not really knowledgeable to review the others, if they are
> > required or how the locking is supposed to work.  But these aren't
> > correct because we're only locking around the writers and not the
> > readers so it could still race.
> 
> Readers are in spi_write(), or what do you imply by this comment?
> I.o.w. I do not see the issue with the idea of locking and how it's
> done (I haven't checked all of the details, though).

Sorry, I meant we don't have locking in ad7816_show_oti(), for example.

regards,
dan carpenter