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From:   William Breathitt Gray <william.gray@linaro.org>
To:     Jonathan Cameron <jic23@kernel.org>,
        Lars-Peter Clausen <lars@metafoo.de>
Cc:     linux-iio@vger.kernel.org, linux-kernel@vger.kernel.org,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        William Breathitt Gray <william.gray@linaro.org>
Subject: [PATCH v4 1/5] iio: addac: stx104: Fix race condition for stx104_write_raw()
Date:   Tue,  4 Apr 2023 10:11:58 -0400
Message-Id: <c95c9a77fcef36b2a052282146950f23bbc1ebdc.1680564468.git.william.gray@linaro.org>
In-Reply-To: <cover.1680564468.git.william.gray@linaro.org>
References: <cover.1680564468.git.william.gray@linaro.org>
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The priv->chan_out_states array and actual DAC value can become
mismatched if stx104_write_raw() is called concurrently. Prevent such a
race condition by utilizing a mutex.

Fixes: 97a445dad37a ("iio: Add IIO support for the DAC on the Apex Embedded Systems STX104")
Signed-off-by: William Breathitt Gray <william.gray@linaro.org>
---
 drivers/iio/addac/stx104.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/drivers/iio/addac/stx104.c b/drivers/iio/addac/stx104.c
index e45b70aa5bb7..4239aafe42fc 100644
--- a/drivers/iio/addac/stx104.c
+++ b/drivers/iio/addac/stx104.c
@@ -15,6 +15,7 @@
 #include <linux/kernel.h>
 #include <linux/module.h>
 #include <linux/moduleparam.h>
+#include <linux/mutex.h>
 #include <linux/spinlock.h>
 #include <linux/types.h>
 
@@ -69,10 +70,12 @@ struct stx104_reg {
 
 /**
  * struct stx104_iio - IIO device private data structure
+ * @lock: synchronization lock to prevent I/O race conditions
  * @chan_out_states:	channels' output states
  * @reg:		I/O address offset for the device registers
  */
 struct stx104_iio {
+	struct mutex lock;
 	unsigned int chan_out_states[STX104_NUM_OUT_CHAN];
 	struct stx104_reg __iomem *reg;
 };
@@ -178,9 +181,12 @@ static int stx104_write_raw(struct iio_dev *indio_dev,
 			if ((unsigned int)val > 65535)
 				return -EINVAL;
 
+			mutex_lock(&priv->lock);
+
 			priv->chan_out_states[chan->channel] = val;
 			iowrite16(val, &priv->reg->dac[chan->channel]);
 
+			mutex_unlock(&priv->lock);
 			return 0;
 		}
 		return -EINVAL;
@@ -351,6 +357,8 @@ static int stx104_probe(struct device *dev, unsigned int id)
 
 	indio_dev->name = dev_name(dev);
 
+	mutex_init(&priv->lock);
+
 	/* configure device for software trigger operation */
 	iowrite8(0, &priv->reg->acr);
 
-- 
2.39.2