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From:   Christian Eggers <ceggers@arri.de>
To:     Jonathan Cameron <jic23@kernel.org>
CC:     Hartmut Knaack <knaack.h@gmx.de>,
        Lars-Peter Clausen <lars@metafoo.de>,
        Peter Meerwald-Stadler <pmeerw@pmeerw.net>,
        <linux-iio@vger.kernel.org>, <linux-kernel@vger.kernel.org>,
        Christian Eggers <ceggers@arri.de>, <stable@vger.kernel.org>
Subject: [PATCH v2] iio: trigger: Don't use RT priority
Date:   Thu, 17 Sep 2020 14:03:33 +0200
Message-ID: <20200917120333.2337-1-ceggers@arri.de>
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Triggers may raise transactions on slow busses like I2C.  Using the
original RT priority of a threaded IRQ may prevent other important IRQ
handlers from being run.

Signed-off-by: Christian Eggers <ceggers@arri.de>
Cc: stable@vger.kernel.org
---
In my particular case (on a RT kernel), the RT priority of the sysfstrig
threaded IRQ handler caused (temporarily) raising the prio of a user
space process which was holding the I2C bus mutex.

Due to a bug in the i2c-imx driver, this process spent 500 ms in a busy-wait
loop and prevented all threaded IRQ handlers from being run during this
time.

v2:
- Use sched_set_normal() instead of sched_setscheduler_nocheck()

 drivers/iio/industrialio-trigger.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/drivers/iio/industrialio-trigger.c b/drivers/iio/industrialio-trigger.c
index 6f16357fd732..7ed00ad695c7 100644
--- a/drivers/iio/industrialio-trigger.c
+++ b/drivers/iio/industrialio-trigger.c
@@ -9,7 +9,10 @@
 #include <linux/err.h>
 #include <linux/device.h>
 #include <linux/interrupt.h>
+#include <linux/irq.h>
+#include <linux/irqdesc.h>
 #include <linux/list.h>
+#include <linux/sched.h>
 #include <linux/slab.h>
 
 #include <linux/iio/iio.h>
@@ -245,6 +248,7 @@ int iio_trigger_attach_poll_func(struct iio_trigger *trig,
 	int ret = 0;
 	bool notinuse
 		= bitmap_empty(trig->pool, CONFIG_IIO_CONSUMERS_PER_TRIGGER);
+	struct irq_desc *irq_desc;
 
 	/* Prevent the module from being removed whilst attached to a trigger */
 	__module_get(pf->indio_dev->driver_module);
@@ -264,6 +268,12 @@ int iio_trigger_attach_poll_func(struct iio_trigger *trig,
 	if (ret < 0)
 		goto out_put_irq;
 
+	/* Triggers may raise transactions on slow busses like I2C.  Using the original RT priority
+	 * of a threaded IRQ may prevent other threaded IRQ handlers from being run.
+	 */
+	irq_desc = irq_to_desc(pf->irq);
+	sched_set_normal(irq_desc->action->thread, 0);
+
 	/* Enable trigger in driver */
 	if (trig->ops && trig->ops->set_trigger_state && notinuse) {
 		ret = trig->ops->set_trigger_state(trig, true);
-- 
Christian Eggers
Embedded software developer

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