On Thu, 17 Sep 2020 14:03:33 +0200 Christian Eggers ceggers@arri.de wrote:
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.
I'm not sure I fully understand the impacts of this yet.
What is the impact on cases where we don't have any nasty side affects due to users of the trigger?
I presume reducing the priority will cause some reduction in performance? If so is there any chance that would count as a regression?
Jonathan
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);