Re: [PATCH] iio:trigger: Fix use_count race condition

[Jonathan Cameron <jic23@kernel.org>]

On 06/11/2013 07:18 PM, Lars-Peter Clausen wrote:
> When using more than one trigger consumer it can happen that multiple threads
> perform a read-modify-update cycle on 'use_count' concurrently. This can cause
> updates to be lost and use_count can get stuck at non-zero value, in which case
> the IIO core assumes that at least one thread is still running and will wait for
> it to finish before running any trigger handlers again. This effectively renders
> the trigger disabled and a reboot is necessary before it can be used again. To
> fix this make use_count an atomic variable. Also set it to the number of
> consumers before starting the first consumer, otherwise it might happen that
> use_count drops to 0 even though not all consumers have been run yet.
> 
I am a little worried there is a different race in here. Can't immediateliy get
my head around whether it can actually occur. It would require a subirq thread
to finish handling the interrupt during either trigger_poll or trigger_poll_chained.

I can't immediately see what prevents this happening..

One nasty option might be to ensure that we only launch num_consumers interrupts
on without caring whether they are the ones we originally counted or not.


> Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
> ---
>  drivers/iio/industrialio-trigger.c | 44 +++++++++++++++++++++++++++-----------
>  include/linux/iio/trigger.h        |  3 ++-
>  2 files changed, 33 insertions(+), 14 deletions(-)
> 
> diff --git a/drivers/iio/industrialio-trigger.c b/drivers/iio/industrialio-trigger.c
> index 4d6c7d8..a02ca65 100644
> --- a/drivers/iio/industrialio-trigger.c
> +++ b/drivers/iio/industrialio-trigger.c
> @@ -126,13 +126,22 @@ static struct iio_trigger *iio_trigger_find_by_name(const char *name,
>  
>  void iio_trigger_poll(struct iio_trigger *trig, s64 time)
>  {
> +	unsigned int num_consumers;
>  	int i;
> -	if (!trig->use_count)
> -		for (i = 0; i < CONFIG_IIO_CONSUMERS_PER_TRIGGER; i++)
> -			if (trig->subirqs[i].enabled) {
> -				trig->use_count++;
> +
> +	if (!atomic_read(&trig->use_count)) {
> +		num_consumers = 0;
> +		for (i = 0; i < CONFIG_IIO_CONSUMERS_PER_TRIGGER; i++) {
> +			if (trig->subirqs[i].enabled)
> +				num_consumers++;
> +		}
> +		atomic_set(&trig->use_count, num_consumers);
> +
Is there any chance the state of subirqs[i].enabled might have changed since
it was queried above?

> +		for (i = 0; i < CONFIG_IIO_CONSUMERS_PER_TRIGGER; i++) {
> +			if (trig->subirqs[i].enabled)
how about,

if (trig->subirqs[i].enabled && num_consumers--)
as that would prevent the case of launching too many irq handlers.

>  				generic_handle_irq(trig->subirq_base + i);
> -			}
> +		}
> +	}
>  }
>  EXPORT_SYMBOL(iio_trigger_poll);
>  
> @@ -145,20 +154,29 @@ EXPORT_SYMBOL(iio_trigger_generic_data_rdy_poll);
>  
>  void iio_trigger_poll_chained(struct iio_trigger *trig, s64 time)
>  {
> +	unsigned int num_consumers;
>  	int i;
> -	if (!trig->use_count)
> -		for (i = 0; i < CONFIG_IIO_CONSUMERS_PER_TRIGGER; i++)
> -			if (trig->subirqs[i].enabled) {
> -				trig->use_count++;
> -				handle_nested_irq(trig->subirq_base + i);
> -			}
> +
> +	if (!atomic_read(&trig->use_count)) {
> +		num_consumers = 0;
> +		for (i = 0; i < CONFIG_IIO_CONSUMERS_PER_TRIGGER; i++) {
> +			if (trig->subirqs[i].enabled)
> +				num_consumers++;
> +		}
> +		atomic_set(&trig->use_count, num_consumers);
> +
> +		for (i = 0; i < CONFIG_IIO_CONSUMERS_PER_TRIGGER; i++) {
> +			if (trig->subirqs[i].enabled)
> +				generic_handle_irq(trig->subirq_base + i);
> +		}
> +	}
>  }
>  EXPORT_SYMBOL(iio_trigger_poll_chained);
>  
>  void iio_trigger_notify_done(struct iio_trigger *trig)
>  {
> -	trig->use_count--;
> -	if (trig->use_count == 0 && trig->ops && trig->ops->try_reenable)
> +	if (atomic_dec_and_test(&trig->use_count) && trig->ops &&
> +		trig->ops->try_reenable)
>  		if (trig->ops->try_reenable(trig))
>  			/* Missed an interrupt so launch new poll now */
>  			iio_trigger_poll(trig, 0);
> diff --git a/include/linux/iio/trigger.h b/include/linux/iio/trigger.h
> index 3869c52..369cf2c 100644
> --- a/include/linux/iio/trigger.h
> +++ b/include/linux/iio/trigger.h
> @@ -8,6 +8,7 @@
>   */
>  #include <linux/irq.h>
>  #include <linux/module.h>
> +#include <linux/atomic.h>
>  
>  #ifndef _IIO_TRIGGER_H_
>  #define _IIO_TRIGGER_H_
> @@ -61,7 +62,7 @@ struct iio_trigger {
>  
>  	struct list_head		list;
>  	struct list_head		alloc_list;
> -	int use_count;
> +	atomic_t			use_count;
>  
>  	struct irq_chip			subirq_chip;
>  	int				subirq_base;
>