On 06/11/2013 10:07 PM, Jonathan Cameron wrote:
> 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@xxxxxxxxxx>
>> ---
>> 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?
hm, right.
>
>> + 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.
That wouldn't fix the case where the subirq was enabled. use_count would end
up with a positive value. We can make a copy of enabled and use it for both
loops. Or use a spinlock protecting the triggers subirqs.
>
>> 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;
>>
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