Hi William,
On Sat, Dec 25, 2021 at 01:07:44PM +0900, William Breathitt Gray wrote:
...
> So the counter_push_event() function interacts with two spinlocks:
> events_list_lock and events_in_lock. The events_list_lock spinlock is
> necessary because userspace can modify the events_list list via the
> counter_enable_events() and counter_disable_events() functions. The
> events_in_lock spinlock is necessary because userspace can modify the
> events kfifo via the counter_events_queue_size_write() function.
>
> A lockless solution for this might be possible if the driver maintains
> its own circular buffer as you suggest. The driver's IRQ handler can
> write to this circular buffer without calling the counter_push_event()
> function, and then flush the buffer to the Counter character device via
> a userspace write to a "flush_events" sysfs attribute or similar; this
> eliminates the need for the events_in_lock spinlock. The state of the
> events_list list can be captured in the driver's events_configure()
> callback and stored locally in the driver for reference, thus
> eliminating the need for the events_list_lock; interrupts can be
> disabled before the driver's local copy of events_list is modified.
>
> With only one reader and one writer operating on the driver's buffer,
> you can use the normal kfifo_in and kfifo_out calls for lockless
> operations. Perhaps that is a way forward for this problem.
As proof of concept, I implemented the double buffered version with the
sysfs flush_events interface. Currently it feels kind of wired, I use
poll and wait until it timeouts to run the sysfs_flush_counter() to
trigger new data.
Here is example:
int main(void)
{
ret = sysfs_enable_counter();
...
fd = open("/dev/counter0", O_RDWR);
...
ret = ioctl(fd, COUNTER_ADD_WATCH_IOCTL, watches);
...
ret = ioctl(fd, COUNTER_ENABLE_EVENTS_IOCTL);
...
for (;;) {
struct pollfd fds[] = {
{
.fd = fd,
.events = POLLIN,
},
};
ssize_t i;
/* wait for 10 sec */
ret = poll(fds, ARRAY_SIZE(fds), DEFAULT_TIMEOUT_MS);
if (ret == -EINTR)
continue;
else if (ret < 0)
return -errno;
else if (ret == 0) {
sysfs_flush_counter(); <---- request to flush queued events from the driver
continue;
}
ret = read(fd, event_data, sizeof(event_data));
...
for (i = 0; i < ret / (ssize_t)sizeof(event_data[0]); i++)
/* process event */
....
}
}
return ret;
}
If it is still the only way to go, I'll send kernel patches.
Regards,
Oleksij
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