Re: [RFCv4] timerfd: add TFD_NOTIFY_CLOCK_SET to watch for clock changes

[Arnd Bergmann <arnd@arndb.de>]

On Wednesday 09 March 2011 15:36:51 Alexander Shishkin wrote:
> This is another attempt to approach notifying userspace about system
> clock changes. The other one is using an eventfd and a syscall [1]. In
> the course of discussing the necessity of a syscall for this kind of
> notifications, it was suggested that this functionality can be achieved
> via timers [2] (and timerfd in particular [3]). This idea got quite
> some support [4], [5], [6] and some vague criticism [7], so I decided
> to try and go a bit further with it.

I don't understand from your description or from the patch how user
space gets notified. From your description, I would have expected
to see a change to the timerfd_poll() function to check if the
clock has changed, possibly returning POLLPRI, but the only such
change I can see is in the timerfd_read() function. Don't you need
to change poll() so that a task knows when to call read()?

> +/* TFD_NOTIFY_CLOCK_SET timers go here */
> +static DEFINE_SPINLOCK(notifiers_lock);
> +static LIST_HEAD(notifiers_list);

Maybe I was the only one to be confused by this, but I think t
he naming is slightly misleading, because it's easy to confuse
with a struct notifier_block. You could of course use the
notifier API instead of building your own, but if you don't,
I'd recommend coming up with a different name.

I also think that a mutex would be better here than a spinlock.
It's unlikely to be under heavy contention, but if you have
a lot of threads, it could be held for a significant amount
of time.

	Arnd