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

[Thomas Gleixner <tglx@linutronix.de>]

On Wed, 9 Mar 2011, Alexander Shishkin wrote:
> The major change since the previous version is the new semantics of
> timerfd_settime() when it's called on a time change notification
> descriptor: it will set the system time to utmr.it_value if the time
> change counter is zero, otherwise it will return EBUSY, this is required
> to prevent a race between setting the time and reading the counter, when
> the time controlling procees changes the time immediately after another
> process in the system did the same (the counter is greater than one),
> that process' time change will be lost. Thus, the time controlling
> process should use timerfd_settime() instead of clock_settime() or
> settimeofday() to ensure that other processes' time changes don't get
> lost.

No, we really don't want to go there and invent another mechanism to
set the time.

>  	/*
> +	 * for the notification timerfd, set current time to it_value
> +	 * if the timer hasn't expired; otherwise someone has changed
> +	 * the system time to the value that we don't know
> +	 */
> +	if (!list_empty(&ctx->notifiers_list) && utmr) {
> +		if (ctx->ticks) {
> +			ret = -EBUSY;
> +			goto out;
> +		}
> +
> +		ret = security_settime(&ktmr.it_value, NULL);
> +		if (ret)
> +			goto out;
> +
> +		spin_unlock_irq(&ctx->wqh.lock);
> +		ret = do_settimeofday(&ktmr.it_value);
> +		goto out1;
> +	}

And how does that solve the problem of multiple processes using that
interface? Not at all. You moved the timer_fd_clock_was_set()
notification into the syscalls so you do not deadlock on the
notifier_lock when you call do_settimeofday() here. So if you have
multiple users of notification fd then they do not notice that you
changed the time here. That's a half thought hack, really.

And you start to overload timerfd in a way which is really horrible.
The proposed semantics of timerfd_settime() with utmr == NULL or utmr
!= NULL depending on the notification flag are so non obvious that Joe
user space programmer is doomed to fail.

The problem you want to solve is:

   Wakeup CLOCK_REALTIME timers which are not yet expired, when the
   time is set backward.

That's at least what you said you wanted to solve. I regret already
that I suggested adding that flag to timerfd, as it was only meant to
provide an interface which wakes a non expired timer whenever clock
was set.

The patch does something different. How is this related to the problem
you wanted to solve in the first place?

Can you please explain which problems you identified aside of the
initial one?

Thanks,

	tglx