Re: [PATCH] ktimers subsystem

[Oleg Nesterov <oleg@tv-sign.ru>]

Thomas Gleixner wrote:
>
> +static int internal_restart_ktimer(struct ktimer *timer, nsec_t *tim,
> +				   int mode)
> +{
> +	struct ktimer_base *base, *new_base;
> +  	unsigned long flags;
> +	int ret;
> +
> +	BUG_ON(!timer->function);
> +
> + retry:
> +	base = lock_ktimer_base(timer, &flags);
> +
> +	/* Remove an active timer from the queue */
> +	ret = remove_ktimer(timer, base);
> +
> +	/* Switch the timer base, if necessary */
> +	new_base = switch_ktimer_base(timer, base);
> +
> +	/* SMP  */
> +	if (ktimer_base_can_change && unlikely(!new_base)){
> +		spin_unlock_irqrestore(&base->lock, flags);
> +		wait_for_ktimer(timer);
> +		goto retry;
> +	}

This is deadlockable.

The caller of modify_ktimer()->internal_restart_ktimer() can hold locks
which would prevent the completion of the ktimer->function().

Also, I don't understand why the timer is deleted _before_ checking that
new_base != NULL. This way you can delete the timer (ret == 1), then goto
retry, then get ret == 0.

Oleg.