Re: [PATCH] Add support for deferrable timers (respun)

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

On 03/27, Venki Pallipadi wrote:
>
>  	for (;;) {
> -		base = timer->base;
> +		tvec_base_t *prelock_base = timer->base;
> +		base = timer_get_base(timer);
>  		if (likely(base != NULL)) {
>  			spin_lock_irqsave(&base->lock, *flags);
> -			if (likely(base == timer->base))
> +			if (likely(prelock_base == timer->base))
>  				return base;

I don't think this is correct, at least in theory.

Suppose that

	tvec_base_t *prelock_base = timer->base;
	base = timer_get_base(timer);

are re-ordered (the second LOAD happens after the first one), and the timer
changes its base in between. Now, we lock the old base, and return it because
"prelock_base == timer->base" == true.

Oleg.