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

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Message-ID: <20070327211145.GB216@tv-sign.ru>
Date:	Wed, 28 Mar 2007 01:11:45 +0400
From:	Oleg Nesterov <oleg@...sign.ru>
To:	Venki Pallipadi <venkatesh.pallipadi@...el.com>
Cc:	linux-kernel <linux-kernel@...r.kernel.org>,
	akpm@...ux-foundation.org, davej@...emonkey.org.uk,
	johnstul@...ibm.com, mingo@...e.hu, tglx@...utronix.de
Subject: Re: [PATCH] Add support for deferrable timers (respun)

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.

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