linux-kernel - Re: timerfd-add-tfd_notify_clock_set-to-watch-for-clock-changes.patch added to -mm tree

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Date:	Fri, 11 Mar 2011 14:50:23 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Alexander Shishkin <virtuoso@...nd.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	Feng Tang <feng.tang@...el.com>,
	Michael Tokarev <mjt@....msk.ru>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	John Stultz <johnstul@...ibm.com>,
	Chris Friesen <chris.friesen@...band.com>,
	Kay Sievers <kay.sievers@...y.org>,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	Artem Bityutskiy <dedekind1@...il.com>,
	Davide Libenzi <davidel@...ilserver.org>,
	Michael Kerrisk <mtk.manpages@...il.com>,
	linux-api@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re:
	timerfd-add-tfd_notify_clock_set-to-watch-for-clock-changes.patch
	added to -mm tree

> @@ -218,10 +266,12 @@ SYSCALL_DEFINE4(timerfd_settime, int, uf
>  	 * it to the new values.
>  	 */
>  	for (;;) {
> +		spin_lock(&notifiers_lock);
>  		spin_lock_irq(&ctx->wqh.lock);
> -		if (hrtimer_try_to_cancel(&ctx->tmr) >= 0)
> +		if (!list_empty(&notifiers_list) || hrtimer_try_to_cancel(&ctx->tmr) >= 0)
>  			break;

Confused. Why do we check the global notifiers_list?

IOW. Suppose that this list is not empty and timerfd_settime() is called
without TFD_NOTIFY_CLOCK_SET. Now we are going to reprogramm the timer
without stopping it?

And. What if timerfd_settime(TFD_NOTIFY_CLOCK_SET, utmr => NULL) is called
twice? timerfd_setup() blindly does list_add(), and we corrupt the list, no?

Oleg.

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