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

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Message-ID: <20110311150857.GJ11410@shisha.kicks-ass.net>
Date:	Fri, 11 Mar 2011 17:08:57 +0200
From:	Alexander Shishkin <virtuoso@...nd.org>
To:	Oleg Nesterov <oleg@...hat.com>
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,
	Alexander Shishkin <virtuoso@...nd.org>
Subject: Re:
 timerfd-add-tfd_notify_clock_set-to-watch-for-clock-changes.patch added to
 -mm tree

On Fri, Mar 11, 2011 at 02:50:23PM +0100, Oleg Nesterov wrote:
> > @@ -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?

You're right, it's a bug, should be &ctx->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?

Another catch.

There will be a quite a bit of rework required, though, considering tglx's
comments.

Thanks!
--
Alex
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