linux-kernel - Re: [PATCH 1/2] hrtimer: update '->active_bases' before calling hrtimer_force

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Thu, 2 Apr 2015 16:16:33 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Viresh Kumar <viresh.kumar@...aro.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Linaro Kernel Mailman List <linaro-kernel@...ts.linaro.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] hrtimer: update '->active_bases' before calling
 hrtimer_force_reprogram()

On Thu, Apr 02, 2015 at 07:23:31PM +0530, Viresh Kumar wrote:
> On 2 April 2015 at 19:17, Peter Zijlstra <peterz@...radead.org> wrote:
> > On Thu, Apr 02, 2015 at 04:21:21PM +0530, Viresh Kumar wrote:
> >> 'active_bases' indicates which clock-base have active timers. While it
> >> is updated (almost) correctly, it is hardly used. Next commit will start
> >> using it to make code more efficient, but before that we need to fix a
> >> problem.
> >>
> >> While removing hrtimers, in __remove_hrtimer():
> >> - We first remove the hrtimer from the queue.
> >> - Then reprogram clockevent device if required
> >>   (hrtimer_force_reprogram()).
> >> - And then finally clear 'active_bases', if no more timers are pending
> >>   on the current clock base (from which we are removing the hrtimer).
> >>
> >> hrtimer_force_reprogram() needs to loop over all active clock bases to
> >> find the next expiry event, and while doing so it will use
> >> 'active_bases' (after next commit). And it will find the current base
> >> active, as we haven't cleared it until now, even if current clock base
> >> has no more hrtimers queued.
> >>
> >> To fix this issue, clear active_bases before calling
> >> hrtimer_force_reprogram().
> >
> > This is a small inefficiency right? Not an actual bug or anything.
> 
> So, what's explained in this patch is a BUG, which isn't harming us today.

So then I'm not seeing how its a bug. Sure __hrtimer_get_next_event()
will iterate all the bases again, and it will not skip the just empty
one. But I don't see how that is anything but an inefficiency. By virtue
of the base being empty it cannot find an event there, so its a
pointless check.

What am I missing?
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/