linux-kernel - Re: [patch 6/13] signal/timer/event fds v9

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Message-ID: <Pine.LNX.4.64.0704021025570.26489@alien.or.mcafeemobile.com>
Date:	Mon, 2 Apr 2007 10:30:11 -0700 (PDT)
From:	Davide Libenzi <davidel@...ilserver.org>
To:	Thomas Gleixner <tglx@...utronix.de>
cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [patch 6/13] signal/timer/event fds v9 - timerfd core ...

On Mon, 2 Apr 2007, Thomas Gleixner wrote:

> The DoS is simple: create a bunch of periodic timers with 10usec period.
> This can be done by any user.

Ok, that's what I immagined. Agreed, that's a problem.



> There is no inaccuracy when you rearm the timer on read: hrtimer_forward
> takes care, that the period is accurate. It does not start the timer out
> of the periodic order, i.e. on a different time frame.
> 
> Where is the win of keeping the timer running, when nobody cares about
> the expiry at all ? It just generates interrupts and events for nothing.

Then you'd lose the ability to know if you lost one or more (yes, you 
could figure it out by reading the time and with a few calculations). I 
think that the capping (to a sane value) idea solves the DoS issue and at 
the same time have the ability to report you missed ticks. What are your 
strong points against that solution?



- Davide


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