linux-kernel - Re: [RFC/PATCH] 2.6.24-rcx: Make sys

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:	Tue, 18 Dec 2007 20:29:34 -0600
From:	Robert Hancock <hancockr@...w.ca>
To:	Karsten Wiese <fzu@...gehoertderstaat.de>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [RFC/PATCH] 2.6.24-rcx: Make sys_poll() wait at least timeout ms

Karsten Wiese wrote:
> Am Mittwoch, 19. Dezember 2007 schrieb Robert Hancock:
>> That seems fishy. What is your value of HZ and what is the timeout value 
>> that was passed in the bad case?
> 
> HZ set to 250, timeout to 4ms.
> Time spent in poll() taken by clock_gettime(CLOCK_MONOTONIC, &time)
> before and after poll()call: i.e 62us.
> Time measured with hpet gave 166us once.

msecs_to_jiffies (kernel/time.c) has this:

#if HZ <= MSEC_PER_SEC && !(MSEC_PER_SEC % HZ)
	/*
	 * HZ is equal to or smaller than 1000, and 1000 is a nice
	 * round multiple of HZ, divide with the factor between them,
	 * but round upwards:
	 */
	return (m + (MSEC_PER_SEC / HZ) - 1) / (MSEC_PER_SEC / HZ);

With HZ=250 and m=4 this gives 7/4 or only 1 jiffy, which is not more 
than 4ms, but if we are already at near the end of the current jiffy it 
could be much less than that (potentially almost no time at all).

Maybe we could convert poll to use a hrtimer for this instead?

-- 
Robert Hancock      Saskatoon, SK, Canada
To email, remove "nospam" from hancockr@...pamshaw.ca
Home Page: http://www.roberthancock.com/
--
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/