From: Andrew Morton <akpm@linux-foundation.org>
To: "H. Peter Anvin" <hpa@zytor.com>
Cc: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] Avoid overflows in kernel/time.c
Date: Tue, 4 Dec 2007 03:29:49 -0800 [thread overview]
Message-ID: <20071204032949.c677c021.akpm@linux-foundation.org> (raw)
In-Reply-To: <200711300019.lAU0Jpbr003807@tazenda.hos.anvin.org>
On Thu, 29 Nov 2007 16:19:51 -0800 "H. Peter Anvin" <hpa@zytor.com> wrote:
> When the conversion factor between jiffies and milli- or microseconds
> is not a single multiply or divide, as for the case of HZ == 300, we
> currently do a multiply followed by a divide. The intervening
> result, however, is subject to overflows, especially since the
> fraction is not simplified (for HZ == 300, we multiply by 300 and
> divide by 1000).
>
> This is exposed to the user when passing a large timeout to poll(),
> for example.
>
> This patch replaces the multiply-divide with a reciprocal
> multiplication on 32-bit platforms. When the input is an unsigned
> long, there is no portable way to do this on 64-bit platforms there is
> no portable way to do this since it requires a 128-bit intermediate
> result (which gcc does support on 64-bit platforms but may generate
> libgcc calls, e.g. on 64-bit s390), but since the output is a 32-bit
> integer in the cases affected, just simplify the multiply-divide
> (*3/10 instead of *300/1000).
>
> The reciprocal multiply used can have off-by-one errors in the upper
> half of the valid output range. This could be avoided at the expense
> of having to deal with a potential 65-bit intermediate result. Since
> the intent is to avoid overflow problems and most of the other time
> conversions are only semiexact, the off-by-one errors were considered
> an acceptable tradeoff.
>
> NOTE: This patch uses a bc(1) script to compute the appropriate
> constants.
My ia64 allmodconfig build has taken
akpm 15700 89.6 0.0 8256 700 pts/4 RN+ 03:09 10:41 bc -q kernel/timeconst.bc
11 minutes so far. fc6/x86_64.
next prev parent reply other threads:[~2007-12-04 11:30 UTC|newest]
Thread overview: 27+ messages / expand[flat|nested] mbox.gz Atom feed top
2007-11-30 0:19 [PATCH] Avoid overflows in kernel/time.c H. Peter Anvin
2007-11-30 1:54 ` Andrew Morton
2007-11-30 3:01 ` H. Peter Anvin
2007-11-30 3:27 ` [PATCH] Documentation/Changes -> Documentation/Requirements H. Peter Anvin
2007-11-30 3:32 ` [PATCH] Documentation/Changes -> Documentation/Requirements (resend without truncated comment text) H. Peter Anvin
2007-11-30 7:16 ` Jarek Poplawski
2007-11-30 17:40 ` H. Peter Anvin
2007-11-30 17:47 ` [PATCH] Documentation/Changes -> Documentation/Requirements H. Peter Anvin
2007-11-30 18:09 ` Robert P. J. Day
2007-11-30 18:20 ` H. Peter Anvin
2007-11-30 1:59 ` [PATCH] Avoid overflows in kernel/time.c Chris Snook
2007-11-30 3:04 ` H. Peter Anvin
2007-11-30 3:40 ` Arjan van de Ven
2007-11-30 3:54 ` H. Peter Anvin
2007-12-02 18:37 ` Pavel Machek
2007-12-03 14:53 ` Jan Engelhardt
2007-12-10 16:37 ` H. Peter Anvin
2007-12-01 0:33 ` Adrian Bunk
2007-12-01 4:19 ` H. Peter Anvin
2007-12-01 13:20 ` Alan Cox
2007-12-01 13:33 ` Alan Cox
2007-12-02 1:53 ` H. Peter Anvin
2007-12-07 0:22 ` Jeremy Fitzhardinge
2007-12-04 11:29 ` Andrew Morton [this message]
2007-12-10 16:46 ` H. Peter Anvin
2007-12-10 18:59 ` H. Peter Anvin
2007-12-10 22:04 ` Andrew Morton
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