From: Ingo Molnar <mingo@elte.hu>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: linux-kernel@vger.kernel.org,
Andrew Morton <akpm@linux-foundation.org>,
Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [git pull] timer fix
Date: Wed, 4 Feb 2009 23:25:43 +0100 [thread overview]
Message-ID: <20090204222543.GA19944@elte.hu> (raw)
In-Reply-To: <alpine.LFD.2.00.0902041400100.3247@localhost.localdomain>
* Linus Torvalds <torvalds@linux-foundation.org> wrote:
> On Wed, 4 Feb 2009, Ingo Molnar wrote:
> >
> > Pavel Emelyanov (1):
> > x86: fix hpet timer reinit for x86_64
> >
> >
> > arch/x86/kernel/hpet.c | 2 +-
> > 1 files changed, 1 insertions(+), 1 deletions(-)
> >
> > diff --git a/arch/x86/kernel/hpet.c b/arch/x86/kernel/hpet.c
> > index 64d5ad0..ec319d1 100644
> > --- a/arch/x86/kernel/hpet.c
> > +++ b/arch/x86/kernel/hpet.c
> > @@ -1075,7 +1075,7 @@ static void hpet_rtc_timer_reinit(void)
> > hpet_t1_cmp += delta;
> > hpet_writel(hpet_t1_cmp, HPET_T1_CMP);
> > lost_ints++;
> > - } while ((long)(hpet_readl(HPET_COUNTER) - hpet_t1_cmp) > 0);
> > + } while ((long)(hpet_readl(HPET_COUNTER) - (u32)hpet_t1_cmp) > 0);
>
> This is bordering on not being correct.
yeah, i had to look twice. The only reason i left it that way was because i
couldnt reproduce the problem and hpet is hellishly fragile and this patch
was tested so i chickened out.
OTOH that fragility is partly because such constructs have piled up so you
very much have a valid point ...
We'll clean this up. I've already added the clean 32-bit casts - which also
has another advantage: it does not actually trust the hw to always return
32-bit values - it explicitly cuts to 32 bits and does signed arithmetics on
that. Will also do the helper function cleanup to abstract the counter
arithmetics away.
> In particular, think about when HPET_COUNTER or hpet_t1_cmp overflows in
> 32 bits, and what you want to happen. If you do the subtract add test in
> 64 bits, it will simply do the wrong thing. Think what happens if
> hpet_t1_cmp is actually _larger_ than HPET_COUNTER, but overflowed in 32
> bits, and you're now looking at:
>
> (long) (0xffffffff - 0x00000001)
>
> which is actually > 0, so the thing will continue to loop INCORRECTLY. It
> should have stopped (and _would_ have stopped on 32-bit x86).
yeah, allowing that to happen is just wrong.
Ingo
next prev parent reply other threads:[~2009-02-04 22:28 UTC|newest]
Thread overview: 59+ messages / expand[flat|nested] mbox.gz Atom feed top
2009-02-04 19:25 [git pull] timer fix Ingo Molnar
2009-02-04 22:11 ` Linus Torvalds
2009-02-04 22:16 ` Linus Torvalds
2009-02-04 22:25 ` Ingo Molnar [this message]
2009-02-04 22:58 ` Ingo Molnar
2009-02-04 23:13 ` H. Peter Anvin
2009-02-05 0:04 ` Ingo Molnar
2009-02-05 7:51 ` Kirill Korotaev
2009-02-05 9:58 ` Pavel Emelyanov
2009-02-05 14:30 ` Ingo Molnar
2009-02-05 16:04 ` Ray Lee
2009-02-17 16:38 Ingo Molnar
2009-06-20 16:55 [GIT PULL] " Ingo Molnar
2009-08-04 19:04 Ingo Molnar
2009-08-09 16:09 Ingo Molnar
2009-09-26 12:27 Ingo Molnar
2009-10-02 12:38 Ingo Molnar
2010-01-31 17:26 Ingo Molnar
2011-02-15 17:06 Ingo Molnar
2011-02-28 17:39 Ingo Molnar
2011-04-29 18:11 Ingo Molnar
2011-10-17 1:39 Linux 3.1-rc9 Linus Torvalds
2011-10-17 10:34 ` Peter Zijlstra
2011-10-17 14:57 ` Linus Torvalds
2011-10-17 17:54 ` Peter Zijlstra
2011-10-17 18:31 ` Linus Torvalds
2011-10-17 19:23 ` Peter Zijlstra
2011-10-17 21:00 ` Thomas Gleixner
2011-10-18 8:39 ` Thomas Gleixner
2011-10-18 9:05 ` Peter Zijlstra
2011-10-18 14:59 ` Linus Torvalds
2011-10-18 18:14 ` [GIT PULL] timer fix Ingo Molnar
2013-09-18 16:22 Ingo Molnar
2013-10-26 12:27 Ingo Molnar
2014-01-15 18:27 Ingo Molnar
2014-03-29 18:44 Ingo Molnar
2015-02-06 18:38 Ingo Molnar
2015-07-18 3:06 Ingo Molnar
2015-08-14 7:13 Ingo Molnar
2016-04-23 11:34 Ingo Molnar
2016-07-13 12:58 Ingo Molnar
2016-10-18 11:18 Ingo Molnar
2016-12-23 22:53 Ingo Molnar
2017-01-18 9:37 Ingo Molnar
2017-05-12 7:35 Ingo Molnar
2017-07-21 10:21 Ingo Molnar
2017-08-26 7:17 Ingo Molnar
2017-09-24 11:25 Ingo Molnar
2018-03-25 9:00 Ingo Molnar
2018-12-21 12:34 Ingo Molnar
2018-12-21 19:30 ` pr-tracker-bot
2018-12-23 19:29 ` Heiko Carstens
2019-01-17 9:51 ` Ingo Molnar
2019-01-17 15:58 ` Heiko Carstens
2019-01-17 16:57 ` Thomas Gleixner
2019-04-12 13:09 Ingo Molnar
2019-04-13 4:05 ` pr-tracker-bot
2019-09-26 20:18 Ingo Molnar
2019-09-26 23:00 ` pr-tracker-bot
2019-10-02 22:06 Ingo Molnar
2019-10-02 23:00 ` pr-tracker-bot
2019-11-16 21:38 Ingo Molnar
2019-11-17 0:35 ` pr-tracker-bot
2020-04-25 10:16 Ingo Molnar
2020-04-25 19:30 ` pr-tracker-bot
2020-06-28 18:39 Ingo Molnar
2020-06-28 22:05 ` pr-tracker-bot
2024-05-10 11:12 Ingo Molnar
2024-05-10 17:29 ` pr-tracker-bot
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