linux-kernel - Re: [PATCH] x86: fix locking and sync bugs in x86_64 RTC code in time

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Message-ID: <473AF3BE.1010006@reed.com>
Date:	Wed, 14 Nov 2007 08:10:22 -0500
From:	"David P. Reed" <dpreed@...d.com>
To:	Thomas Gleixner <tglx@...utronix.de>
CC:	linux-kernel@...r.kernel.org,
	Allessandro Zummo <a.zummo@...ertech.it>
Subject: Re: [PATCH] x86: fix locking and sync bugs in x86_64 RTC code in
 time_64.c

Will make two patches and resend.

Thomas Gleixner wrote:
> David,
>
> On Mon, 12 Nov 2007, David P. Reed wrote:
>
>   
>> From: David P. Reed
>>
>> Fix two bugs in arch/x86/kernel/time_64.c that affect the x86_64 kernel. 1) a
>> repeatable hard freeze due to interrupts when the ntpd service calls
>> update_persistent_clock(), 2) potentially unstable PC RTC timer values when
>> timer is read.
>>     
>
> 1) Please send separate patches for separate problems.
>
> 2) Your patch is white space damaged and does not apply. Please run it
>    through scripts/checkpatch.pl before submitting, maybe send it to
>    yourself first and verify that it applies correctly.
>
>   
>> Signed-off-by: David P. Reed <dpreed@...d.com>
>> ---
>> More explanation:
>> 1) A repeatable but randomly timed freeze has been happening in Fedora 6 and 7
>> for the last year, whenever I run the ntpd service on my AMD64x2 HP Pavilion
>> dv9000z laptop.  This freeze is due to the use of spin_lock(&rtc_lock) under
>> the assumption (per a bad comment) that set_rtc_mmss is called only with
>> interrupts disabled.  The call from ntp.c to update_persistent_clock is made
>> with interrupts enabled.
>>     
>
> Good catch.
>  
>   
>> 2) the use of an incorrect technique for reading the standard PC RTC timer,
>> which is documented to "disconnect" time registers from the bus while updates
>> are in progress.  The use of UIP flag while interrupts are disabled to protect
>> a 244 microsecond window is one of the Motorola spec sheet's documented ways
>> to read the RTC time registers reliably.  I realize that not all "clones" of
>> the
>>     
>
> of the what ?
>
> Also put the detailed description into the patch comment, please.
>
>   
>> The patch updates the misleading comments and minimizes the amount of time
>> that the kernel disables interrupts.
>>
>> I have thoroughly tested this patch on a number of x86_64 machines with
>> various numbers of cores and chipsets, using 2.6.24rc2 kernel source. Note
>> that while testing the ntp code I found another bug in kernel/time/ntp.c which
>> is independent of this fix - neither patch requires the other.
>>
>> If possible, I'd love to see the patch merged with 2.6.24, and even with
>> 2.6.23.
>>     
>
> Care to resend ?
>
> Thanks,
>
>      tglx
>
>   
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