Re: [RFC][PATCH] Use proper casting with signed timespec.tv_nsec values
From: George Anzinger (george_at_mvista.com)
Date: 09/02/05
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Date: Thu, 01 Sep 2005 18:14:33 -0700 To: john stultz <johnstul@us.ibm.com>
john stultz wrote:
> All,
> I recently ran into a bug with an older kernel where xtime's tv_nsec
> field had accumulated more then 2 seconds worth of time. The timespec's
> tv_nsec is a signed long, however gettimeofday() treats it as an
> unsigned long. Thus when the failure occured, very strange and difficult
> to debug time problems occurred.
>
> The main cause of the problem I was seeing is already fixed in mainline,
> however just to be safe, I figured the following patch would be wise.
>
> I only audited i386 and x86_64, however other arches probably could have
> similar signed problems as well.
>
> Please let me know if you have any further comments or feedback.
John,
There is a problem in the way this code handles the conversion to usec.
There is a conversion here and also in the get_offset code. If the
nanoseconds are carrier until after the addition of the two about 25% of
the time you will end up with an additional usec in time. I strongly
suggest changing to convert to usec after the addition of xtime and
get_offset time to avoid this. If the "correct" thing is done in
clock_gettime() (i.e. get_offset is in nanoseconds) this actually turns
up as a back step in time WRT gettimeofday and clock_gettime().
George
--
>
> thanks
> -john
>
> linux-2.6.13_signed-tv_nsec_A0.patch
> ====================================
> diff --git a/arch/i386/kernel/time.c b/arch/i386/kernel/time.c
> --- a/arch/i386/kernel/time.c
> +++ b/arch/i386/kernel/time.c
> @@ -156,7 +156,7 @@ void do_gettimeofday(struct timeval *tv)
> usec += lost * (USEC_PER_SEC / HZ);
>
> sec = xtime.tv_sec;
> - usec += (xtime.tv_nsec / 1000);
> + usec += (unsigned long)xtime.tv_nsec / 1000;
> } while (read_seqretry(&xtime_lock, seq));
>
> while (usec >= 1000000) {
> diff --git a/arch/x86_64/kernel/time.c b/arch/x86_64/kernel/time.c
> --- a/arch/x86_64/kernel/time.c
> +++ b/arch/x86_64/kernel/time.c
> @@ -128,7 +128,7 @@ void do_gettimeofday(struct timeval *tv)
> seq = read_seqbegin(&xtime_lock);
>
> sec = xtime.tv_sec;
> - usec = xtime.tv_nsec / 1000;
> + usec = (unsigned long)xtime.tv_nsec / 1000;
>
> /* i386 does some correction here to keep the clock
> monotonous even when ntpd is fixing drift.
> diff --git a/kernel/timer.c b/kernel/timer.c
> --- a/kernel/timer.c
> +++ b/kernel/timer.c
> @@ -824,7 +824,7 @@ static void update_wall_time(unsigned lo
> do {
> ticks--;
> update_wall_time_one_tick();
> - if (xtime.tv_nsec >= 1000000000) {
> + if ((unsigned long)xtime.tv_nsec >= 1000000000) {
> xtime.tv_nsec -= 1000000000;
> xtime.tv_sec++;
> second_overflow();
>
>
> -
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>
--
George Anzinger george@mvista.com
HRT (High-res-timers): http://sourceforge.net/projects/high-res-timers/
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- Previous message: Damir Perisa: "2.6.13-mm1 - drivers/serial/jsm/jsm_tty broken too"
- In reply to: john stultz: "[RFC][PATCH] Use proper casting with signed timespec.tv_nsec values"
- Next in thread: Christoph Lameter: "Re: [RFC][PATCH] Use proper casting with signed timespec.tv_nsec values"
- Messages sorted by: [ date ] [ thread ] [ subject ] [ author ]
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