RE: [chrony-dev] [PATCH] More reliable method of reading rtc for initial trim

["Hattink, Tjalling [FINT]" <T.Hattink@xxxxxxxx>]

Hi,

Looks like my patch was still malformed, you have to remove some
line-ending breaks to make it work. I hoped that the max length was 80,
but everything after 70 is moved to the next line.

Tjalling Hattink

> -----Original Message-----
> From: Hattink, Tjalling [FINT] [mailto:T.Hattink@xxxxxxxx]
> Sent: Wednesday, May 28, 2014 13:24
> To: chrony-dev@xxxxxxxxxxxxxxxxxxxx
> Subject: [chrony-dev] [PATCH] More reliable method of reading rtc for
> initial trim
> 
> When chrony reads in the linux rtc for the first time to trim the
> system
> clock, it only reads it once. As it is possible that the rtc updates
> itself
> during the read operation, the reported rtc time could be false. To
> prevent
> this I've added a loop that reads the rtc clock twice, if the seconds
> do
> not
> match retry the two read operations. If they match you can assume the
> read
> operation was successful.
> 
> This is based on the hwclock implementation of reading the rtc clock
> from the
> util-linux package.
> ---
>  rtc_linux.c | 12 ++++++++++--
>  1 file changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/rtc_linux.c b/rtc_linux.c
> index 91b0cac..add828f 100644
> --- a/rtc_linux.c
> +++ b/rtc_linux.c
> @@ -971,7 +971,7 @@ void
>  RTC_Linux_TimePreInit(void)
>  {
>    int fd, status;
> -  struct rtc_time rtc_raw;
> +  struct rtc_time rtc_raw, rtc_raw_retry;
>    struct tm rtc_tm;
>    time_t rtc_t, estimated_correct_rtc_t;
>    long interval;
> @@ -989,7 +989,15 @@ RTC_Linux_TimePreInit(void)
>      return; /* Can't open it, and won't be able to later */
>    }
> 
> -  status = ioctl(fd, RTC_RD_TIME, &rtc_raw);
> +  /* Retry reading the rtc until both read attempts give the same sec
> value.
> +     This way the race condition is prevented that the RTC has
updated
> itself
> +     during the first read operation. */
> +  do {
> +    status = ioctl(fd, RTC_RD_TIME, &rtc_raw);
> +    if (status >= 0) {
> +      status = ioctl(fd, RTC_RD_TIME, &rtc_raw_retry);
> +    }
> +  } while (status >= 0 && rtc_raw.tm_sec != rtc_raw_retry.tm_sec);
> 
>    if (status >= 0) {
>      /* Convert to seconds since 1970 */
> --
> 1.9.1


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