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From: Warner Losh <imp@bsdimp.com>
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Cc: Konstantin Belousov <kostikbel@gmail.com>, freebsd-arch@FreeBSD.org
Subject: Re: Fast gettimeofday(2) and clock_gettime(2)
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On Jun 6, 2012, at 12:23 PM, John Baldwin wrote:
> 2) You read tk->tk_boottime without the tk_current protection in your
>   non-uptime routines.  This is racey as the kernel alters the
>   boottime when it skews time for large adjustments from ntp, etc.

One of the 'etc' is leap seconds.

Warner