From: John Stultz <john.stultz@linaro.org>
Date: Fri, 2 Mar 2012 06:11:09 +0000 (-0800)
Subject: time: x86: Fix race switching from vsyscall to non-vsyscall clock
X-Git-Tag: v3.4-rc1~44^2~17
X-Git-Url: http://git.openpandora.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=a939e817aa7e199d2fff05a67cb745be32dd5c2d;p=pandora-kernel.git

time: x86: Fix race switching from vsyscall to non-vsyscall clock

When switching from a vsyscall capable to a non-vsyscall capable
clocksource, there was a small race, where the last vsyscall
gettimeofday before the switch might return a invalid time value
using the new non-vsyscall enabled clocksource values after the
switch is complete.

This is due to the vsyscall code checking the vclock_mode once
outside of the seqcount protected section. After it reads the
vclock mode, it doesn't re-check that the sampled clock data
that is obtained in the seqcount critical section still matches.

The fix is to sample vclock_mode inside the protected section,
and as long as it isn't VCLOCK_NONE, return the calculated
value. If it has changed and is now VCLOCK_NONE, fall back
to the syscall gettime calculation.

v2:
  * Cleanup checks as suggested by tglx
  * Also fix same issue present in gettimeofday path

CC: Andy Lutomirski <luto@amacapital.net>
CC: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: John Stultz <john.stultz@linaro.org>
---

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