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    Subject[patch 046/108] x86: move rdtsc_barrier() into the TSC vread method
    2.6.30-stable review patch.  If anyone has any objections, please let us know.

    ------------------

    From: Petr Tesarik <ptesarik@suse.cz>

    commit 7d96fd41cadc55f4e00231c8c71b8e25c779f122 upstream.

    The *fence instructions were moved to vsyscall_64.c by commit
    cb9e35dce94a1b9c59d46224e8a94377d673e204. But this breaks the
    vDSO, because vread methods are also called from there.

    Besides, the synchronization might be unnecessary for other
    time sources than TSC.

    [ Impact: fix potential time warp in VDSO ]

    Signed-off-by: Petr Tesarik <ptesarik@suse.cz>
    LKML-Reference: <9d0ea9ea0f866bdc1f4d76831221ae117f11ea67.1243241859.git.ptesarik@suse.cz>
    Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
    Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

    ---
    arch/x86/kernel/tsc.c | 11 ++++++++++-
    arch/x86/kernel/vsyscall_64.c | 8 --------
    2 files changed, 10 insertions(+), 9 deletions(-)

    --- a/arch/x86/kernel/tsc.c
    +++ b/arch/x86/kernel/tsc.c
    @@ -710,7 +710,16 @@ static cycle_t read_tsc(struct clocksour
    #ifdef CONFIG_X86_64
    static cycle_t __vsyscall_fn vread_tsc(void)
    {
    - cycle_t ret = (cycle_t)vget_cycles();
    + cycle_t ret;
    +
    + /*
    + * Surround the RDTSC by barriers, to make sure it's not
    + * speculated to outside the seqlock critical section and
    + * does not cause time warps:
    + */
    + rdtsc_barrier();
    + ret = (cycle_t)vget_cycles();
    + rdtsc_barrier();

    return ret >= __vsyscall_gtod_data.clock.cycle_last ?
    ret : __vsyscall_gtod_data.clock.cycle_last;
    --- a/arch/x86/kernel/vsyscall_64.c
    +++ b/arch/x86/kernel/vsyscall_64.c
    @@ -132,15 +132,7 @@ static __always_inline void do_vgettimeo
    return;
    }

    - /*
    - * Surround the RDTSC by barriers, to make sure it's not
    - * speculated to outside the seqlock critical section and
    - * does not cause time warps:
    - */
    - rdtsc_barrier();
    now = vread();
    - rdtsc_barrier();
    -
    base = __vsyscall_gtod_data.clock.cycle_last;
    mask = __vsyscall_gtod_data.clock.mask;
    mult = __vsyscall_gtod_data.clock.mult;



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