Re: [PATCH 2/2] arm64: VDSO: fix coarse clock monotonicity regression

[Catalin Marinas]

On Mon, Aug 10, 2015 at 10:22:53AM +0100, Will Deacon wrote:
> On Sat, Aug 08, 2015 at 03:03:23AM +0100, Nathan Lynch wrote:
> > Since 906c55579a63 ("timekeeping: Copy the shadow-timekeeper over the
> > real timekeeper last") it has become possible on arm64 to:
> > 
> > - Obtain a CLOCK_MONOTONIC_COARSE or CLOCK_REALTIME_COARSE timestamp
> >   via syscall.
> > - Subsequently obtain a timestamp for the same clock ID via VDSO which
> >   predates the first timestamp (by one jiffy).
> > 
> > This is because arm64's update_vsyscall is deriving the coarse time
> > using the __current_kernel_time interface, when it should really be
> > using the timekeeper object provided to it by the timekeeping core.
> > It happened to work before only because __current_kernel_time would
> > access the same timekeeper object which had been passed to
> > update_vsyscall.  This is no longer the case.
> > 
> > Signed-off-by: Nathan Lynch <nathan_lynch@xxxxxxxxxx>
> > ---
> >  arch/arm64/kernel/vdso.c | 7 +++----
> >  1 file changed, 3 insertions(+), 4 deletions(-)
> > 
> > diff --git a/arch/arm64/kernel/vdso.c b/arch/arm64/kernel/vdso.c
> > index ec37ab3f524f..97bc68f4c689 100644
> > --- a/arch/arm64/kernel/vdso.c
> > +++ b/arch/arm64/kernel/vdso.c
> > @@ -199,16 +199,15 @@ int arch_setup_additional_pages(struct linux_binprm *bprm,
> >   */
> >  void update_vsyscall(struct timekeeper *tk)
> >  {
> > -	struct timespec xtime_coarse;
> >  	u32 use_syscall = strcmp(tk->tkr_mono.clock->name, "arch_sys_counter");
> >  
> >  	++vdso_data->tb_seq_count;
> >  	smp_wmb();
> >  
> > -	xtime_coarse = __current_kernel_time();
> >  	vdso_data->use_syscall			= use_syscall;
> > -	vdso_data->xtime_coarse_sec		= xtime_coarse.tv_sec;
> > -	vdso_data->xtime_coarse_nsec		= xtime_coarse.tv_nsec;
> > +	vdso_data->xtime_coarse_sec		= tk->xtime_sec;
> > +	vdso_data->xtime_coarse_nsec		= tk->tkr_mono.xtime_nsec >>
> > +							tk->tkr_mono.shift;
> >  	vdso_data->wtm_clock_sec		= tk->wall_to_monotonic.tv_sec;
> >  	vdso_data->wtm_clock_nsec		= tk->wall_to_monotonic.tv_nsec;
> 
> Looks good,
> 
>   Acked-by: Will Deacon <will.deacon@xxxxxxx>
> 
> There's probably still time for Catalin to pick this up for 4.2.

Applied, I'll send a pull request today/tomorrow. Thanks.

-- 
Catalin
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