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

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Date:	Mon, 10 Aug 2015 10:22:53 +0100
From:	Will Deacon <will.deacon@....com>
To:	Nathan Lynch <nathan_lynch@...tor.com>
Cc:	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Russell King <linux@....linux.org.uk>,
	John Stultz <john.stultz@...aro.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	catalin.marinas@....com
Subject: Re: [PATCH 2/2] arm64: VDSO: fix coarse clock monotonicity regression

Hi Nathan,

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@...tor.com>
> ---
>  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@....com>

There's probably still time for Catalin to pick this up for 4.2.

Will
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