linux-kernel - Re: [PATCH] vsyscall: use __iter_div_u64

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Date:   Wed, 10 Jul 2019 09:56:51 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        linux-arch@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-mips@...r.kernel.org, linux-kselftest@...r.kernel.org,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Russell King <linux@...linux.org.uk>,
        Ralf Baechle <ralf@...ux-mips.org>,
        Paul Burton <paul.burton@...s.com>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Mark Salyzyn <salyzyn@...roid.com>,
        Peter Collingbourne <pcc@...gle.com>,
        Shuah Khan <shuah@...nel.org>,
        Dmitry Safonov <0x7f454c46@...il.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Huw Davies <huw@...eweavers.com>, linux-kernel@...r.kernel.org,
        clang-built-linux@...glegroups.com
Subject: Re: [PATCH] vsyscall: use __iter_div_u64_rem()

On Wed, Jul 10, 2019 at 03:01:53PM +0200, Arnd Bergmann wrote:
> On 32-bit x86 when building with clang-9, the loop gets turned back into
> an inefficient division that causes a link error:
> 
> kernel/time/vsyscall.o: In function `update_vsyscall':
> vsyscall.c:(.text+0xe3): undefined reference to `__udivdi3'
> 
> Use the provided __iter_div_u64_rem() function that is meant to address
> the same case in other files.
> 
> Fixes: 44f57d788e7d ("timekeeping: Provide a generic update_vsyscall() implementation")
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> ---
>  kernel/time/vsyscall.c | 6 +-----
>  1 file changed, 1 insertion(+), 5 deletions(-)
> 
> diff --git a/kernel/time/vsyscall.c b/kernel/time/vsyscall.c
> index a80893180826..8cf3596a4ce6 100644
> --- a/kernel/time/vsyscall.c
> +++ b/kernel/time/vsyscall.c
> @@ -104,11 +104,7 @@ void update_vsyscall(struct timekeeper *tk)
>  	vdso_ts->sec	= tk->xtime_sec + tk->wall_to_monotonic.tv_sec;
>  	nsec		= tk->tkr_mono.xtime_nsec >> tk->tkr_mono.shift;
>  	nsec		= nsec + tk->wall_to_monotonic.tv_nsec;
> -	while (nsec >= NSEC_PER_SEC) {
> -		nsec = nsec - NSEC_PER_SEC;
> -		vdso_ts->sec++;
> -	}
> -	vdso_ts->nsec	= nsec;
> +	vdso_ts->sec	+= __iter_div_u64_rem(nsec, NSEC_PER_SEC, &vdso_ts->nsec);
>  
>  	if (__arch_use_vsyscall(vdata))
>  		update_vdso_data(vdata, tk);
> -- 
> 2.20.0
> 

What an interesting function. Looks good to me and I can confirm it
fixes the link error.

Reviewed-by: Nathan Chancellor <natechancellor@...il.com>
Tested-by: Nathan Chancellor <natechancellor@...il.com>