linux-kernel - Re: [PATCH 2/3] x86/vdso: Rearrange do

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Message-Id: <9CF548E6-37EC-450D-A670-7826BC9F7F08@amacapital.net>
Date:   Fri, 5 Oct 2018 06:05:19 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Andy Lutomirski <luto@...nel.org>, x86@...nel.org,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/3] x86/vdso: Rearrange do_hres() to improve code generation



> On Oct 4, 2018, at 11:00 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
> 
>> On Thu, 4 Oct 2018, Andy Lutomirski wrote:
>> index 18c8a78d1ec9..419de7552c2f 100644
>> --- a/arch/x86/entry/vdso/vclock_gettime.c
>> +++ b/arch/x86/entry/vdso/vclock_gettime.c
>> @@ -147,10 +147,9 @@ notrace static int do_hres(clockid_t clk, struct timespec *ts)
>> 
>>    do {
>>        seq = gtod_read_begin(gtod);
>> -        ts->tv_sec = base->sec;
>> +        cycles = vgetcyc(gtod->vclock_mode);
>>        ns = base->nsec;
>>        last = gtod->cycle_last;
>> -        cycles = vgetcyc(gtod->vclock_mode);
>>        if (unlikely((s64)cycles < 0))
>>            return vdso_fallback_gettime(clk, ts);
>>        if (cycles > last)
>> @@ -158,7 +157,7 @@ notrace static int do_hres(clockid_t clk, struct timespec *ts)
>>        ns >>= gtod->shift;
>>    } while (unlikely(gtod_read_retry(gtod, seq)));
>> 
>> -    ts->tv_sec += __iter_div_u64_rem(ns, NSEC_PER_SEC, &ns);
>> +    ts->tv_sec = base->sec + __iter_div_u64_rem(ns, NSEC_PER_SEC, &ns);
> 
> You cannot access base->sec outside of the seqcount protected region. It
> might have been incremented by now and you'll get a time jump by a full
> second.

Duh. Let me try this again.

> 
> Thanks,
> 
>    tglx
> 
>