linux-kernel - Re: [PATCH 1/3] lib/vdso: Delay mask application in do

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Date:   Wed, 26 Jun 2019 10:25:20 +0100
From:   Vincenzo Frascino <vincenzo.frascino@....com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     linux-arch@...r.kernel.org,
        LAK <linux-arm-kernel@...ts.infradead.org>,
        LKML <linux-kernel@...r.kernel.org>, linux-mips@...r.kernel.org,
        linux-kselftest@...r.kernel.org, catalin.marinas@....com,
        Will Deacon <will.deacon@....com>,
        Arnd Bergmann <arnd@...db.de>, linux@...linux.org.uk,
        Ralf Baechle <ralf@...ux-mips.org>, paul.burton@...s.com,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        salyzyn@...roid.com, pcc@...gle.com, shuah@...nel.org,
        0x7f454c46@...il.com, linux@...musvillemoes.dk,
        huw@...eweavers.com, sthotton@...vell.com, andre.przywara@....com,
        Andy Lutomirski <luto@...nel.org>
Subject: Re: [PATCH 1/3] lib/vdso: Delay mask application in do_hres()

Hi Thomas,

On 26/06/2019 07:38, Thomas Gleixner wrote:
> On Tue, 25 Jun 2019, Thomas Gleixner wrote:
>> On Tue, 25 Jun 2019, Vincenzo Frascino wrote:
>>> do_hres() in the vDSO generic library masks the hw counter value
>>> immediately after reading it.
>>>
>>> Postpone the mask application after checking if the syscall fallback is
>>> enabled, in order to be able to detect a possible fallback for the
>>> architectures that have masks smaller than ULLONG_MAX.
>>
>> Right. This only worked on x86 because the mask is there ULLONG_MAX for all
>> VDSO capable clocksources, i.e. that ever worked just by chance.
> 
> But it's actually worse than that:
> 
>>> +		cycles &= vd->mask;
>>>  		if (cycles > last)
>>>  			ns += (cycles - last) * vd->mult;
>>>  		ns >>= vd->shift;
> 
> This is broken for any clocksource which can legitimately wrap around. The
> core timekeeping does the right thing:
> 
>      		 (cycles - last) & mask
> 
> That makes sure that a wraparound is correctly handled. With the above the
> wrap around would be ignored due to
> 
>      	    if (cycles > last)
> 

You are right. Thanks for spotting it.


...

-- 
Regards,
Vincenzo