linux-kernel - Re: [PATCH 7/7] clocksource/arm_arch_timer: Use arch_timer_read

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Message-ID: <c8449908-1f8a-a10e-bfce-95bd7415e523@linaro.org>
Date:   Fri, 3 May 2019 22:32:24 +0200
From:   Daniel Lezcano <daniel.lezcano@...aro.org>
To:     Valentin Schneider <valentin.schneider@....com>,
        Marc Zyngier <marc.zyngier@....com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Cc:     Russell King <linux@....linux.org.uk>,
        Will Deacon <will.deacon@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Mark Rutland <mark.rutland@....com>,
        Wim Van Sebroeck <wim@...ux-watchdog.org>,
        Guenter Roeck <linux@...ck-us.net>
Subject: Re: [PATCH 7/7] clocksource/arm_arch_timer: Use
 arch_timer_read_counter to access stable counters


Hi Valentin,

On 30/04/2019 17:39, Valentin Schneider wrote:
> Hi,
> 
> On 30/04/2019 16:27, Marc Zyngier wrote:
> [...]
>>>> @@ -372,6 +392,7 @@ static u32 notrace sun50i_a64_read_cntv_tval_el0(void)
>>>>  DEFINE_PER_CPU(const struct arch_timer_erratum_workaround *, timer_unstable_counter_workaround);
>>>>  EXPORT_SYMBOL_GPL(timer_unstable_counter_workaround);
>>>>  
>>>> +static atomic_t timer_unstable_counter_workaround_in_use = ATOMIC_INIT(0);
>>>
>>> Wouldn't make sense to READ_ONCE / WRITE_ONCE instead of using an atomic?
>>
>> I don't think *_ONCE says anything about the atomicity of the access. It
>> only instruct the compiler that this should only be accessed once, and
>> not reloaded/rewritten.
> 
> FWIW 7bd3e239d6c6 ("locking: Remove atomicy checks from {READ,WRITE}_ONCE")
> points this out.

Interesting, thanks for the pointer.

  -- Daniel


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