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

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Date:   Tue, 30 Apr 2019 16:39:24 +0100
From:   Valentin Schneider <valentin.schneider@....com>
To:     Marc Zyngier <marc.zyngier@....com>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        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,

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.

[...]