linux-kernel - Re: [PATCH v2 01/22] rtc: pm8xxx: fix set-alarm race

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Date:   Mon, 6 Feb 2023 19:12:43 -0800
From:   David Collins <quic_collinsd@...cinc.com>
To:     Johan Hovold <johan+linaro@...nel.org>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        Bjorn Andersson <andersson@...nel.org>
CC:     Andy Gross <agross@...nel.org>,
        Konrad Dybcio <konrad.dybcio@...aro.org>,
        Alessandro Zummo <a.zummo@...ertech.it>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Maximilian Luz <luzmaximilian@...il.com>,
        <linux-arm-msm@...r.kernel.org>, <linux-rtc@...r.kernel.org>,
        <devicetree@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <stable@...r.kernel.org>
Subject: Re: [PATCH v2 01/22] rtc: pm8xxx: fix set-alarm race

On 2/2/23 07:54, Johan Hovold wrote:
> Make sure to disable the alarm before updating the four alarm time
> registers to avoid spurious alarms during the update.

What scenario can encounter a spurious alarm triggering upon writing the
new alarm time inside of pm8xxx_rtc_set_alarm()?

> Note that the disable needs to be done outside of the ctrl_reg_lock
> section to prevent a racing alarm interrupt from disabling the newly set
> alarm when the lock is released.

What scenario shows the IRQ race issue that you mentioned?  How does not
protecting this register write with a lock avoid the race condition?

> Fixes: 9a9a54ad7aa2 ("drivers/rtc: add support for Qualcomm PMIC8xxx RTC")
> Cc: stable@...r.kernel.org      # 3.1
> Signed-off-by: Johan Hovold <johan+linaro@...nel.org>
> ---
>  drivers/rtc/rtc-pm8xxx.c | 24 ++++++++++--------------
>  1 file changed, 10 insertions(+), 14 deletions(-)

Note that since locking is removed later in the patch series, my
questions above are mainly for the sake of curiosity.


Reviewed-by: David Collins <quic_collinsd@...cinc.com>

Thanks,
David