linux-kernel - Re: [PATCH v1.1 2/2] thermal: sysfs: Simplifications of trip point attribute callbacks

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Message-ID: <CAJZ5v0jKOarHuhVgS_Q8Rtamt6uP8LKdbVC=moYNO=WT+Kueog@mail.gmail.com>
Date:   Thu, 30 Nov 2023 20:51:08 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:     Daniel Lezcano <daniel.lezcano@...aro.org>,
        Lukasz Luba <lukasz.luba@....com>,
        Linux PM <linux-pm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        Zhang Rui <rui.zhang@...el.com>
Subject: Re: [PATCH v1.1 2/2] thermal: sysfs: Simplifications of trip point
 attribute callbacks

Wrong subject, sorry for the noise.  Will resend.

On Thu, Nov 30, 2023 at 8:42 PM Rafael J. Wysocki <rjw@...ysocki.net> wrote:
>
> From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
>
> The _show() callback functions of the trip point sysfs attributes,
> temperature, hysteresis and type, need not use thermal zone locking,
> because the layout of the data structures they access does not change
> after the thermal zone registration.
>
> Namely, they all need to access a specific entry in the thermal
> zone's trips[] table that is always present for non-tripless thermal
> zones and its size cannot change after the thermal zone has been
> registered.  Thus it is always safe to access the trips[] table of a
> registered thermal zone from each of the sysfs attributes in question.
>
> Moreover, the type of a trip point does not change after registering its
> thermal zone, and while its temperature and hysteresis can change, for
> example due to a firmware-induced thermal zone update, holding the zone
> lock around reading them is pointless, because it does not prevent stale
> values from being returned to user space.  For example, a trip point
> temperature can always change ater trip_point_temp_show() has read it
> and before the function's return statement is executed, regardless of
> whether or not zone locking is used.
>
> For this reason, drop the zone locking from trip_point_type_show(),
> trip_point_temp_show(), and trip_point_hyst_show().