linux-kernel - Re: [PATCH v1 1/3] thermal: core: Make thermal_zone_device_unregister() return after freeing the zone

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Message-ID: <CAJZ5v0heWWM8Jt-8ksVDF8R4Uc6FvsspEOAEeN-nQDgwPfHU8Q@mail.gmail.com>
Date:   Mon, 11 Dec 2023 18:54:54 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Daniel Lezcano <daniel.lezcano@...aro.org>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Linux PM <linux-pm@...r.kernel.org>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        Zhang Rui <rui.zhang@...el.com>,
        Linux ACPI <linux-acpi@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Lukasz Luba <lukasz.luba@....com>
Subject: Re: [PATCH v1 1/3] thermal: core: Make thermal_zone_device_unregister()
 return after freeing the zone

On Mon, Dec 11, 2023 at 6:35 PM Daniel Lezcano
<daniel.lezcano@...aro.org> wrote:
>
> On 11/12/2023 17:42, Rafael J. Wysocki wrote:
> > On Mon, Dec 11, 2023 at 5:28 PM Daniel Lezcano
> > <daniel.lezcano@...aro.org> wrote:
> >>
> >> On 08/12/2023 20:13, Rafael J. Wysocki wrote:
> >>> From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> >>>
> >>> Make thermal_zone_device_unregister() wait until all of the references
> >>> to the given thermal zone object have been dropped and free it before
> >>> returning.
> >>>
> >>> This guarantees that when thermal_zone_device_unregister() returns,
> >>> there is no leftover activity regarding the thermal zone in question
> >>> which is required by some of its callers (for instance, modular driver
> >>> code that wants to know when it is safe to let the module go away).
> >>>
> >>> Subsequently, this will allow some confusing device_is_registered()
> >>> checks to be dropped from the thermal sysfs and core code.
> >>>
> >>> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> >>> ---
> >>
> >> Definitively agree on the change
> >>
> >> Acked-by: Daniel Lezcano <daniel.lezcano@...aro.org>
> >
> > Thanks!
> >
> >> Would it make sense to use kref_get/put ?
> >
> > Why and where?
>
> Well it is a general question. Usually this kind of removal is tied with
> a refcount

It is tied to a refcount already, but the problem is that the last
reference can be dropped from a thread concurrent to the removal one.

The completion effectively causes the removal thread to wait for the
refcont to become 0.