linux-kernel - [PATCH v1] thermal: trip: Remove lockdep assertion from for_each_thermal

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Message-ID: <2709769.mvXUDI8C0e@kreacher>
Date:   Sat, 07 Oct 2023 13:34:35 +0200
From:   "Rafael J. Wysocki" <rjw@...ysocki.net>
To:     Linux PM <linux-pm@...r.kernel.org>
Cc:     Daniel Lezcano <daniel.lezcano@...aro.org>,
        Linux ACPI <linux-acpi@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        Zhang Rui <rui.zhang@...el.com>
Subject: [PATCH v1] thermal: trip: Remove lockdep assertion from for_each_thermal_trip()

From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>

The lockdep assertion in for_each_thermal_trip() was added to possibly
catch incorrect usage of that function without the thermal zone lock.
However, it turns out that the ACPI thermal driver has a legitimate
reason to call for_each_thermal_trip() without locking.

Namely, it is called by acpi_thermal_bind_unbind_cdev() in the thermal
zone registration and unregistration paths.  That function cannot acquire
the thermal zone lock by itself, because it calls functions that acquire
it, thermal_bind_cdev_to_trip() or thermal_unbind_cdev_from_trip().
However, it is invoked when the ACPI notify handler for the thermal
zone in question has not been registered yet (in the registration path)
or after that handler has been unregistered (in the unregistration
path).  Therefore, when for_each_thermal_trip() is called by
acpi_thermal_bind_unbind_cdev(), thermal trip changes induced by the
platform firmware cannot take place and so the thermal zone's trips[]
table is effectively immutable.  Hence, it is valid to call
for_each_thermal_trip() from acpi_thermal_bind_unbind_cdev() without
locking and the lockdep assertion in the former is in fact incorrect, so
remove it.

Fixes: d5ea889246b1 ("ACPI: thermal: Do not use trip indices for cooling device binding")
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
---
 drivers/thermal/thermal_trip.c |    2 --
 1 file changed, 2 deletions(-)

Index: linux-pm/drivers/thermal/thermal_trip.c
===================================================================
--- linux-pm.orig/drivers/thermal/thermal_trip.c
+++ linux-pm/drivers/thermal/thermal_trip.c
@@ -15,8 +15,6 @@ int for_each_thermal_trip(struct thermal
 {
 	int i, ret;

-	lockdep_assert_held(&tz->lock);
-
 	for (i = 0; i < tz->num_trips; i++) {
 		ret = cb(&tz->trips[i], data);
 		if (ret)