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devfreq: governor_bw_hwmon: fix deadlock warning due to state_lock usage

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lockdep is detecting possible circular locking dependency
due to state_lock mutex as shown below:

Possible unsafe locking scenario:

      CPU0                    CPU1
      ----                    ----
 lock(devfreq_list_lock);
                              lock(state_lock#2);
                              lock(devfreq_list_lock);
 lock(state_lock#2);

*** DEADLOCK ***

Below is partial call stacks (in reverse order) showing relevant
locking paths:

Call stack for CPU0:
devfreq_bw_hwmon_ev_handler+0x4c/0x5e0	[may acquire &state_lock]
devfreq_add_device+0x418/0x538		[may acquire &devfreq_list_lock]
devfreq_add_icc+0x41c/0x528
devfreq_icc_probe+0x20/0x30

Call stack for CPU1:
devfreq_add_governor+0x3c/0x260 	[may acquire &devfreq_list_lock]
register_bw_hwmon+0x1e8/0x248		[may acquire &state_lock]
bimc_bwmon_driver_probe+0x310/0x408

Practically, this race is not possible, since devfreq_add_device first
tries to find the governor, and only if it is succeeds,
devfreq_bw_hwmon_ev_handler is called. And the governor would be found
only if devfreq_add_governor has added it priorly.
We have mechanism(initcall_level) in place to make sure that
governor is added before devfreq_add_device happens.

In attempt to quiet the lockdep warning, below fix is adopted:
Since state_lock mutex has different purpose, introduce a new
event_handle_lock mutex for devfreq_bw_hwmon_ev_handler
to avoid this warning.

Change-Id: I53c4514aa2357bf39e9405683f5e73ada0160ba5
Signed-off-by: Abhishek Shah <abhshah@codeaurora.org>
This commit is contained in:
Abhishek Shah 2021-08-16 16:47:52 +05:30 • committed by Alexander Winkowski
commit ae1efd457b
No known key found for this signature in database
GPG key ID: 72762A66704CDE44
1 changed files with 5 additions and 3 deletions
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8
drivers/devfreq/governor_bw_hwmon.c
View file

@ -1,6 +1,6 @@
// SPDX-License-Identifier: GPL-2.0-only
/*
* Copyright (c) 2013-2020, The Linux Foundation. All rights reserved.
* Copyright (c) 2013-2021, The Linux Foundation. All rights reserved.
*/
#define pr_fmt(fmt) "bw-hwmon: " fmt
@ -80,6 +80,8 @@ static DEFINE_MUTEX(list_lock);
static int use_cnt;
static DEFINE_MUTEX(state_lock);
static DEFINE_MUTEX(event_handle_lock);
#define show_attr(name) \
static ssize_t name##_show(struct device *dev, \
struct device_attribute *attr, char *buf) \
@ -865,7 +867,7 @@ static int devfreq_bw_hwmon_ev_handler(struct devfreq *df,
struct hwmon_node *node;
struct bw_hwmon *hw;
mutex_lock(&state_lock);
mutex_lock(&event_handle_lock);
switch (event) {
case DEVFREQ_GOV_START:
@ -939,7 +941,7 @@ static int devfreq_bw_hwmon_ev_handler(struct devfreq *df,
}
out:
mutex_unlock(&state_lock);
mutex_unlock(&event_handle_lock);
return ret;
}