linux-kernel - [PATCH v3] cpufreq: fix locking order in store_local

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Message-ID: <20250430160943.2836-1-ImanDevel@gmail.com>
Date: Wed, 30 Apr 2025 12:09:43 -0400
From: Seyediman Seyedarab <imandevel@...il.com>
To: rafael@...nel.org,
	viresh.kumar@...aro.org
Cc: linux-pm@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	skhan@...uxfoundation.org,
	linux-kernel-mentees@...ts.linux.dev,
	Seyediman Seyedarab <ImanDevel@...il.com>
Subject: [PATCH v3] cpufreq: fix locking order in store_local_boost to prevent deadlock

Lockdep reports a possible circular locking dependency[1] when
writing to /sys/devices/system/cpu/cpufreq/policyN/boost,
triggered by power-profiles-daemon at boot.

store_local_boost() used to acquire cpu_hotplug_lock *after*
the policy lock had already been taken by the store() handler.
However, the expected locking hierarchy is to acquire
cpu_hotplug_lock before the policy guard. This inverted lock order
creates a *theoretical* deadlock possibility.

Acquire cpu_hotplug_lock in the store() handler *only* for the
local_boost attribute, before entering the policy guard block,
and remove the cpus_read_lock/unlock() calls from store_local_boost().
Also switch from guard() to scoped_guard() to allow explicitly wrapping
the policy guard inside the cpu_hotplug_lock critical section.

 [1]
 ======================================================
 WARNING: possible circular locking dependency detected
 6.15.0-rc4-debug #28 Not tainted
 ------------------------------------------------------
 power-profiles-/596 is trying to acquire lock:
 ffffffffb147e910 (cpu_hotplug_lock){++++}-{0:0}, at: store_local_boost+0x6a/0xd0

 but task is already holding lock:
 ffff9eaa48377b80 (&policy->rwsem){++++}-{4:4}, at: store+0x37/0x90

 which lock already depends on the new lock.

 the existing dependency chain (in reverse order) is:

 -> #2 (&policy->rwsem){++++}-{4:4}:
        down_write+0x29/0xb0
        cpufreq_online+0x841/0xa00
        cpufreq_add_dev+0x71/0x80
        subsys_interface_register+0x14b/0x170
        cpufreq_register_driver+0x154/0x250
        amd_pstate_register_driver+0x36/0x70
        amd_pstate_init+0x1e7/0x270
        do_one_initcall+0x67/0x2c0
        kernel_init_freeable+0x230/0x270
        kernel_init+0x15/0x130
        ret_from_fork+0x2c/0x50
        ret_from_fork_asm+0x11/0x20

 -> #1 (subsys mutex#3){+.+.}-{4:4}:
        __mutex_lock+0xc2/0x930
        subsys_interface_register+0x83/0x170
        cpufreq_register_driver+0x154/0x250
        amd_pstate_register_driver+0x36/0x70
        amd_pstate_init+0x1e7/0x270
        do_one_initcall+0x67/0x2c0
        kernel_init_freeable+0x230/0x270
        kernel_init+0x15/0x130
        ret_from_fork+0x2c/0x50
        ret_from_fork_asm+0x11/0x20

 -> #0 (cpu_hotplug_lock){++++}-{0:0}:
        __lock_acquire+0x1087/0x17e0
        lock_acquire.part.0+0x66/0x1b0
        cpus_read_lock+0x2a/0xc0
        store_local_boost+0x6a/0xd0
        store+0x50/0x90
        kernfs_fop_write_iter+0x135/0x200
        vfs_write+0x2ab/0x540
        ksys_write+0x6c/0xe0
        do_syscall_64+0xbb/0x1d0
        entry_SYSCALL_64_after_hwframe+0x56/0x5e

Signed-off-by: Seyediman Seyedarab <ImanDevel@...il.com>
---
Changes in v3:
 - Rebased over PM tree's linux-next branch
 - Added a comment to explain why this piece of code is required
 - Switched from guard() to scoped_guard() to allow explicitly wrapping
   the policy guard inside the cpu_hotplug_lock critical section.

Changes in v2:
 - Restrict cpu_hotplug_lock acquisition to only
   the local_boost attribute in store() handler.

Regards,
Seyediman

 drivers/cpufreq/cpufreq.c | 23 ++++++++++++++++-------
 1 file changed, 16 insertions(+), 7 deletions(-)

diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
index 21fa733a2..b349adbeb 100644
--- a/drivers/cpufreq/cpufreq.c
+++ b/drivers/cpufreq/cpufreq.c
@@ -622,10 +622,7 @@ static ssize_t store_local_boost(struct cpufreq_policy *policy,
 	if (!policy->boost_supported)
 		return -EINVAL;
 
-	cpus_read_lock();
 	ret = policy_set_boost(policy, enable);
-	cpus_read_unlock();
-
 	if (!ret)
 		return count;
 
@@ -1006,16 +1003,28 @@ static ssize_t store(struct kobject *kobj, struct attribute *attr,
 {
 	struct cpufreq_policy *policy = to_policy(kobj);
 	struct freq_attr *fattr = to_attr(attr);
+	int ret = -EBUSY;
 
 	if (!fattr->store)
 		return -EIO;
 
-	guard(cpufreq_policy_write)(policy);
+	/*
+	 * store_local_boost() requires cpu_hotplug_lock to be held, and must be
+	 * called with that lock acquired *before* taking policy->rwsem to avoid
+	 * lock ordering violations.
+	 */
+	if (fattr == &local_boost)
+		cpus_read_lock();
 
-	if (likely(!policy_is_inactive(policy)))
-		return fattr->store(policy, buf, count);
+	scoped_guard(cpufreq_policy_write, policy) {
+		if (likely(!policy_is_inactive(policy)))
+			ret = fattr->store(policy, buf, count);
+	}
 
-	return -EBUSY;
+	if (fattr == &local_boost)
+		cpus_read_unlock();
+
+	return ret;
 }
 
 static void cpufreq_sysfs_release(struct kobject *kobj)
-- 
2.49.0