Message-ID: <20260206203712.1989610-2-longman@redhat.com>
Date: Fri,  6 Feb 2026 15:37:09 -0500
From: Waiman Long <longman@...hat.com>
To: Chen Ridong <chenridong@...weicloud.com>,
	Tejun Heo <tj@...nel.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Michal Koutný <mkoutny@...e.com>,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Juri Lelli <juri.lelli@...hat.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ben Segall <bsegall@...gle.com>,
	Mel Gorman <mgorman@...e.de>,
	Valentin Schneider <vschneid@...hat.com>,
	Anna-Maria Behnsen <anna-maria@...utronix.de>,
	Frederic Weisbecker <frederic@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Shuah Khan <shuah@...nel.org>
Cc: cgroups@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	linux-kselftest@...r.kernel.org,
	Waiman Long <longman@...hat.com>
Subject: [PATCH/for-next v4 1/4] cgroup/cpuset: Clarify exclusion rules for cpuset internal variables

Clarify the locking rules associated with file level internal variables
inside the cpuset code. There is no functional change.

Signed-off-by: Waiman Long <longman@...hat.com>
---
 kernel/cgroup/cpuset.c | 105 ++++++++++++++++++++++++-----------------
 1 file changed, 61 insertions(+), 44 deletions(-)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index c43efef7df71..a4c6386a594d 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -61,6 +61,58 @@ static const char * const perr_strings[] = {
 	[PERR_REMOTE]    = "Have remote partition underneath",
 };
 
+/*
+ * CPUSET Locking Convention
+ * -------------------------
+ *
+ * Below are the three global locks guarding cpuset structures in lock
+ * acquisition order:
+ *  - cpu_hotplug_lock (cpus_read_lock/cpus_write_lock)
+ *  - cpuset_mutex
+ *  - callback_lock (raw spinlock)
+ *
+ * A task must hold all the three locks to modify externally visible or
+ * used fields of cpusets, though some of the internally used cpuset fields
+ * and internal variables can be modified without holding callback_lock. If only
+ * reliable read access of the externally used fields are needed, a task can
+ * hold either cpuset_mutex or callback_lock which are exposed to other
+ * external subsystems.
+ *
+ * If a task holds cpu_hotplug_lock and cpuset_mutex, it blocks others,
+ * ensuring that it is the only task able to also acquire callback_lock and
+ * be able to modify cpusets.  It can perform various checks on the cpuset
+ * structure first, knowing nothing will change. It can also allocate memory
+ * without holding callback_lock. While it is performing these checks, various
+ * callback routines can briefly acquire callback_lock to query cpusets.  Once
+ * it is ready to make the changes, it takes callback_lock, blocking everyone
+ * else.
+ *
+ * Calls to the kernel memory allocator cannot be made while holding
+ * callback_lock which is a spinlock, as the memory allocator may sleep or
+ * call back into cpuset code and acquire callback_lock.
+ *
+ * Now, the task_struct fields mems_allowed and mempolicy may be changed
+ * by other task, we use alloc_lock in the task_struct fields to protect
+ * them.
+ *
+ * The cpuset_common_seq_show() handlers only hold callback_lock across
+ * small pieces of code, such as when reading out possibly multi-word
+ * cpumasks and nodemasks.
+ */
+
+static DEFINE_MUTEX(cpuset_mutex);
+
+/*
+ * File level internal variables below follow one of the following exclusion
+ * rules.
+ *
+ * RWCS: Read/write-able by holding either cpus_write_lock or both
+ *       cpus_read_lock and cpuset_mutex.
+ *
+ * CSCB: Readable by holding either cpuset_mutex or callback_lock. Writable
+ *	 by holding both cpuset_mutex and callback_lock.
+ */
+
 /*
  * For local partitions, update to subpartitions_cpus & isolated_cpus is done
  * in update_parent_effective_cpumask(). For remote partitions, it is done in
@@ -70,19 +122,18 @@ static const char * const perr_strings[] = {
  * Exclusive CPUs distributed out to local or remote sub-partitions of
  * top_cpuset
  */
-static cpumask_var_t	subpartitions_cpus;
+static cpumask_var_t	subpartitions_cpus;	/* RWCS */
 
 /*
- * Exclusive CPUs in isolated partitions
+ * Exclusive CPUs in isolated partitions (shown in cpuset.cpus.isolated)
  */
-static cpumask_var_t	isolated_cpus;
+static cpumask_var_t	isolated_cpus;		/* CSCB */
 
 /*
- * isolated_cpus updating flag (protected by cpuset_mutex)
- * Set if isolated_cpus is going to be updated in the current
- * cpuset_mutex crtical section.
+ * Set if isolated_cpus is being updated in the current cpuset_mutex
+ * critical section.
  */
-static bool isolated_cpus_updating;
+static bool		isolated_cpus_updating;	/* RWCS */
 
 /*
  * A flag to force sched domain rebuild at the end of an operation.
@@ -98,7 +149,7 @@ static bool isolated_cpus_updating;
  * Note that update_relax_domain_level() in cpuset-v1.c can still call
  * rebuild_sched_domains_locked() directly without using this flag.
  */
-static bool force_sd_rebuild;
+static bool force_sd_rebuild;			/* RWCS */
 
 /*
  * Partition root states:
@@ -218,42 +269,6 @@ struct cpuset top_cpuset = {
 	.partition_root_state = PRS_ROOT,
 };
 
-/*
- * There are two global locks guarding cpuset structures - cpuset_mutex and
- * callback_lock. The cpuset code uses only cpuset_mutex. Other kernel
- * subsystems can use cpuset_lock()/cpuset_unlock() to prevent change to cpuset
- * structures. Note that cpuset_mutex needs to be a mutex as it is used in
- * paths that rely on priority inheritance (e.g. scheduler - on RT) for
- * correctness.
- *
- * A task must hold both locks to modify cpusets.  If a task holds
- * cpuset_mutex, it blocks others, ensuring that it is the only task able to
- * also acquire callback_lock and be able to modify cpusets.  It can perform
- * various checks on the cpuset structure first, knowing nothing will change.
- * It can also allocate memory while just holding cpuset_mutex.  While it is
- * performing these checks, various callback routines can briefly acquire
- * callback_lock to query cpusets.  Once it is ready to make the changes, it
- * takes callback_lock, blocking everyone else.
- *
- * Calls to the kernel memory allocator can not be made while holding
- * callback_lock, as that would risk double tripping on callback_lock
- * from one of the callbacks into the cpuset code from within
- * __alloc_pages().
- *
- * If a task is only holding callback_lock, then it has read-only
- * access to cpusets.
- *
- * Now, the task_struct fields mems_allowed and mempolicy may be changed
- * by other task, we use alloc_lock in the task_struct fields to protect
- * them.
- *
- * The cpuset_common_seq_show() handlers only hold callback_lock across
- * small pieces of code, such as when reading out possibly multi-word
- * cpumasks and nodemasks.
- */
-
-static DEFINE_MUTEX(cpuset_mutex);
-
 /**
  * cpuset_lock - Acquire the global cpuset mutex
  *
@@ -1163,6 +1178,8 @@ static void reset_partition_data(struct cpuset *cs)
 static void isolated_cpus_update(int old_prs, int new_prs, struct cpumask *xcpus)
 {
 	WARN_ON_ONCE(old_prs == new_prs);
+	lockdep_assert_held(&callback_lock);
+	lockdep_assert_held(&cpuset_mutex);
 	if (new_prs == PRS_ISOLATED)
 		cpumask_or(isolated_cpus, isolated_cpus, xcpus);
 	else
-- 
2.52.0

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