[PATCH -next RFC 01/11] cpuset: move the root cpuset write check earlier

[Chen Ridong <chenridong@huaweicloud.com>]

From: Chen Ridong <chenridong@huawei.com>

The 'cpus' or 'mems' lists of the top_cpuset cannot be modified.
This check can be moved before acquiring any locks as a common code
block to improve efficiency and maintainability.

Signed-off-by: Chen Ridong <chenridong@huawei.com>
---
 kernel/cgroup/cpuset.c | 22 +++++++++-------------
 1 file changed, 9 insertions(+), 13 deletions(-)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index a78ccd11ce9b..7e6a40e361ea 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -337,6 +337,11 @@ static inline bool cpuset_v2(void)
 		cgroup_subsys_on_dfl(cpuset_cgrp_subsys);
 }
 
+static inline bool cpuset_is_root(struct cpuset *cs)
+{
+	return (cs == &top_cpuset);
+}
+
 /*
  * Cgroup v2 behavior is used on the "cpus" and "mems" control files when
  * on default hierarchy or when the cpuset_v2_mode flag is set by mounting
@@ -2334,10 +2339,6 @@ static int update_cpumask(struct cpuset *cs, struct cpuset *trialcs,
 	bool force = false;
 	int old_prs = cs->partition_root_state;
 
-	/* top_cpuset.cpus_allowed tracks cpu_active_mask; it's read-only */
-	if (cs == &top_cpuset)
-		return -EACCES;
-
 	/*
 	 * An empty cpus_allowed is ok only if the cpuset has no tasks.
 	 * Since cpulist_parse() fails on an empty mask, we special case
@@ -2783,15 +2784,6 @@ static int update_nodemask(struct cpuset *cs, struct cpuset *trialcs,
 {
 	int retval;
 
-	/*
-	 * top_cpuset.mems_allowed tracks node_stats[N_MEMORY];
-	 * it's read-only
-	 */
-	if (cs == &top_cpuset) {
-		retval = -EACCES;
-		goto done;
-	}
-
 	/*
 	 * An empty mems_allowed is ok iff there are no tasks in the cpuset.
 	 * Since nodelist_parse() fails on an empty mask, we special case
@@ -3257,6 +3249,10 @@ ssize_t cpuset_write_resmask(struct kernfs_open_file *of,
 	struct cpuset *trialcs;
 	int retval = -ENODEV;
 
+	/* root is read-only */
+	if (cpuset_is_root(cs))
+		return -EACCES;
+
 	buf = strstrip(buf);
 	cpuset_full_lock();
 	if (!is_cpuset_online(cs))
-- 
2.34.1