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Message-Id: <e1c56f0ac5826010dbbddf781641306a0cf666cd.1757486368.git.escape@linux.alibaba.com>
Date: Wed, 10 Sep 2025 14:59:33 +0800
From: Yi Tao <escape@...ux.alibaba.com>
To: tj@...nel.org,
hannes@...xchg.org,
mkoutny@...e.com
Cc: cgroups@...r.kernel.org,
linux-kernel@...r.kernel.org
Subject: [PATCH v5 1/3] cgroup: refactor the cgroup_attach_lock code to make it clearer
Dynamic cgroup migration involving threadgroup locks can be in one of
two states: no lock held, or holding the global lock. Explicitly
declaring the different lock modes to make the code easier to
understand and facilitates future extensions of the lock modes.
Signed-off-by: Yi Tao <escape@...ux.alibaba.com>
---
include/linux/cgroup-defs.h | 8 ++++
kernel/cgroup/cgroup-internal.h | 9 +++--
kernel/cgroup/cgroup-v1.c | 14 +++----
kernel/cgroup/cgroup.c | 67 ++++++++++++++++++++++++---------
4 files changed, 69 insertions(+), 29 deletions(-)
diff --git a/include/linux/cgroup-defs.h b/include/linux/cgroup-defs.h
index 6b93a64115fe..213b0d01464f 100644
--- a/include/linux/cgroup-defs.h
+++ b/include/linux/cgroup-defs.h
@@ -140,6 +140,14 @@ enum {
__CFTYPE_ADDED = (1 << 18),
};
+enum cgroup_attach_lock_mode {
+ /* Default */
+ CGRP_ATTACH_LOCK_GLOBAL,
+
+ /* When pid=0 && threadgroup=false, see comments in cgroup_procs_write_start */
+ CGRP_ATTACH_LOCK_NONE,
+};
+
/*
* cgroup_file is the handle for a file instance created in a cgroup which
* is used, for example, to generate file changed notifications. This can
diff --git a/kernel/cgroup/cgroup-internal.h b/kernel/cgroup/cgroup-internal.h
index b14e61c64a34..a6d6f30b6f65 100644
--- a/kernel/cgroup/cgroup-internal.h
+++ b/kernel/cgroup/cgroup-internal.h
@@ -249,12 +249,13 @@ int cgroup_migrate(struct task_struct *leader, bool threadgroup,
int cgroup_attach_task(struct cgroup *dst_cgrp, struct task_struct *leader,
bool threadgroup);
-void cgroup_attach_lock(bool lock_threadgroup);
-void cgroup_attach_unlock(bool lock_threadgroup);
+void cgroup_attach_lock(enum cgroup_attach_lock_mode lock_mode);
+void cgroup_attach_unlock(enum cgroup_attach_lock_mode lock_mode);
struct task_struct *cgroup_procs_write_start(char *buf, bool threadgroup,
- bool *locked)
+ enum cgroup_attach_lock_mode *lock_mode)
__acquires(&cgroup_threadgroup_rwsem);
-void cgroup_procs_write_finish(struct task_struct *task, bool locked)
+void cgroup_procs_write_finish(struct task_struct *task,
+ enum cgroup_attach_lock_mode lock_mode)
__releases(&cgroup_threadgroup_rwsem);
void cgroup_lock_and_drain_offline(struct cgroup *cgrp);
diff --git a/kernel/cgroup/cgroup-v1.c b/kernel/cgroup/cgroup-v1.c
index 2a4a387f867a..f3838888ea6f 100644
--- a/kernel/cgroup/cgroup-v1.c
+++ b/kernel/cgroup/cgroup-v1.c
@@ -68,7 +68,7 @@ int cgroup_attach_task_all(struct task_struct *from, struct task_struct *tsk)
int retval = 0;
cgroup_lock();
- cgroup_attach_lock(true);
+ cgroup_attach_lock(CGRP_ATTACH_LOCK_GLOBAL);
for_each_root(root) {
struct cgroup *from_cgrp;
@@ -80,7 +80,7 @@ int cgroup_attach_task_all(struct task_struct *from, struct task_struct *tsk)
if (retval)
break;
}
- cgroup_attach_unlock(true);
+ cgroup_attach_unlock(CGRP_ATTACH_LOCK_GLOBAL);
cgroup_unlock();
return retval;
@@ -117,7 +117,7 @@ int cgroup_transfer_tasks(struct cgroup *to, struct cgroup *from)
cgroup_lock();
- cgroup_attach_lock(true);
+ cgroup_attach_lock(CGRP_ATTACH_LOCK_GLOBAL);
/* all tasks in @from are being moved, all csets are source */
spin_lock_irq(&css_set_lock);
@@ -153,7 +153,7 @@ int cgroup_transfer_tasks(struct cgroup *to, struct cgroup *from)
} while (task && !ret);
out_err:
cgroup_migrate_finish(&mgctx);
- cgroup_attach_unlock(true);
+ cgroup_attach_unlock(CGRP_ATTACH_LOCK_GLOBAL);
cgroup_unlock();
return ret;
}
@@ -502,13 +502,13 @@ static ssize_t __cgroup1_procs_write(struct kernfs_open_file *of,
struct task_struct *task;
const struct cred *cred, *tcred;
ssize_t ret;
- bool locked;
+ enum cgroup_attach_lock_mode lock_mode;
cgrp = cgroup_kn_lock_live(of->kn, false);
if (!cgrp)
return -ENODEV;
- task = cgroup_procs_write_start(buf, threadgroup, &locked);
+ task = cgroup_procs_write_start(buf, threadgroup, &lock_mode);
ret = PTR_ERR_OR_ZERO(task);
if (ret)
goto out_unlock;
@@ -531,7 +531,7 @@ static ssize_t __cgroup1_procs_write(struct kernfs_open_file *of,
ret = cgroup_attach_task(cgrp, task, threadgroup);
out_finish:
- cgroup_procs_write_finish(task, locked);
+ cgroup_procs_write_finish(task, lock_mode);
out_unlock:
cgroup_kn_unlock(of->kn);
diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index 312c6a8b55bb..2b88c7abaa00 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -2459,7 +2459,7 @@ EXPORT_SYMBOL_GPL(cgroup_path_ns);
/**
* cgroup_attach_lock - Lock for ->attach()
- * @lock_threadgroup: whether to down_write cgroup_threadgroup_rwsem
+ * @lock_mode: whether to down_write cgroup_threadgroup_rwsem
*
* cgroup migration sometimes needs to stabilize threadgroups against forks and
* exits by write-locking cgroup_threadgroup_rwsem. However, some ->attach()
@@ -2480,21 +2480,39 @@ EXPORT_SYMBOL_GPL(cgroup_path_ns);
* write-locking cgroup_threadgroup_rwsem. This allows ->attach() to assume that
* CPU hotplug is disabled on entry.
*/
-void cgroup_attach_lock(bool lock_threadgroup)
+void cgroup_attach_lock(enum cgroup_attach_lock_mode lock_mode)
{
cpus_read_lock();
- if (lock_threadgroup)
+
+ switch (lock_mode) {
+ case CGRP_ATTACH_LOCK_NONE:
+ break;
+ case CGRP_ATTACH_LOCK_GLOBAL:
percpu_down_write(&cgroup_threadgroup_rwsem);
+ break;
+ default:
+ pr_warn("cgroup: Unexpected attach lock mode.");
+ break;
+ }
}
/**
* cgroup_attach_unlock - Undo cgroup_attach_lock()
- * @lock_threadgroup: whether to up_write cgroup_threadgroup_rwsem
+ * @lock_mode: whether to up_write cgroup_threadgroup_rwsem
*/
-void cgroup_attach_unlock(bool lock_threadgroup)
+void cgroup_attach_unlock(enum cgroup_attach_lock_mode lock_mode)
{
- if (lock_threadgroup)
+ switch (lock_mode) {
+ case CGRP_ATTACH_LOCK_NONE:
+ break;
+ case CGRP_ATTACH_LOCK_GLOBAL:
percpu_up_write(&cgroup_threadgroup_rwsem);
+ break;
+ default:
+ pr_warn("cgroup: Unexpected attach lock mode.");
+ break;
+ }
+
cpus_read_unlock();
}
@@ -2968,7 +2986,7 @@ int cgroup_attach_task(struct cgroup *dst_cgrp, struct task_struct *leader,
}
struct task_struct *cgroup_procs_write_start(char *buf, bool threadgroup,
- bool *threadgroup_locked)
+ enum cgroup_attach_lock_mode *lock_mode)
{
struct task_struct *tsk;
pid_t pid;
@@ -2985,8 +3003,13 @@ struct task_struct *cgroup_procs_write_start(char *buf, bool threadgroup,
* Therefore, we can skip the global lock.
*/
lockdep_assert_held(&cgroup_mutex);
- *threadgroup_locked = pid || threadgroup;
- cgroup_attach_lock(*threadgroup_locked);
+
+ if (pid || threadgroup)
+ *lock_mode = CGRP_ATTACH_LOCK_GLOBAL;
+ else
+ *lock_mode = CGRP_ATTACH_LOCK_NONE;
+
+ cgroup_attach_lock(*lock_mode);
rcu_read_lock();
if (pid) {
@@ -3017,14 +3040,15 @@ struct task_struct *cgroup_procs_write_start(char *buf, bool threadgroup,
goto out_unlock_rcu;
out_unlock_threadgroup:
- cgroup_attach_unlock(*threadgroup_locked);
- *threadgroup_locked = false;
+ cgroup_attach_unlock(*lock_mode);
+ *lock_mode = CGRP_ATTACH_LOCK_NONE;
out_unlock_rcu:
rcu_read_unlock();
return tsk;
}
-void cgroup_procs_write_finish(struct task_struct *task, bool threadgroup_locked)
+void cgroup_procs_write_finish(struct task_struct *task,
+ enum cgroup_attach_lock_mode lock_mode)
{
struct cgroup_subsys *ss;
int ssid;
@@ -3032,7 +3056,7 @@ void cgroup_procs_write_finish(struct task_struct *task, bool threadgroup_locked
/* release reference from cgroup_procs_write_start() */
put_task_struct(task);
- cgroup_attach_unlock(threadgroup_locked);
+ cgroup_attach_unlock(lock_mode);
for_each_subsys(ss, ssid)
if (ss->post_attach)
@@ -3088,6 +3112,7 @@ static int cgroup_update_dfl_csses(struct cgroup *cgrp)
struct cgroup_subsys_state *d_css;
struct cgroup *dsct;
struct css_set *src_cset;
+ enum cgroup_attach_lock_mode lock_mode;
bool has_tasks;
int ret;
@@ -3119,7 +3144,13 @@ static int cgroup_update_dfl_csses(struct cgroup *cgrp)
* write-locking can be skipped safely.
*/
has_tasks = !list_empty(&mgctx.preloaded_src_csets);
- cgroup_attach_lock(has_tasks);
+
+ if (has_tasks)
+ lock_mode = CGRP_ATTACH_LOCK_GLOBAL;
+ else
+ lock_mode = CGRP_ATTACH_LOCK_NONE;
+
+ cgroup_attach_lock(lock_mode);
/* NULL dst indicates self on default hierarchy */
ret = cgroup_migrate_prepare_dst(&mgctx);
@@ -3140,7 +3171,7 @@ static int cgroup_update_dfl_csses(struct cgroup *cgrp)
ret = cgroup_migrate_execute(&mgctx);
out_finish:
cgroup_migrate_finish(&mgctx);
- cgroup_attach_unlock(has_tasks);
+ cgroup_attach_unlock(lock_mode);
return ret;
}
@@ -5241,13 +5272,13 @@ static ssize_t __cgroup_procs_write(struct kernfs_open_file *of, char *buf,
struct task_struct *task;
const struct cred *saved_cred;
ssize_t ret;
- bool threadgroup_locked;
+ enum cgroup_attach_lock_mode lock_mode;
dst_cgrp = cgroup_kn_lock_live(of->kn, false);
if (!dst_cgrp)
return -ENODEV;
- task = cgroup_procs_write_start(buf, threadgroup, &threadgroup_locked);
+ task = cgroup_procs_write_start(buf, threadgroup, &lock_mode);
ret = PTR_ERR_OR_ZERO(task);
if (ret)
goto out_unlock;
@@ -5273,7 +5304,7 @@ static ssize_t __cgroup_procs_write(struct kernfs_open_file *of, char *buf,
ret = cgroup_attach_task(dst_cgrp, task, threadgroup);
out_finish:
- cgroup_procs_write_finish(task, threadgroup_locked);
+ cgroup_procs_write_finish(task, lock_mode);
out_unlock:
cgroup_kn_unlock(of->kn);
--
2.32.0.3.g01195cf9f
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