* [PATCH 1/2 v3] cgroup: Remove unnecessary task_lock before fetching css_set on migration
[not found] ` <1324494199-23928-1-git-send-email-fweisbec-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
@ 2011-12-21 19:03 ` Frederic Weisbecker
2011-12-21 19:03 ` [PATCH 2/2 v3] cgroup: Drop task_lock(parent) on cgroup_fork() Frederic Weisbecker
1 sibling, 0 replies; 3+ messages in thread
From: Frederic Weisbecker @ 2011-12-21 19:03 UTC (permalink / raw)
To: Tejun Heo, Li Zefan
Cc: Frederic Weisbecker, Containers, Oleg Nesterov, LKML, Cgroups,
Andrew Morton, Paul Menage
When we fetch the css_set of the tasks on cgroup migration, we don't need
anymore to synchronize against cgroup_exit() that could swap the old one
with init_css_set. Now that we are using threadgroup_lock() during
the migrations, we don't need to worry about it anymore.
Signed-off-by: Frederic Weisbecker <fweisbec-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
Reviewed-by: Mandeep Singh Baines <msb-F7+t8E8rja9g9hUCZPvPmw@public.gmane.org>
Reviewed-by: Li Zefan <lizf-BthXqXjhjHXQFUHtdCDX3A@public.gmane.org>
Cc: Tejun Heo <tj-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
Cc: Containers <containers-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org>
Cc: Cgroups <cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu-+CUm20s59erQFUHtdCDX3A@public.gmane.org>
Cc: Oleg Nesterov <oleg-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
Cc: Andrew Morton <akpm-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>
Cc: Paul Menage <paul-inf54ven1CmVyaH7bEyXVA@public.gmane.org>
---
kernel/cgroup.c | 20 ++++++++++----------
1 files changed, 10 insertions(+), 10 deletions(-)
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index bc3caff..24f6d6f 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -1850,14 +1850,14 @@ static int cgroup_task_migrate(struct cgroup *cgrp, struct cgroup *oldcgrp,
struct css_set *newcg;
/*
- * get old css_set. we need to take task_lock and refcount it, because
- * an exiting task can change its css_set to init_css_set and drop its
- * old one without taking cgroup_mutex.
+ * get old css_set. We are synchronized through threadgroup_lock()
+ * against PF_EXITING setting such that we can't race against
+ * cgroup_exit() changing the css_set to init_css_set and dropping the
+ * old one.
*/
- task_lock(tsk);
+ WARN_ON_ONCE(tsk->flags & PF_EXITING);
oldcg = tsk->cgroups;
get_css_set(oldcg);
- task_unlock(tsk);
/* locate or allocate a new css_set for this task. */
if (guarantee) {
@@ -1879,9 +1879,7 @@ static int cgroup_task_migrate(struct cgroup *cgrp, struct cgroup *oldcgrp,
}
put_css_set(oldcg);
- /* @tsk can't exit as its threadgroup is locked */
task_lock(tsk);
- WARN_ON_ONCE(tsk->flags & PF_EXITING);
rcu_assign_pointer(tsk->cgroups, newcg);
task_unlock(tsk);
@@ -2182,11 +2180,13 @@ int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader)
/* nothing to do if this task is already in the cgroup */
if (tc->cgrp == cgrp)
continue;
- /* get old css_set pointer */
- task_lock(tc->task);
+ /*
+ * get old css_set pointer. threadgroup is locked so this is
+ * safe against concurrent cgroup_exit() changing this to
+ * init_css_set.
+ */
oldcg = tc->task->cgroups;
get_css_set(oldcg);
- task_unlock(tc->task);
/* see if the new one for us is already in the list? */
if (css_set_check_fetched(cgrp, tc->task, oldcg, &newcg_list)) {
/* was already there, nothing to do. */
--
1.7.5.4
^ permalink raw reply related [flat|nested] 3+ messages in thread* [PATCH 2/2 v3] cgroup: Drop task_lock(parent) on cgroup_fork()
[not found] ` <1324494199-23928-1-git-send-email-fweisbec-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
2011-12-21 19:03 ` [PATCH 1/2 v3] cgroup: Remove unnecessary task_lock before fetching css_set on migration Frederic Weisbecker
@ 2011-12-21 19:03 ` Frederic Weisbecker
[not found] ` <1324494199-23928-3-git-send-email-fweisbec-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
1 sibling, 1 reply; 3+ messages in thread
From: Frederic Weisbecker @ 2011-12-21 19:03 UTC (permalink / raw)
To: Tejun Heo, Li Zefan
Cc: Frederic Weisbecker, Containers, Oleg Nesterov, LKML,
Mandeep Singh Baines, Cgroups, Andrew Morton, Paul Menage
We don't need to hold the parent task_lock() on the
parent in cgroup_fork() because we are already synchronized
against the two places that may change the parent css_set
concurrently:
- cgroup_exit(), but the parent obviously can't exit concurrently
- cgroup migration: we are synchronized against threadgroup_lock()
So we can safely remove the task_lock() there.
Signed-off-by: Frederic Weisbecker <fweisbec-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
Cc: Tejun Heo <tj-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
Cc: Li Zefan <lizf-BthXqXjhjHXQFUHtdCDX3A@public.gmane.org>
Cc: Containers <containers-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org>
Cc: Cgroups <cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu-+CUm20s59erQFUHtdCDX3A@public.gmane.org>
Cc: Oleg Nesterov <oleg-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
Cc: Andrew Morton <akpm-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>
Cc: Paul Menage <paul-inf54ven1CmVyaH7bEyXVA@public.gmane.org>
Cc: Mandeep Singh Baines <msb-F7+t8E8rja9g9hUCZPvPmw@public.gmane.org>
---
kernel/cgroup.c | 23 +++++++++++++++++------
1 files changed, 17 insertions(+), 6 deletions(-)
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 24f6d6f..4936d88 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -4556,20 +4556,31 @@ static const struct file_operations proc_cgroupstats_operations = {
*
* A pointer to the shared css_set was automatically copied in
* fork.c by dup_task_struct(). However, we ignore that copy, since
- * it was not made under the protection of RCU or cgroup_mutex, so
- * might no longer be a valid cgroup pointer. cgroup_attach_task() might
- * have already changed current->cgroups, allowing the previously
- * referenced cgroup group to be removed and freed.
+ * it was not made under the protection of RCU, cgroup_mutex or
+ * threadgroup_change_begin(), so it might no longer be a valid
+ * cgroup pointer. cgroup_attach_task() might have already changed
+ * current->cgroups, allowing the previously referenced cgroup
+ * group to be removed and freed.
+ *
+ * Outside the pointer validity we also need to process the css_set
+ * inheritance between threadgoup_change_begin() and
+ * threadgoup_change_end(), this way there is no leak in any process
+ * wide migration performed by cgroup_attach_proc() that could otherwise
+ * miss a thread because it is too early or too late in the fork stage.
*
* At the point that cgroup_fork() is called, 'current' is the parent
* task, and the passed argument 'child' points to the child task.
*/
void cgroup_fork(struct task_struct *child)
{
- task_lock(current);
+ /*
+ * We don't need to task_lock() current because current->cgroups
+ * can't be changed concurrently here. The parent obviously hasn't
+ * exited and called cgroup_exit(), and we are synchronized against
+ * cgroup migration through threadgroup_change_begin().
+ */
child->cgroups = current->cgroups;
get_css_set(child->cgroups);
- task_unlock(current);
INIT_LIST_HEAD(&child->cg_list);
}
--
1.7.5.4
^ permalink raw reply related [flat|nested] 3+ messages in thread