Messages in this thread Patch in this message | | | From | Frederic Weisbecker <> | Subject | [RFC PATCH] cgroup: Remove task_lock() from cgroup_post_fork() | Date | Thu, 22 Dec 2011 02:18:31 +0100 |
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cgroup_post_fork() is protected between threadgroup_change_begin() and threadgroup_change_end() against concurrent changes of the child's css_set in cgroup_task_migrate(). Also the child can't exit and call cgroup_exit() at this stage, this means it's css_set can't be changed with init_css_set concurrently.
For these reasons, we don't need to hold task_lock() on the child because it's css_set can only remain stable in this place.
Let's remove the lock there.
NOTE: We could do something else: move threadgroup_change_end() before cgroup_post_fork() and keep the task_lock() which, combined with the css_set_lock, would be enough to synchronize against cgroup_task_migrate()'s change on child->cgroup and its cglist. Because outside that, the threadgroup lock doesn't appear to be needed on cgroup_post_fork().
Let's debate!
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com> Cc: Li Zefan <lizf@cn.fujitsu.com> Cc: Tejun Heo <tj@kernel.org> Cc: Containers <containers@lists.linux-foundation.org> Cc: Cgroups <cgroups@vger.kernel.org> Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> Cc: Oleg Nesterov <oleg@redhat.com> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Paul Menage <paul@paulmenage.org> Cc: Mandeep Singh Baines <msb@chromium.org> --- kernel/cgroup.c | 11 ++++++++--- 1 files changed, 8 insertions(+), 3 deletions(-)
diff --git a/kernel/cgroup.c b/kernel/cgroup.c index 4936d88..d0dbf72 100644 --- a/kernel/cgroup.c +++ b/kernel/cgroup.c @@ -4622,10 +4622,15 @@ void cgroup_post_fork(struct task_struct *child) { if (use_task_css_set_links) { write_lock(&css_set_lock); - task_lock(child); - if (list_empty(&child->cg_list)) + if (list_empty(&child->cg_list)) { + /* + * It's safe to use child->cgroups without task_lock() + * here because we are protected through + * threadgroup_change_begin() against concurrent + * css_set change in cgroup_task_migrate() + */ list_add(&child->cg_list, &child->cgroups->tasks); - task_unlock(child); + } write_unlock(&css_set_lock); } } -- 1.7.5.4
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