* Re: [RFC PATCH] cgroup: Remove task_lock() from cgroup_post_fork()
[not found] ` <1324516711-26913-1-git-send-email-fweisbec-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
@ 2011-12-22 8:55 ` Li Zefan
0 siblings, 0 replies; 3+ messages in thread
From: Li Zefan @ 2011-12-22 8:55 UTC (permalink / raw)
To: Frederic Weisbecker
Cc: Containers, LKML, Oleg Nesterov, Mandeep Singh Baines, Tejun Heo,
Cgroups, Andrew Morton, Paul Menage
Frederic Weisbecker wrote:
> 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().
>
To narrow the scope of the threadgroup lock? I think it's preferable to keep
cgroup_post_fork() inside the lock, to make things simpler and we have
the same lock rule for both cgroup_fork() and cgroup_post_fork().
> Let's debate!
>
> Signed-off-by: Frederic Weisbecker <fweisbec-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
> Cc: 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>
> Cc: Mandeep Singh Baines <msb-F7+t8E8rja9g9hUCZPvPmw@public.gmane.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()
> + */
Also explain why it won't race with cgroup_exit()? You were not quite confident
about that before Oleg's explanation. ;)
> list_add(&child->cg_list, &child->cgroups->tasks);
> - task_unlock(child);
> + }
> write_unlock(&css_set_lock);
> }
> }
^ permalink raw reply [flat|nested] 3+ messages in thread
[parent not found: <4EF2F095.90207@cn.fujitsu.com>]
* [RFC PATCH] cgroup: Remove task_lock() from cgroup_post_fork()
@ 2011-12-22 1:18 Frederic Weisbecker
0 siblings, 0 replies; 3+ messages in thread
From: Frederic Weisbecker @ 2011-12-22 1:18 UTC (permalink / raw)
To: Li Zefan, Tejun Heo
Cc: Frederic Weisbecker, Containers, Oleg Nesterov, LKML,
Mandeep Singh Baines, Cgroups, Andrew Morton, Paul Menage
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-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
Cc: 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>
Cc: Mandeep Singh Baines <msb-F7+t8E8rja9g9hUCZPvPmw@public.gmane.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
^ permalink raw reply related [flat|nested] 3+ messages in thread
end of thread, other threads:[~2011-12-22 9:33 UTC | newest]
Thread overview: 3+ messages (download: mbox.gz follow: Atom feed
-- links below jump to the message on this page --
[not found] <1324516711-26913-1-git-send-email-fweisbec@gmail.com>
[not found] ` <1324516711-26913-1-git-send-email-fweisbec-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
2011-12-22 8:55 ` [RFC PATCH] cgroup: Remove task_lock() from cgroup_post_fork() Li Zefan
[not found] ` <4EF2F095.90207@cn.fujitsu.com>
[not found] ` <4EF2F095.90207-BthXqXjhjHXQFUHtdCDX3A@public.gmane.org>
2011-12-22 9:33 ` Frederic Weisbecker
2011-12-22 1:18 Frederic Weisbecker
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox