Re: [PATCH] cgroup/cpuset: fix circular locking dependency

[Prateek Sood]

On 12/13/2017 09:10 PM, Tejun Heo wrote:
Hi TJ,

> Hello, Prateek.
> 
> On Wed, Dec 13, 2017 at 07:58:24PM +0530, Prateek Sood wrote:
>> Did you mean something like below. If not then could you
>> please share a patch for this problem in
>> cgroup_transfer_tasks().
> 
> Oh we surely can add a new iterator but we can just count in
> cgroup_transfer_tasks() too, right?
I did not get what you meant by this. Could you please share a
patch for this.
> 
> Thanks.
> 

Following are two ways to improve cgroup_transfer_tasks(). In
both cases task in PF_EXITING state would be left in source
cgroup. It would be removed from cgroup_exit() in exit path.

diff --git a/kernel/cgroup/cgroup-v1.c b/kernel/cgroup/cgroup-v1.c
index 024085d..e2bdcdb 100644
--- a/kernel/cgroup/cgroup-v1.c
+++ b/kernel/cgroup/cgroup-v1.c
@@ -123,7 +123,10 @@ int cgroup_transfer_tasks(struct cgroup *to, struct cgroup *from)
         */
        do {
                css_task_iter_start(&from->self, 0, &it);
-               task = css_task_iter_next(&it);
+               do {
+                       task = css_task_iter_next(&it);
+               } while (task && (task & PF_EXITING))
+
                if (task)
                        get_task_struct(task);
                css_task_iter_end(&it);

----------------------------8<---------------------------------

diff --git a/kernel/cgroup/cgroup-v1.c b/kernel/cgroup/cgroup-v1.c
index 024085d..843b8bb 100644
--- a/kernel/cgroup/cgroup-v1.c
+++ b/kernel/cgroup/cgroup-v1.c
@@ -121,12 +121,11 @@ int cgroup_transfer_tasks(struct cgroup *to, struct cgroup *from)
         * Migrate tasks one-by-one until @from is empty.  This fails iff
         * ->can_attach() fails.
         */
+       css_task_iter_start(&from->self, 0, &it);
        do {
-               css_task_iter_start(&from->self, 0, &it);
                task = css_task_iter_next(&it);
                if (task)
                        get_task_struct(task);
-               css_task_iter_end(&it);

                if (task) {
                        ret = cgroup_migrate(task, false, &mgctx);
@@ -135,6 +134,7 @@ int cgroup_transfer_tasks(struct cgroup *to, struct cgroup *from)
                        put_task_struct(task);
                }
        } while (task && !ret);
+       css_task_iter_end(&it);
 out_err:
        cgroup_migrate_finish(&mgctx);
        percpu_up_write(&cgroup_threadgroup_rwsem);


Thanks


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