Commits
Frederic Weisbecker committed 3ce3230a0cf
cgroup: Walk task list under tasklist_lock in cgroup_enable_task_cg_list
Walking through the tasklist in cgroup_enable_task_cg_list() inside
an RCU read side critical section is not enough because:
- RCU is not (yet) safe against while_each_thread()
- If we use only RCU, a forking task that has passed cgroup_post_fork()
without seeing use_task_css_set_links == 1 is not guaranteed to have
its child immediately visible in the tasklist if we walk through it
remotely with RCU. In this case it will be missing in its css_set's
task list.
Thus we need to traverse the list (unfortunately) under the
tasklist_lock. It makes us safe against while_each_thread() and also
make sure we see all forked task that have been added to the tasklist.
As a secondary effect, reading and writing use_task_css_set_links are
now well ordered against tasklist traversing and modification. The new
layout is:
CPU 0 CPU 1
use_task_css_set_links = 1 write_lock(tasklist_lock)
read_lock(tasklist_lock) add task to tasklist
do_each_thread() { write_unlock(tasklist_lock)
add thread to css set links if (use_task_css_set_links)
} while_each_thread() add thread to css set links
read_unlock(tasklist_lock)
If CPU 0 traverse the list after the task has been added to the tasklist
then it is correctly added to the css set links. OTOH if CPU 0 traverse
the tasklist before the new task had the opportunity to be added to the
tasklist because it was too early in the fork process, then CPU 1
catches up and add the task to the css set links after it added the task
to the tasklist. The right value of use_task_css_set_links is guaranteed
to be visible from CPU 1 due to the LOCK/UNLOCK implicit barrier properties:
the read_unlock on CPU 0 makes the write on use_task_css_set_links happening
and the write_lock on CPU 1 make the read of use_task_css_set_links that comes
afterward to return the correct value.
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Acked-by: Li Zefan <lizf@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Mandeep Singh Baines <msb@chromium.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>