[PATCH 4.2 078/258] Revert "cgroup: simplify threadgroup locking"

From: Greg Kroah-Hartman
Date: Sun Oct 18 2015 - 00:40:43 EST


4.2-stable review patch. If anyone has any objections, please let me know.

------------------

From: Tejun Heo <tj@xxxxxxxxxx>

commit f9f9e7b776142fb1c0782cade004cc8e0147a199 upstream.

This reverts commit b5ba75b5fc0e8404e2c50cb68f39bb6a53fc916f.

d59cfc09c32a ("sched, cgroup: replace signal_struct->group_rwsem with
a global percpu_rwsem") and b5ba75b5fc0e ("cgroup: simplify
threadgroup locking") changed how cgroup synchronizes against task
fork and exits so that it uses global percpu_rwsem instead of
per-process rwsem; unfortunately, the write [un]lock paths of
percpu_rwsem always involve synchronize_rcu_expedited() which turned
out to be too expensive.

Improvements for percpu_rwsem are scheduled to be merged in the coming
v4.4-rc1 merge window which alleviates this issue. For now, revert
the two commits to restore per-process rwsem. They will be re-applied
for the v4.4-rc1 merge window.

Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
Link: http://lkml.kernel.org/g/55F8097A.7000206@xxxxxxxxxx
Reported-by: Christian Borntraeger <borntraeger@xxxxxxxxxx>
Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
kernel/cgroup.c | 45 +++++++++++++++++++++++++++++++++------------
1 file changed, 33 insertions(+), 12 deletions(-)

--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -2452,13 +2452,14 @@ static ssize_t __cgroup_procs_write(stru
if (!cgrp)
return -ENODEV;

- percpu_down_write(&cgroup_threadgroup_rwsem);
+retry_find_task:
rcu_read_lock();
if (pid) {
tsk = find_task_by_vpid(pid);
if (!tsk) {
+ rcu_read_unlock();
ret = -ESRCH;
- goto out_unlock_rcu;
+ goto out_unlock_cgroup;
}
} else {
tsk = current;
@@ -2474,23 +2475,37 @@ static ssize_t __cgroup_procs_write(stru
*/
if (tsk == kthreadd_task || (tsk->flags & PF_NO_SETAFFINITY)) {
ret = -EINVAL;
- goto out_unlock_rcu;
+ rcu_read_unlock();
+ goto out_unlock_cgroup;
}

get_task_struct(tsk);
rcu_read_unlock();

+ percpu_down_write(&cgroup_threadgroup_rwsem);
+ if (threadgroup) {
+ if (!thread_group_leader(tsk)) {
+ /*
+ * a race with de_thread from another thread's exec()
+ * may strip us of our leadership, if this happens,
+ * there is no choice but to throw this task away and
+ * try again; this is
+ * "double-double-toil-and-trouble-check locking".
+ */
+ percpu_up_write(&cgroup_threadgroup_rwsem);
+ put_task_struct(tsk);
+ goto retry_find_task;
+ }
+ }
+
ret = cgroup_procs_write_permission(tsk, cgrp, of);
if (!ret)
ret = cgroup_attach_task(cgrp, tsk, threadgroup);

- put_task_struct(tsk);
- goto out_unlock_threadgroup;
-
-out_unlock_rcu:
- rcu_read_unlock();
-out_unlock_threadgroup:
percpu_up_write(&cgroup_threadgroup_rwsem);
+
+ put_task_struct(tsk);
+out_unlock_cgroup:
cgroup_kn_unlock(of->kn);
return ret ?: nbytes;
}
@@ -2635,8 +2650,6 @@ static int cgroup_update_dfl_csses(struc

lockdep_assert_held(&cgroup_mutex);

- percpu_down_write(&cgroup_threadgroup_rwsem);
-
/* look up all csses currently attached to @cgrp's subtree */
down_read(&css_set_rwsem);
css_for_each_descendant_pre(css, cgroup_css(cgrp, NULL)) {
@@ -2692,8 +2705,17 @@ static int cgroup_update_dfl_csses(struc
goto out_finish;
last_task = task;

+ percpu_down_write(&cgroup_threadgroup_rwsem);
+ /* raced against de_thread() from another thread? */
+ if (!thread_group_leader(task)) {
+ percpu_up_write(&cgroup_threadgroup_rwsem);
+ put_task_struct(task);
+ continue;
+ }
+
ret = cgroup_migrate(src_cset->dfl_cgrp, task, true);

+ percpu_up_write(&cgroup_threadgroup_rwsem);
put_task_struct(task);

if (WARN(ret, "cgroup: failed to update controllers for the default hierarchy (%d), further operations may crash or hang\n", ret))
@@ -2703,7 +2725,6 @@ static int cgroup_update_dfl_csses(struc

out_finish:
cgroup_migrate_finish(&preloaded_csets);
- percpu_up_write(&cgroup_threadgroup_rwsem);
return ret;
}



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