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Date: Wed, 16 Sep 2015 11:55:45 -0400
From: Tejun Heo <tj@...nel.org>
To: Christian Borntraeger <borntraeger@...ibm.com>
Cc: Ingo Molnar <mingo@...hat.com>,
Peter Zijlstra <peterz@...radead.org>,
"linux-kernel@...r.kernel.org >> Linux Kernel Mailing List"
<linux-kernel@...r.kernel.org>, Oleg Nesterov <oleg@...hat.com>,
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
Paolo Bonzini <pbonzini@...hat.com>,
KVM list <kvm@...r.kernel.org>
Subject: [PATCH cgroup/for-4.3-fixes 1/2] Revert "cgroup: simplify
threadgroup locking"
>From f9f9e7b776142fb1c0782cade004cc8e0147a199 Mon Sep 17 00:00:00 2001
From: Tejun Heo <tj@...nel.org>
Date: Wed, 16 Sep 2015 11:51:12 -0400
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@...nel.org>
Link: http://lkml.kernel.org/g/55F8097A.7000206@de.ibm.com
Reported-by: Christian Borntraeger <borntraeger@...ibm.com>
Cc: Oleg Nesterov <oleg@...hat.com>
Cc: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Paolo Bonzini <pbonzini@...hat.com>
Cc: stable@...r.kernel.org # v4.2+
---
Hello,
These are the two reverts that I'm pushing through
cgroup/for-4.3-fixes. I'll re-apply the reverted patches on the
for-4.4 branch so that they can land together with percpu_rwsem
updates during the next merge window.
Thanks.
kernel/cgroup.c | 45 +++++++++++++++++++++++++++++++++------------
1 file changed, 33 insertions(+), 12 deletions(-)
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 2cf0f79..115091e 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -2460,13 +2460,14 @@ static ssize_t __cgroup_procs_write(struct kernfs_open_file *of, char *buf,
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;
@@ -2482,23 +2483,37 @@ static ssize_t __cgroup_procs_write(struct kernfs_open_file *of, char *buf,
*/
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;
}
@@ -2643,8 +2658,6 @@ static int cgroup_update_dfl_csses(struct cgroup *cgrp)
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)) {
@@ -2700,8 +2713,17 @@ static int cgroup_update_dfl_csses(struct cgroup *cgrp)
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))
@@ -2711,7 +2733,6 @@ static int cgroup_update_dfl_csses(struct cgroup *cgrp)
out_finish:
cgroup_migrate_finish(&preloaded_csets);
- percpu_up_write(&cgroup_threadgroup_rwsem);
return ret;
}
--
2.4.3
--
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