Re: 3.10.16 cgroup_mutex deadlock

From: Hugh Dickins <hidden>
Date: 2013-11-18 02:17:55
Also in: lkml

On Fri, 15 Nov 2013, Tejun Heo wrote:

Hello,

Shawn, Hugh, can you please verify whether the attached patch makes
the deadlock go away?

Thanks a lot, Tejun: report below.

quoted hunk ↗ jump to hunk

Thanks.

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index e0839bc..dc9dc06 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c

@@ -90,6 +90,14 @@ static DEFINE_MUTEX(cgroup_mutex);
 static DEFINE_MUTEX(cgroup_root_mutex);
 
 /*
+ * cgroup destruction makes heavy use of work items and there can be a lot
+ * of concurrent destructions.  Use a separate workqueue so that cgroup
+ * destruction work items don't end up filling up max_active of system_wq
+ * which may lead to deadlock.
+ */
+static struct workqueue_struct *cgroup_destroy_wq;
+
+/*
  * Generate an array of cgroup subsystem pointers. At boot time, this is
  * populated with the built in subsystems, and modular subsystems are
  * registered after that. The mutable section of this array is protected by

@@ -871,7 +879,7 @@ static void cgroup_free_rcu(struct rcu_head *head)
 	struct cgroup *cgrp = container_of(head, struct cgroup, rcu_head);
 
 	INIT_WORK(&cgrp->destroy_work, cgroup_free_fn);
-	schedule_work(&cgrp->destroy_work);
+	queue_work(cgroup_destroy_wq, &cgrp->destroy_work);
 }
 
 static void cgroup_diput(struct dentry *dentry, struct inode *inode)

@@ -4254,7 +4262,7 @@ static void css_free_rcu_fn(struct rcu_head *rcu_head)
 	 * css_put().  dput() requires process context which we don't have.
 	 */
 	INIT_WORK(&css->destroy_work, css_free_work_fn);
-	schedule_work(&css->destroy_work);
+	queue_work(cgroup_destroy_wq, &css->destroy_work);
 }
 
 static void css_release(struct percpu_ref *ref)

@@ -4544,7 +4552,7 @@ static void css_killed_ref_fn(struct percpu_ref *ref)
 		container_of(ref, struct cgroup_subsys_state, refcnt);
 
 	INIT_WORK(&css->destroy_work, css_killed_work_fn);
-	schedule_work(&css->destroy_work);
+	queue_work(cgroup_destroy_wq, &css->destroy_work);
 }
 
 /**

@@ -5025,6 +5033,17 @@ int __init cgroup_init(void)
 	if (err)
 		return err;
 
+	/*
+	 * There isn't much point in executing destruction path in
+	 * parallel.  Good chunk is serialized with cgroup_mutex anyway.
+	 * Use 1 for @max_active.
+	 */
+	cgroup_destroy_wq = alloc_workqueue("cgroup_destroy", 0, 1);
+	if (!cgroup_destroy_wq) {
+		err = -ENOMEM;
+		goto out;
+	}
+
 	for_each_builtin_subsys(ss, i) {
 		if (!ss->early_init)
 			cgroup_init_subsys(ss);

@@ -5062,9 +5081,11 @@ int __init cgroup_init(void)
 	proc_create("cgroups", 0, NULL, &proc_cgroupstats_operations);
 
 out:
-	if (err)
+	if (err) {
+		if (cgroup_destroy_wq)
+			destroy_workqueue(cgroup_destroy_wq);
 		bdi_destroy(&cgroup_backing_dev_info);
-
+	}
 	return err;
 }

Sorry for the delay: I was on the point of reporting success last
night, when I tried a debug kernel: and that didn't work so well
(got spinlock bad magic report in pwd_adjust_max_active(), and
tests wouldn't run at all).

Even the non-early cgroup_init() is called well before the
early_initcall init_workqueues(): though only the debug (lockdep
and spinlock debug) kernel appeared to have a problem with that.

Here's the patch I ended up with successfully on a 3.11.7-based
kernel (though below I've rediffed it against 3.11.8): the
schedule_work->queue_work hunks are slightly different on 3.11
than in your patch against current, and I did alloc_workqueue()
from a separate core_initcall.

The interval between cgroup_init and that is a bit of a worry;
but we don't seem to have suffered from the interval between
cgroup_init and init_workqueues before (when system_wq is NULL)
- though you may have more courage than I to reorder them!

Initially I backed out my system_highpri_wq workaround, and
verified that it was still easy to reproduce the problem with
one of our cgroup stresstests.  Yes it was, then your modified
patch below convincingly fixed it.

I ran with Johannes's patch adding extra mem_cgroup_reparent_charges:
as I'd expected, that didn't solve this issue (though it's worth
our keeping it in to rule out another source of problems).  And I
checked back on dumps of failures: they indeed show the tell-tale
256 kworkers doing cgroup_offline_fn, just as you predicted.

Thanks!
Hugh

---
 kernel/cgroup.c | 30 +++++++++++++++++++++++++++---
 1 file changed, 27 insertions(+), 3 deletions(-)

--- 3.11.8/kernel/cgroup.c	2013-11-17 17:40:54.200640692 -0800
+++ linux/kernel/cgroup.c	2013-11-17 17:43:10.876643941 -0800

@@ -89,6 +89,14 @@ static DEFINE_MUTEX(cgroup_mutex);
 static DEFINE_MUTEX(cgroup_root_mutex);
 
 /*
+ * cgroup destruction makes heavy use of work items and there can be a lot
+ * of concurrent destructions.  Use a separate workqueue so that cgroup
+ * destruction work items don't end up filling up max_active of system_wq
+ * which may lead to deadlock.
+ */
+static struct workqueue_struct *cgroup_destroy_wq;
+
+/*
  * Generate an array of cgroup subsystem pointers. At boot time, this is
  * populated with the built in subsystems, and modular subsystems are
  * registered after that. The mutable section of this array is protected by

@@ -890,7 +898,7 @@ static void cgroup_free_rcu(struct rcu_h
 	struct cgroup *cgrp = container_of(head, struct cgroup, rcu_head);
 
 	INIT_WORK(&cgrp->destroy_work, cgroup_free_fn);
-	schedule_work(&cgrp->destroy_work);
+	queue_work(cgroup_destroy_wq, &cgrp->destroy_work);
 }
 
 static void cgroup_diput(struct dentry *dentry, struct inode *inode)

@@ -4205,7 +4213,7 @@ static void css_release(struct percpu_re
 	struct cgroup_subsys_state *css =
 		container_of(ref, struct cgroup_subsys_state, refcnt);
 
-	schedule_work(&css->dput_work);
+	queue_work(cgroup_destroy_wq, &css->dput_work);
 }
 
 static void init_cgroup_css(struct cgroup_subsys_state *css,

@@ -4439,7 +4447,7 @@ static void cgroup_css_killed(struct cgr
 
 	/* percpu ref's of all css's are killed, kick off the next step */
 	INIT_WORK(&cgrp->destroy_work, cgroup_offline_fn);
-	schedule_work(&cgrp->destroy_work);
+	queue_work(cgroup_destroy_wq, &cgrp->destroy_work);
 }
 
 static void css_ref_killed_fn(struct percpu_ref *ref)

@@ -4967,6 +4975,22 @@ out:
 	return err;
 }
 
+static int __init cgroup_destroy_wq_init(void)
+{
+	/*
+	 * There isn't much point in executing destruction path in
+	 * parallel.  Good chunk is serialized with cgroup_mutex anyway.
+	 * Use 1 for @max_active.
+	 *
+	 * We would prefer to do this in cgroup_init() above, but that
+	 * is called before init_workqueues(): so leave this until after.
+	 */
+	cgroup_destroy_wq = alloc_workqueue("cgroup_destroy", 0, 1);
+	BUG_ON(!cgroup_destroy_wq);
+	return 0;
+}
+core_initcall(cgroup_destroy_wq_init);
+
 /*
  * proc_cgroup_show()
  *  - Print task's cgroup paths into seq_file, one line for each hierarchy

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