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Subject[PATCH] cgroup/cpuset: remove circular dependency deadlock
Date
Remove circular dependency deadlock in a scenario where hotplug of CPU is
being done while there is updation in cgroup and cpuset triggered from
userspace.

Example scenario:
kworker/0:0 => kthreadd => init:729 => init:1 => kworker/0:0

kworker/0:0 - lock(cpuhotplug.mutex) [held]
flush(work) [no high prio workqueue available on CPU]
wait_for_completion()

kthreadd - percpu_down_read(cgroup_threadgroup_rwsem) [waiting]

init:729 - percpu_down_write(cgroup_threadgroup_rwsem) [held]
lock(cpuset_mutex) [waiting]

init:1 - lock(cpuset_mutex) [held]
lock(cpuhotplug.mutex) [waiting]

Eliminate this dependecy by reordering locking of cpuset_mutex
and cpuhotplug.mutex in following order
1. Acquire cpuhotplug.mutex
2. Acquire cpuset_mutex

Signed-off-by: Prateek Sood <prsood@codeaurora.org>
---
kernel/cgroup/cpuset.c | 70 +++++++++++++++++++++++++++++++++++++++++++-------
1 file changed, 61 insertions(+), 9 deletions(-)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 2f4039b..c7a3901 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -843,10 +843,41 @@ static void rebuild_sched_domains_locked(void)
out:
put_online_cpus();
}
+
+/*
+ * Rebuild scheduler domains.
+ * Call with following mutex held in the order
+ * 1. cpuhotplug.mutex
+ * 2. cpuset_mutex
+ */
+static void rebuild_sched_domains_unlocked(void)
+{
+ struct sched_domain_attr *attr;
+ cpumask_var_t *doms;
+ int ndoms;
+
+ /*
+ * We have raced with CPU hotplug. Don't do anything to avoid
+ * passing doms with offlined cpu to partition_sched_domains().
+ * Anyways, hotplug work item will rebuild sched domains.
+ */
+ if (!cpumask_equal(top_cpuset.effective_cpus, cpu_active_mask))
+ return;
+
+ /* Generate domain masks and attrs */
+ ndoms = generate_sched_domains(&doms, &attr);
+
+ /* Have scheduler rebuild the domains */
+ partition_sched_domains(ndoms, doms, attr);
+}
#else /* !CONFIG_SMP */
static void rebuild_sched_domains_locked(void)
{
}
+
+static void rebuild_sched_domains_unlocked(void)
+{
+}
#endif /* CONFIG_SMP */

void rebuild_sched_domains(void)
@@ -885,7 +916,9 @@ static void update_tasks_cpumask(struct cpuset *cs)
*
* On legacy hierachy, effective_cpus will be the same with cpu_allowed.
*
- * Called with cpuset_mutex held
+ * Called with following mutex held in order
+ * 1. cpuhotplug.mutex
+ * 2. cpuset_mutex
*/
static void update_cpumasks_hier(struct cpuset *cs, struct cpumask *new_cpus)
{
@@ -940,7 +973,7 @@ static void update_cpumasks_hier(struct cpuset *cs, struct cpumask *new_cpus)
rcu_read_unlock();

if (need_rebuild_sched_domains)
- rebuild_sched_domains_locked();
+ rebuild_sched_domains_unlocked();
}

/**
@@ -1262,6 +1295,11 @@ int current_cpuset_is_being_rebound(void)
return ret;
}

+/*
+ * Call with following mutex held in order
+ * 1. cpuhotplug.mutex
+ * 2. cpuset_mutex
+ */
static int update_relax_domain_level(struct cpuset *cs, s64 val)
{
#ifdef CONFIG_SMP
@@ -1273,7 +1311,7 @@ static int update_relax_domain_level(struct cpuset *cs, s64 val)
cs->relax_domain_level = val;
if (!cpumask_empty(cs->cpus_allowed) &&
is_sched_load_balance(cs))
- rebuild_sched_domains_locked();
+ rebuild_sched_domains_unlocked();
}

return 0;
@@ -1304,9 +1342,10 @@ static void update_tasks_flags(struct cpuset *cs)
* cs: the cpuset to update
* turning_on: whether the flag is being set or cleared
*
- * Call with cpuset_mutex held.
+ * Call with following mutex held in order
+ * 1. cpuhotplug.mutex
+ * 2. cpuset_mutex
*/
-
static int update_flag(cpuset_flagbits_t bit, struct cpuset *cs,
int turning_on)
{
@@ -1339,7 +1378,7 @@ static int update_flag(cpuset_flagbits_t bit, struct cpuset *cs,
spin_unlock_irq(&callback_lock);

if (!cpumask_empty(trialcs->cpus_allowed) && balance_flag_changed)
- rebuild_sched_domains_locked();
+ rebuild_sched_domains_unlocked();

if (spread_flag_changed)
update_tasks_flags(cs);
@@ -1607,6 +1646,7 @@ static int cpuset_write_u64(struct cgroup_subsys_state *css, struct cftype *cft,
cpuset_filetype_t type = cft->private;
int retval = 0;

+ get_online_cpus();
mutex_lock(&cpuset_mutex);
if (!is_cpuset_online(cs)) {
retval = -ENODEV;
@@ -1644,6 +1684,7 @@ static int cpuset_write_u64(struct cgroup_subsys_state *css, struct cftype *cft,
}
out_unlock:
mutex_unlock(&cpuset_mutex);
+ put_online_cpus();
return retval;
}

@@ -1654,6 +1695,7 @@ static int cpuset_write_s64(struct cgroup_subsys_state *css, struct cftype *cft,
cpuset_filetype_t type = cft->private;
int retval = -ENODEV;

+ get_online_cpus();
mutex_lock(&cpuset_mutex);
if (!is_cpuset_online(cs))
goto out_unlock;
@@ -1668,6 +1710,7 @@ static int cpuset_write_s64(struct cgroup_subsys_state *css, struct cftype *cft,
}
out_unlock:
mutex_unlock(&cpuset_mutex);
+ put_online_cpus();
return retval;
}

@@ -1706,6 +1749,7 @@ static ssize_t cpuset_write_resmask(struct kernfs_open_file *of,
kernfs_break_active_protection(of->kn);
flush_work(&cpuset_hotplug_work);

+ get_online_cpus();
mutex_lock(&cpuset_mutex);
if (!is_cpuset_online(cs))
goto out_unlock;
@@ -1731,6 +1775,7 @@ static ssize_t cpuset_write_resmask(struct kernfs_open_file *of,
free_trial_cpuset(trialcs);
out_unlock:
mutex_unlock(&cpuset_mutex);
+ put_online_cpus();
kernfs_unbreak_active_protection(of->kn);
css_put(&cs->css);
flush_workqueue(cpuset_migrate_mm_wq);
@@ -2031,13 +2076,14 @@ static int cpuset_css_online(struct cgroup_subsys_state *css)
/*
* If the cpuset being removed has its flag 'sched_load_balance'
* enabled, then simulate turning sched_load_balance off, which
- * will call rebuild_sched_domains_locked().
+ * will call rebuild_sched_domains_unlocked().
*/

static void cpuset_css_offline(struct cgroup_subsys_state *css)
{
struct cpuset *cs = css_cs(css);

+ get_online_cpus();
mutex_lock(&cpuset_mutex);

if (is_sched_load_balance(cs))
@@ -2047,6 +2093,7 @@ static void cpuset_css_offline(struct cgroup_subsys_state *css)
clear_bit(CS_ONLINE, &cs->flags);

mutex_unlock(&cpuset_mutex);
+ put_online_cpus();
}

static void cpuset_css_free(struct cgroup_subsys_state *css)
@@ -2341,8 +2388,13 @@ static void cpuset_hotplug_workfn(struct work_struct *work)
}

/* rebuild sched domains if cpus_allowed has changed */
- if (cpus_updated)
- rebuild_sched_domains();
+ if (cpus_updated) {
+ get_online_cpus();
+ mutex_lock(&cpuset_mutex);
+ rebuild_sched_domains_unlocked();
+ mutex_unlock(&cpuset_mutex);
+ put_online_cpus();
+ }
}

void cpuset_update_active_cpus(void)
--
Qualcomm India Private Limited, on behalf of Qualcomm Innovation Center, Inc.,
is a member of Code Aurora Forum, a Linux Foundation Collaborative Project.
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