[Natty, Oneiric, Precise SRU] Fix race in task_group()
Stefan Bader
stefan.bader at canonical.com
Tue Jul 3 13:01:58 UTC 2012
Though reproducing this somehow required (or at least made much
more likely) a certain setup (two CPUs and the described loop of
a certain command), the result (crash) is probably bad enough to
warrant applying it before it appears upstream. Peter queued it
up but needs Ingo to push it onwards.
Also this may cause other subtle to find issues as the current
code calls task_group() four times to assign values to cfs and
rt schdeduler elements. So while the observed crash happened when
cfs elements were inconsistent, there could be other problems when
the inconsistency hits cfs/rt or rt only.
Quantal still has the same race but at least the code was changed
to call task_group only once. So the value could be old but at least
it is always old (or the new one).
This backport should apply to Natty till Precise (maybe a bit of
fuzz in Natty). But later that 3.3 all the files moved around.
-Stefan
>From 97dc9f26a508f140cc97eadedddab493619eeee4 Mon Sep 17 00:00:00 2001
From: Peter Zijlstra <peterz at infradead.org>
Date: Fri, 22 Jun 2012 13:36:00 +0200
Subject: [PATCH] UBUNTU: (pre-stable) sched: Fix race in task_group()
Stefan reported a crash on a kernel before a3e5d1091c1 ("sched: Don't
call task_group() too many times in set_task_rq()"), he found the reason
to be that the multiple task_group() invocations in set_task_rq()
returned different values.
Looking at all that I found a lack of serialization and plain wrong
comments.
The below tries to fix it using an extra pointer which is updated under
the appropriate scheduler locks. Its not pretty, but I can't really see
another way given how all the cgroup stuff works.
Reported-by: Stefan Bader <stefan.bader at canonical.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra at chello.nl>
BugLink: http://bugs.launchpad.net/bugs/999755
[backported from patch posted on mailing list]
Signed-off-by: Stefan Bader <stefan.bader at canonical.com>
---
include/linux/init_task.h | 12 +++++++++++-
include/linux/sched.h | 5 ++++-
kernel/sched.c | 32 ++++++++++++++++++--------------
3 files changed, 33 insertions(+), 16 deletions(-)
diff --git a/include/linux/init_task.h b/include/linux/init_task.h
index 32574ee..13b2684 100644
--- a/include/linux/init_task.h
+++ b/include/linux/init_task.h
@@ -117,8 +117,17 @@ extern struct group_info init_groups;
extern struct cred init_cred;
+extern struct task_group root_task_group;
+
+#ifdef CONFIG_CGROUP_SCHED
+# define INIT_CGROUP_SCHED(tsk) \
+ .sched_task_group = &root_task_group,
+#else
+# define INIT_CGROUP_SCHED(tsk)
+#endif
+
#ifdef CONFIG_PERF_EVENTS
-# define INIT_PERF_EVENTS(tsk) \
+# define INIT_PERF_EVENTS(tsk) \
.perf_event_mutex = \
__MUTEX_INITIALIZER(tsk.perf_event_mutex), \
.perf_event_list = LIST_HEAD_INIT(tsk.perf_event_list),
@@ -155,6 +164,7 @@ extern struct cred init_cred;
}, \
.tasks = LIST_HEAD_INIT(tsk.tasks), \
INIT_PUSHABLE_TASKS(tsk) \
+ INIT_CGROUP_SCHED(tsk) \
.ptraced = LIST_HEAD_INIT(tsk.ptraced), \
.ptrace_entry = LIST_HEAD_INIT(tsk.ptrace_entry), \
.real_parent = &tsk, \
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 56de5c1..ac51641 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1242,6 +1242,9 @@ struct task_struct {
const struct sched_class *sched_class;
struct sched_entity se;
struct sched_rt_entity rt;
+#ifdef CONFIG_CGROUP_SCHED
+ struct task_group *sched_task_group;
+#endif
#ifdef CONFIG_PREEMPT_NOTIFIERS
/* list of struct preempt_notifier: */
@@ -2646,7 +2649,7 @@ extern int sched_group_set_rt_period(struct task_group *tg,
extern long sched_group_rt_period(struct task_group *tg);
extern int sched_rt_can_attach(struct task_group *tg, struct task_struct *tsk);
#endif
-#endif
+#endif /* CONFIG_CGROUP_SCHED */
extern int task_can_switch_user(struct user_struct *up,
struct task_struct *tsk);
diff --git a/kernel/sched.c b/kernel/sched.c
index aae0c1d..b99a61e 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -746,22 +746,19 @@ static inline int cpu_of(struct rq *rq)
/*
* Return the group to which this tasks belongs.
*
- * We use task_subsys_state_check() and extend the RCU verification with
- * pi->lock and rq->lock because cpu_cgroup_attach() holds those locks for each
- * task it moves into the cgroup. Therefore by holding either of those locks,
- * we pin the task to the current cgroup.
+ * We cannot use task_subsys_state() and friends because the cgroup
+ * subsystem changes that value before the cgroup_subsys::attach() method
+ * is called, therefore we cannot pin it and might observe the wrong value.
+ *
+ * The same is true for autogroup's p->signal->autogroup->tg, the autogroup
+ * core changes this before calling sched_move_task().
+ *
+ * Instead we use a 'copy' which is updated from sched_move_task() while
+ * holding both task_struct::pi_lock and rq::lock.
*/
static inline struct task_group *task_group(struct task_struct *p)
{
- struct task_group *tg;
- struct cgroup_subsys_state *css;
-
- css = task_subsys_state_check(p, cpu_cgroup_subsys_id,
- lockdep_is_held(&p->pi_lock) ||
- lockdep_is_held(&task_rq(p)->lock));
- tg = container_of(css, struct task_group, css);
-
- return autogroup_task_group(p, tg);
+ return p->sched_task_group;
}
/* Change a task's cfs_rq and parent entity if it moves across CPUs/groups */
@@ -2373,7 +2370,7 @@ void set_task_cpu(struct task_struct *p, unsigned int new_cpu)
* a task's CPU. ->pi_lock for waking tasks, rq->lock for runnable tasks.
*
* sched_move_task() holds both and thus holding either pins the cgroup,
- * see set_task_rq().
+ * see task_group().
*
* Furthermore, all task_rq users should acquire both locks, see
* task_rq_lock().
@@ -8765,6 +8762,7 @@ void sched_destroy_group(struct task_group *tg)
*/
void sched_move_task(struct task_struct *tsk)
{
+ struct task_group *tg;
int on_rq, running;
unsigned long flags;
struct rq *rq;
@@ -8779,6 +8777,12 @@ void sched_move_task(struct task_struct *tsk)
if (unlikely(running))
tsk->sched_class->put_prev_task(rq, tsk);
+ tg = container_of(task_subsys_state_check(tsk, cpu_cgroup_subsys_id,
+ lockdep_is_held(&tsk->sighand->siglock)),
+ struct task_group, css);
+ tg = autogroup_task_group(tsk, tg);
+ tsk->sched_task_group = tg;
+
#ifdef CONFIG_FAIR_GROUP_SCHED
if (tsk->sched_class->task_move_group)
tsk->sched_class->task_move_group(tsk, on_rq);
--
1.7.9.5
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