2.6.35-rc2-git1 - include/linux/cgroup.h:534 invoked rcu_dereference_check() without protection!

Hi All,

I just reproduced a warning I reported quite a while ago.  Is a patch for this in the pipeline?

[    0.167267] [ INFO: suspicious rcu_dereference_check() usage. ] [    0.167396] --------------------------------------------------- [    0.167526] include/linux/cgroup.h:534 invoked rcu_dereference_check() without protection!
[    0.167728]
[    0.167729] other info that might help us debug this:
[    0.167731]
[    0.168092]
[    0.168093] rcu_scheduler_active = 1, debug_locks = 1
[    0.168337] no locks held by watchdog/0/5.
[    0.168462]
[    0.168463] stack backtrace:
[    0.168704] Pid: 5, comm: watchdog/0 Not tainted 2.6.35-rc2-git1 #8 [    0.168834] Call Trace:
[    0.168965]  [<ffffffff81064e9c>] lockdep_rcu_dereference+0x9d/0xa5 [    0.169100]  [<ffffffff8102c1ce>] task_subsys_state+0x59/0x70 [    0.169232]  [<ffffffff8103189b>] __sched_setscheduler+0x19d/0x2f8 [    0.169365]  [<ffffffff8102a5ef>] ? need_resched+0x1e/0x28 [    0.169497]  [<ffffffff813c7d01>] ? schedule+0x586/0x619 [    0.169628]  [<ffffffff81081c33>] ? watchdog+0x0/0x8c
[    0.169758]  [<ffffffff81031a11>] sched_setscheduler+0xe/0x10 [    0.169889]  [<ffffffff81081c5d>] watchdog+0x2a/0x8c
[    0.170010]  [<ffffffff81081c33>] ? watchdog+0x0/0x8c
[    0.170141]  [<ffffffff81054a82>] kthread+0x89/0x91
[    0.170274]  [<ffffffff81003054>] kernel_thread_helper+0x4/0x10 [    0.170405]  [<ffffffff813ca480>] ? restore_args+0x0/0x30 [    0.170536]  [<ffffffff810549f9>] ? kthread+0x0/0x91
[    0.170667]  [<ffffffff81003050>] ? kernel_thread_helper+0x0/0x10

[    0.176751] lockdep: fixing up alternatives.

Quoted 4 lines · Miles Lane

Hi All,

I just reproduced a warning I reported quite a while ago.  Is a patch for this in the pipeline?

I proposed a patch, thinking that it was a false positive.  Peter Zijlstra pointed out that there was a real race, and proposed an alternative patch, which may be found at http://lkml.org/lkml/2010/4/22/603.

Could you please test Peter's patch and let us know if it cures the problem?

                                                       Thanx, Paul

Quoted 29 lines

[    0.167267] [ INFO: suspicious rcu_dereference_check() usage. ] [    0.167396] --------------------------------------------------- [    0.167526] include/linux/cgroup.h:534 invoked rcu_dereference_check() without protection!
[    0.167728]
[    0.167729] other info that might help us debug this:
[    0.167731]
[    0.168092]
[    0.168093] rcu_scheduler_active = 1, debug_locks = 1
[    0.168337] no locks held by watchdog/0/5.
[    0.168462]
[    0.168463] stack backtrace:
[    0.168704] Pid: 5, comm: watchdog/0 Not tainted 2.6.35-rc2-git1 #8 [    0.168834] Call Trace:
[    0.168965]  [<ffffffff81064e9c>] lockdep_rcu_dereference+0x9d/0xa5 [    0.169100]  [<ffffffff8102c1ce>] task_subsys_state+0x59/0x70 [    0.169232]  [<ffffffff8103189b>] __sched_setscheduler+0x19d/0x2f8 [    0.169365]  [<ffffffff8102a5ef>] ? need_resched+0x1e/0x28 [    0.169497]  [<ffffffff813c7d01>] ? schedule+0x586/0x619 [    0.169628]  [<ffffffff81081c33>] ? watchdog+0x0/0x8c
[    0.169758]  [<ffffffff81031a11>] sched_setscheduler+0xe/0x10 [    0.169889]  [<ffffffff81081c5d>] watchdog+0x2a/0x8c
[    0.170010]  [<ffffffff81081c33>] ? watchdog+0x0/0x8c
[    0.170141]  [<ffffffff81054a82>] kthread+0x89/0x91
[    0.170274]  [<ffffffff81003054>] kernel_thread_helper+0x4/0x10 [    0.170405]  [<ffffffff813ca480>] ? restore_args+0x0/0x30 [    0.170536]  [<ffffffff810549f9>] ? kthread+0x0/0x91
[    0.170667]  [<ffffffff81003050>] ? kernel_thread_helper+0x0/0x10 [    0.176751] lockdep: fixing up alternatives.

Quoted 6 lines · Paul E. McKenney

Quoted 4 lines · Miles Lane

Hi All,

I just reproduced a warning I reported quite a while ago. Is a patch for this in the pipeline?

Quoted 6 lines

I proposed a patch, thinking that it was a false positive. Peter Zijlstra pointed out that there was a real race, and proposed an alternative patch, which may be found at http://lkml.org/lkml/2010/4/22/603.

Could you please test Peter's patch and let us know if it cures the problem?

Quoted 8 lines · Miles Lane

Quoted 6 lines · Paul E. McKenney

Quoted 4 lines · Miles Lane

Hi All,

I just reproduced a warning I reported quite a while ago.  Is a patch for this in the pipeline?

Quoted 6 lines

Quoted 6 lines

I proposed a patch, thinking that it was a false positive.  Peter Zijlstra pointed out that there was a real race, and proposed an alternative patch, which may be found at http://lkml.org/lkml/2010/4/22/603.

Could you please test Peter's patch and let us know if it cures the problem?

Gah, this task_group() stuff is annoying, how about something like the below which teaches task_group() about the task_rq()->lock rule?

---
 include/linux/cgroup.h |   20 +++++++++++----
 kernel/sched.c         |   61 +++++++++++++++++++++++++----------------------

 2 files changed, 46 insertions(+), 35 deletions(-)

include/linux/cgroup.h +14 -6

index 0c62160..1efd212 100644
@@ -525,13 +525,21 @@ static inline struct cgroup_subsys_state *cgroup_subsys_state(
        return cgrp->subsys[subsys_id];
  }
 
-static inline struct cgroup_subsys_state *task_subsys_state(
-       struct task_struct *task, int subsys_id)
+/*
+ * function to get the cgroup_subsys_state which allows for extra
+ * rcu_dereference_check() conditions, such as locks used during the
+ * cgroup_subsys::attach() methods.
+ */
+#define task_subsys_state_check(task, subsys_id, __c)                  \
+       rcu_dereference_check(task->cgroups->subsys[subsys_id],         \
+                             rcu_read_lock_held() ||                   \
+                             lockdep_is_held(&task->alloc_lock) ||     \
+                             cgroup_lock_is_held() || (__c))
+
+static inline struct cgroup_subsys_state *
+task_subsys_state(struct task_struct *task, int subsys_id)
  {
-       return rcu_dereference_check(task->cgroups->subsys[subsys_id],
-                                    rcu_read_lock_held() ||
-                                    lockdep_is_held(&task->alloc_lock) ||
-                                    cgroup_lock_is_held());
+       return task_subsys_state_check(task, subsys_id, false);
  }
 
  static inline struct cgroup* task_cgroup(struct task_struct *task,

kernel/sched.c +25 -29

index f8b8996..e01bb45 100644
@@ -306,32 +306,26 @@ static int init_task_group_load = INIT_TASK_GROUP_LOAD;
  */
  struct task_group init_task_group;
 
-/* return group to which a task belongs */
+/*
+ * Return the group to which this tasks belongs.
+ *
+ * We use task_subsys_state_check() and extend the RCU verification
+ * with lockdep_is_held(&task_rq(p)->lock) because cpu_cgroup_attach()
+ * holds that lock for each task it moves into the cgroup. Therefore
+ * by holding that lock, we pin the task to the current cgroup.
+ */
  static inline struct task_group *task_group(struct task_struct *p)
  {
-       struct task_group *tg;
+       struct cgroup_subsys_state *css;
 
-#ifdef CONFIG_CGROUP_SCHED
-       tg = container_of(task_subsys_state(p, cpu_cgroup_subsys_id),
-                               struct task_group, css);
-#else
-       tg = &init_task_group;
-#endif
-       return tg;
+       css = task_subsys_state_check(p, cpu_cgroup_subsys_id,
+                       lockdep_is_held(&task_rq(p)->lock));
+       return container_of(css, struct task_group, css);
  }
 
  /* Change a task's cfs_rq and parent entity if it moves across CPUs/groups */
  static inline void set_task_rq(struct task_struct *p, unsigned int cpu)
  {
-       /*
-        * Strictly speaking this rcu_read_lock() is not needed since the
-        * task_group is tied to the cgroup, which in turn can never go away
-        * as long as there are tasks attached to it.
-        *
-        * However since task_group() uses task_subsys_state() which is an
-        * rcu_dereference() user, this quiets CONFIG_PROVE_RCU.
-        */
-       rcu_read_lock();
  #ifdef CONFIG_FAIR_GROUP_SCHED
        p->se.cfs_rq = task_group(p)->cfs_rq[cpu];
        p->se.parent = task_group(p)->se[cpu];

@@ -341,7 +335,6 @@ static inline void set_task_rq(struct task_struct *p, unsigned int cpu)
        p->rt.rt_rq  = task_group(p)->rt_rq[cpu];
        p->rt.parent = task_group(p)->rt_se[cpu];
  #endif
-       rcu_read_unlock();
  }
 
  #else

@@ -4465,16 +4458,6 @@ recheck:
        }
 
        if (user) {
-#ifdef CONFIG_RT_GROUP_SCHED
-               /*
-                * Do not allow realtime tasks into groups that have no runtime
-                * assigned.
-                */
-               if (rt_bandwidth_enabled() && rt_policy(policy) &&
-                               task_group(p)->rt_bandwidth.rt_runtime == 0)
-                       return -EPERM;
-#endif
-
                retval = security_task_setscheduler(p, policy, param);
                if (retval)
                        return retval;

@@ -4490,6 +4473,26 @@ recheck:
         * runqueue lock must be held.
         */
        rq = __task_rq_lock(p);
+
+       retval = 0;
+#ifdef CONFIG_RT_GROUP_SCHED
+       if (user) {
+               /*
+                * Do not allow realtime tasks into groups that have no runtime
+                * assigned.
+                */
+               if (rt_bandwidth_enabled() && rt_policy(policy) &&
+                               task_group(p)->rt_bandwidth.rt_runtime == 0)
+                       retval = -EPERM;
+
+               if (retval) {

CC kernel/sched.o
kernel/sched.c: In function ‘task_group’:
kernel/sched.c:321: error: implicit declaration of function ‘task_rq’ kernel/sched.c:321: error: invalid type argument of ‘->’ (have ‘int’) make[1]: *** [kernel/sched.o] Error 1

I had to apply with fuzz. Did it mess up?

Quoted 7 lines · Miles Lane

  CC      kernel/sched.o
kernel/sched.c: In function ‘task_group’:
kernel/sched.c:321: error: implicit declaration of function ‘task_rq’ kernel/sched.c:321: error: invalid type argument of ‘->’ (have ‘int’) make[1]: *** [kernel/sched.o] Error 1

I had to apply with fuzz.  Did it mess up?

No, I probably did.. task_rq() is defined on line 636 or thereabouts, and this function landed around line 320.

Ahh, and it compiled here because I have CGROUP_SCHED=y, but PROVE_RCU=n, so that whole check expression disappears and is never evaluated...

/me fixes

---
Subject: sched: PROVE_RCU vs cpu_cgroup
From: Peter Zijlstra <[email protected]>
Date: Tue Jun 08 11:40:42 CEST 2010

PROVE_RCU has a few issues with the cpu_cgroup because the scheduler typically holds rq->lock around the css rcu derefs but the generic cgroup code doesn't (and can't) know about that lock.

Provide means to add extra checks to the css dereference and use that in the scheduler to annotate its users.

The addition of rq->lock to these checks is correct because the cgroup_subsys::attach() method takes the rq->lock for each task it moves, therefore by holding that lock, we ensure the task is pinned to the current cgroup and the RCU dereference is valid.

That leaves one genuine race in __sched_setscheduler() where we used task_group() without holding any of the required locks and thus raced with the cgroup code. Solve this by moving the check under the rq->lock.

Signed-off-by Peter Zijlstra <[email protected]>

---
 include/linux/cgroup.h |   20 +++++---
 kernel/sched.c         |  115 +++++++++++++++++++++++++------------------------

 2 files changed, 73 insertions(+), 62 deletions(-)

Index: linux-2.6/include/linux/cgroup.h
===================================================================

include/linux/cgroup.h +14 -6

@@ -525,13 +525,21 @@ static inline struct cgroup_subsys_state
        return cgrp->subsys[subsys_id];
  }
 
-static inline struct cgroup_subsys_state *task_subsys_state(
-       struct task_struct *task, int subsys_id)
+/*
+ * function to get the cgroup_subsys_state which allows for extra
+ * rcu_dereference_check() conditions, such as locks used during the
+ * cgroup_subsys::attach() methods.
+ */
+#define task_subsys_state_check(task, subsys_id, __c)                  \
+       rcu_dereference_check(task->cgroups->subsys[subsys_id],         \
+                             rcu_read_lock_held() ||                   \
+                             lockdep_is_held(&task->alloc_lock) ||     \
+                             cgroup_lock_is_held() || (__c))
+
+static inline struct cgroup_subsys_state *
+task_subsys_state(struct task_struct *task, int subsys_id)
  {
-       return rcu_dereference_check(task->cgroups->subsys[subsys_id],
-                                    rcu_read_lock_held() ||
-                                    lockdep_is_held(&task->alloc_lock) ||
-                                    cgroup_lock_is_held());
+       return task_subsys_state_check(task, subsys_id, false);
  }
 
  static inline struct cgroup* task_cgroup(struct task_struct *task,

Index: linux-2.6/kernel/sched.c
===================================================================

kernel/sched.c +52 -56

@@ -306,52 +306,6 @@ static int init_task_group_load = INIT_T
  */
  struct task_group init_task_group;
 
-/* return group to which a task belongs */
-static inline struct task_group *task_group(struct task_struct *p)
-{
-       struct task_group *tg;
-
-#ifdef CONFIG_CGROUP_SCHED
-       tg = container_of(task_subsys_state(p, cpu_cgroup_subsys_id),
-                               struct task_group, css);
-#else
-       tg = &init_task_group;
-#endif
-       return tg;
-}
-
-/* Change a task's cfs_rq and parent entity if it moves across CPUs/groups */
-static inline void set_task_rq(struct task_struct *p, unsigned int cpu)
-{
-       /*
-        * Strictly speaking this rcu_read_lock() is not needed since the
-        * task_group is tied to the cgroup, which in turn can never go away
-        * as long as there are tasks attached to it.
-        *
-        * However since task_group() uses task_subsys_state() which is an
-        * rcu_dereference() user, this quiets CONFIG_PROVE_RCU.
-        */
-       rcu_read_lock();
-#ifdef CONFIG_FAIR_GROUP_SCHED
-       p->se.cfs_rq = task_group(p)->cfs_rq[cpu];
-       p->se.parent = task_group(p)->se[cpu];
-#endif
-
-#ifdef CONFIG_RT_GROUP_SCHED
-       p->rt.rt_rq  = task_group(p)->rt_rq[cpu];
-       p->rt.parent = task_group(p)->rt_se[cpu];
-#endif
-       rcu_read_unlock();
-}
-
-#else
-
-static inline void set_task_rq(struct task_struct *p, unsigned int cpu) { }
-static inline struct task_group *task_group(struct task_struct *p)
-{
-       return NULL;
-}
-
  #endif /* CONFIG_CGROUP_SCHED */
 
  /* CFS-related fields in a runqueue */

@@ -644,6 +598,49 @@ static inline int cpu_of(struct rq *rq)
  #define cpu_curr(cpu)          (cpu_rq(cpu)->curr)
  #define raw_rq()               (&__raw_get_cpu_var(runqueues))
 
+#ifdef CONFIG_CGROUP_SCHED
+
+/*
+ * Return the group to which this tasks belongs.
+ *
+ * We use task_subsys_state_check() and extend the RCU verification
+ * with lockdep_is_held(&task_rq(p)->lock) because cpu_cgroup_attach()
+ * holds that lock for each task it moves into the cgroup. Therefore
+ * by holding that lock, we pin the task to the current cgroup.
+ */
+static inline struct task_group *task_group(struct task_struct *p)
+{
+       struct cgroup_subsys_state *css;
+
+       css = task_subsys_state_check(p, cpu_cgroup_subsys_id,
+                       lockdep_is_held(&task_rq(p)->lock));
+       return container_of(css, struct task_group, css);
+}
+
+/* Change a task's cfs_rq and parent entity if it moves across CPUs/groups */
+static inline void set_task_rq(struct task_struct *p, unsigned int cpu)
+{
+#ifdef CONFIG_FAIR_GROUP_SCHED
+       p->se.cfs_rq = task_group(p)->cfs_rq[cpu];
+       p->se.parent = task_group(p)->se[cpu];
+#endif
+
+#ifdef CONFIG_RT_GROUP_SCHED
+       p->rt.rt_rq  = task_group(p)->rt_rq[cpu];
+       p->rt.parent = task_group(p)->rt_se[cpu];
+#endif
+}
+
+#else /* CONFIG_CGROUP_SCHED */
+
+static inline void set_task_rq(struct task_struct *p, unsigned int cpu) { }
+static inline struct task_group *task_group(struct task_struct *p)
+{
+       return NULL;
+}
+
+#endif /* CONFIG_CGROUP_SCHED */
+
  inline void update_rq_clock(struct rq *rq)
  {
        if (!rq->skip_clock_update)

@@ -4465,16 +4462,6 @@ recheck:
        }
 
        if (user) {
-#ifdef CONFIG_RT_GROUP_SCHED
-               /*
-                * Do not allow realtime tasks into groups that have no runtime
-                * assigned.
-                */
-               if (rt_bandwidth_enabled() && rt_policy(policy) &&
-                               task_group(p)->rt_bandwidth.rt_runtime == 0)
-                       return -EPERM;
-#endif
-
                retval = security_task_setscheduler(p, policy, param);
                if (retval)
                        return retval;

@@ -4490,6 +4477,22 @@ recheck:
         * runqueue lock must be held.
         */
        rq = __task_rq_lock(p);
+
+#ifdef CONFIG_RT_GROUP_SCHED
+       if (user) {
+               /*
+                * Do not allow realtime tasks into groups that have no runtime
+                * assigned.
+                */
+               if (rt_bandwidth_enabled() && rt_policy(policy) &&
+                               task_group(p)->rt_bandwidth.rt_runtime == 0) {

Sorry. I misunderstood this message when I first read it. I didn't realize this message include a new version of the patch. Anyhow, I just tried to apply the patch to 2.6.35-rc2-git3 and got this:

# patch -p1 -l -F 20 --dry-run < ../5.patch
patching file include/linux/cgroup.h
patching file kernel/sched.c
Hunk #1 succeeded at 306 with fuzz 1.
Hunk #3 FAILED at 4462.
Hunk #4 succeeded at 4487 with fuzz 3.
1 out of 4 hunks FAILED -- saving rejects to file kernel/sched.c.rej

Quoted 5 lines · Miles Lane

Sorry.  I misunderstood this message when I first read it.  I didn't realize this message include a new version of the patch. Anyhow, I just tried to apply the patch to 2.6.35-rc2-git3 and got this:

Quoted 7 lines

# patch -p1 -l -F 20 --dry-run < ../5.patch
patching file include/linux/cgroup.h
patching file kernel/sched.c
Hunk #1 succeeded at 306 with fuzz 1.
Hunk #3 FAILED at 4462.
Hunk #4 succeeded at 4487 with fuzz 3.
1 out of 4 hunks FAILED -- saving rejects to file kernel/sched.c.rej

Weird.. it seems to apply without trouble to Linus' git tree.

root@twins:/usr/src/linux-2.6# git checkout -f origin/master
HEAD is now at 84f7586... Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/ieee1394/linux1394-2.6
root@twins:/usr/src/linux-2.6# quilt push
Applying patch patches/sched-rcu-validation.patch
patching file include/linux/cgroup.h
patching file kernel/sched.c

Now at patch patches/sched-rcu-validation.patch

root@twins:/usr/src/linux-2.6# git describe
v2.6.35-rc2-54-g84f7586

Quoted 23 lines · Peter Zijlstra

Quoted 5 lines · Miles Lane

Sorry.  I misunderstood this message when I first read it.  I didn't realize this message include a new version of the patch. Anyhow, I just tried to apply the patch to 2.6.35-rc2-git3 and got this:

Quoted 7 lines

# patch -p1 -l -F 20 --dry-run < ../5.patch
patching file include/linux/cgroup.h
patching file kernel/sched.c
Hunk #1 succeeded at 306 with fuzz 1.
Hunk #3 FAILED at 4462.
Hunk #4 succeeded at 4487 with fuzz 3.
1 out of 4 hunks FAILED -- saving rejects to file kernel/sched.c.rej

Weird.. it seems to apply without trouble to Linus' git tree.

root@twins:/usr/src/linux-2.6# git checkout -f origin/master
HEAD is now at 84f7586... Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/ieee1394/linux1394-2.6
root@twins:/usr/src/linux-2.6# quilt push
Applying patch patches/sched-rcu-validation.patch
patching file include/linux/cgroup.h
patching file kernel/sched.c

Quoted 3 lines

Now at patch patches/sched-rcu-validation.patch

root@twins:/usr/src/linux-2.6# git describe
v2.6.35-rc2-54-g84f7586

Oh.  Duh. I know what is going on.  I had received another patch to sched.c.  They must conflict.  I will test with just your patch now.

ACPI: Core revision 20100428

[    0.061088]
[    0.061090] ===================================================
[    0.062009] [ INFO: suspicious rcu_dereference_check() usage. ]
[    0.062138] ---------------------------------------------------
[    0.062268] kernel/sched.c:616 invoked rcu_dereference_check()
without protection!
[    0.062470]
[    0.062471] other info that might help us debug this:
[    0.062472]
[    0.062835]
[    0.062836] rcu_scheduler_active = 1, debug_locks = 1
[    0.063009] no locks held by swapper/0.
[    0.063134]
[    0.063135] stack backtrace:
[    0.063378] Pid: 0, comm: swapper Not tainted 2.6.35-rc2-git3 #3
[    0.063507] Call Trace:
[    0.063638]  [<ffffffff81072205>] lockdep_rcu_dereference+0x9d/0xa5
[    0.063773]  [<ffffffff810379f9>] task_group+0x7b/0x8a
[    0.064012]  [<ffffffff81037a1d>] set_task_rq+0x15/0x6e
[    0.064143]  [<ffffffff8103e50f>] set_task_cpu+0xa9/0xba
[    0.064274]  [<ffffffff81042dbb>] sched_fork+0x10a/0x1b3
[    0.064405]  [<ffffffff810446f9>] copy_process+0x617/0x10e6
[    0.064537]  [<ffffffff8104533d>] do_fork+0x175/0x39b
[    0.064670]  [<ffffffff8106589b>] ? up+0xf/0x39
[    0.064800]  [<ffffffff8106589b>] ? up+0xf/0x39
[    0.065013]  [<ffffffff811dbf73>] ? do_raw_spin_lock+0x79/0x13e
[    0.065148]  [<ffffffff81011526>] kernel_thread+0x70/0x72

[    0.065279]  [<ffffffff816cc5e4>] ? kernel_init+0x0/0x1ce
[    0.065411]  [<ffffffff8100aba0>] ? kernel_thread_helper+0x0/0x10
[    0.065545]  [<ffffffff81096bea>] ? rcu_scheduler_starting+0x2a/0x4c
[    0.065679]  [<ffffffff813a8a4d>] rest_init+0x21/0xde
[    0.065810]  [<ffffffff816cce28>] start_kernel+0x448/0x453
[    0.066013]  [<ffffffff816cc2c8>] x86_64_start_reservations+0xb3/0xb7
[    0.066148]  [<ffffffff816cc418>] x86_64_start_kernel+0x14c/0x15b
[    0.066499] Setting APIC routing to flat

Quoted 6 lines · Peter Zijlstra

+       /*
+        * We're not in the pid-hash yet so no cgroup attach races, and the
+        * cgroup is pinned by the parent running this.
+        *
+        * Silence PROVE_RCU.
+        */

Hum,.. not sure that's actually true though, the parent itself is still susceptible to races afaict..

Quoted 8 lines · Peter Zijlstra

Quoted 6 lines · Peter Zijlstra

kernel/sched.c +6 -0

+       /*
+        * We're not in the pid-hash yet so no cgroup attach races, and the
+        * cgroup is pinned by the parent running this.
+        *
+        * Silence PROVE_RCU.
+        */

Quoted 2 lines

Hum,.. not sure that's actually true though, the parent itself is still susceptible to races afaict..

Do you want what you sent earlier tested, or should I wait for another patch?

Paul Menage, Li Zefan, any comments?

---
Because cgroup_fork() is ran before sched_fork() [ from copy_process() ] and the child's pid is not yet visible the child is pinned to its cgroup. Therefore we can silence this warning.

A nicer solution would be moving cgroup_fork() to right after dup_task_struct() and exclude PF_STARTING from task_subsys_state().

Signed-off-by Peter Zijlstra <[email protected]>

---
 kernel/sched.c |    9 +++++++++
 1 files changed, 9 insertions(+), 0 deletions(-)

kernel/sched.c +9 -0

index b697606..2e79518 100644
@@ -2561,7 +2561,16 @@ void sched_fork(struct task_struct *p, int clone_flags)
 	if (p->sched_class->task_fork)
 		p->sched_class->task_fork(p);
 
+	/*
+	 * The child is not yet in the pid-hash so no cgroup attach races,
+	 * and the cgroup is pinned to this child due to cgroup_fork()
+	 * is ran before sched_fork().
+	 *
+	 * Silence PROVE_RCU.
+	 */
+	rcu_read_lock();
 	set_task_cpu(p, cpu);
+	rcu_read_unlock();
 
 #if defined(CONFIG_SCHEDSTATS) || defined(CONFIG_TASK_DELAY_ACCT)
 	if (likely(sched_info_on()))