Re: [cgroups] suspicious rcu_dereference_check() usage!

[Tejun Heo]

Hello, Mike.

Sorry about the delay.  Was on vacation and then had a strep throat.

On Mon, Feb 20, 2017 at 11:15:23AM +0100, Mike Galbraith wrote:
> Running LTP on master.today (v4.10) with a seriously bloated PREEMPT
> config inspired box to emit the below.
> 
> [ 7160.458996] ===============================
> [ 7160.463195] [ INFO: suspicious RCU usage. ]
> [ 7160.467387] 4.10.0-default #100 Tainted: G            E  
> [ 7160.472808] -------------------------------
> [ 7160.476999] ./include/linux/cgroup.h:435 suspicious rcu_dereference_check() usage!
> [ 7160.484576] 
> [ 7160.484576] other info that might help us debug this:
> [ 7160.484576] 
> [ 7160.492577] 
> [ 7160.492577] rcu_scheduler_active = 2, debug_locks = 1
> [ 7160.499113] 1 lock held by pids_task1/19308:
> [ 7160.503390]  #0:  (&cgroup_threadgroup_rwsem){+++++.}, at: [<ffffffff81095b60>] _do_fork+0xf0/0x710
> [ 7160.512450] 
> [ 7160.512450] stack backtrace:
> [ 7160.516810] CPU: 5 PID: 19308 Comm: pids_task1 Tainted: G            E   4.10.0-default #100
> [ 7160.525239] Hardware name: IBM System x3550 M3 -[7944K3G]-/69Y5698     , BIOS -[D6E150AUS-1.10]- 12/15/2010
> [ 7160.534965] Call Trace:
> [ 7160.537414]  dump_stack+0x85/0xc9
> [ 7160.540732]  lockdep_rcu_suspicious+0xd5/0x110
> [ 7160.545177]  task_css.constprop.7+0x88/0x90
> [ 7160.549357]  pids_can_fork+0x132/0x160
> [ 7160.553106]  cgroup_can_fork+0x63/0xc0

That gotta be a spurious one coming from the incorrect
!CONFIG_PROVE_RCU definition of the task_css_set_check() macro.
Something like the following should fix it.

If you still have the .config around, can you please attach it?  I'll
verify the fix and send out the fix.

Thanks.

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index f6b43fb..870ea9a 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -396,7 +396,7 @@ extern spinlock_t css_set_lock;
 		((task)->flags & PF_EXITING) || (__c))
 #else
 #define task_css_set_check(task, __c)					\
-	rcu_dereference((task)->cgroups)
+	rcu_dereference_check((task)->cgroups, (__c))
 #endif
 
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