suspicious RCU usage in cgroup

On boot in Linus' current tree..

===============================
[ INFO: suspicious RCU usage. ]
3.6.0+ #22 Not tainted
-------------------------------
include/linux/cgroup.h:566 suspicious rcu_dereference_check() usage!

other info that might help us debug this:

rcu_scheduler_active = 1, debug_locks = 1
2 locks on stack by kdevtmpfs/59:
#0: blocked: (sb_writers){.+.+.+}, instance: ffff88014595a9a0, at: [<ffffffff81207bc4>] mnt_want_write+0x24/0x50
#1: held: (&sb->s_type->i_mutex_key#3/1){+.+.+.}, instance: ffff880145b1f5c8, at: [<ffffffff811f26d4>] kern_path_create+0x84/0x170

stack backtrace:
Pid: 59, comm: kdevtmpfs Not tainted 3.6.0+ #22
Call Trace:
 [<ffffffff810df1f2>] lockdep_rcu_suspicious+0xe2/0x130
 [<ffffffff81317cad>] devcgroup_inode_mknod+0x19d/0x240
 [<ffffffff812f54d8>] ? security_capable+0x18/0x20
 [<ffffffff8107e214>] ? ns_capable+0x44/0x80
 [<ffffffff811f3b0e>] vfs_mknod+0x7e/0x140
 [<ffffffff81452967>] handle_create.isra.2+0x77/0x210
 [<ffffffff810ddd6c>] ? lock_release_holdtime.part.26+0xcc/0x140
 [<ffffffff810afbf1>] ? get_parent_ip+0x11/0x50
 [<ffffffff816bacd9>] ? sub_preempt_count+0x79/0xd0
 [<ffffffff81452c47>] devtmpfsd+0x147/0x170
 [<ffffffff81452b00>] ? handle_create.isra.2+0x210/0x210

 [<ffffffff8109bf8d>] kthread+0xed/0x100
 [<ffffffff816bacd9>] ? sub_preempt_count+0x79/0xd0
 [<ffffffff816c04a4>] kernel_thread_helper+0x4/0x10
 [<ffffffff810a9fdc>] ? finish_task_switch+0x7c/0x120
 [<ffffffff816b62db>] ? _raw_spin_unlock_irq+0x4b/0x80
 [<ffffffff816b6d37>] ? retint_restore_args+0x13/0x13
 [<ffffffff8109bea0>] ? kthread_create_on_node+0x160/0x160
 [<ffffffff816c04a0>] ? gs_change+0x13/0x13

Hi Dave,

Quoted 29 lines · Dave Jones

On boot in Linus' current tree..

===============================
[ INFO: suspicious RCU usage. ]
3.6.0+ #22 Not tainted
-------------------------------
include/linux/cgroup.h:566 suspicious rcu_dereference_check() usage!

other info that might help us debug this:

rcu_scheduler_active = 1, debug_locks = 1
2 locks on stack by kdevtmpfs/59:
#0: blocked: (sb_writers){.+.+.+}, instance: ffff88014595a9a0, at: [<ffffffff81207bc4>] mnt_want_write+0x24/0x50
#1: held: (&sb->s_type->i_mutex_key#3/1){+.+.+.}, instance: ffff880145b1f5c8, at: [<ffffffff811f26d4>] kern_path_create+0x84/0x170

stack backtrace:
Pid: 59, comm: kdevtmpfs Not tainted 3.6.0+ #22
Call Trace:
 [<ffffffff810df1f2>] lockdep_rcu_suspicious+0xe2/0x130
 [<ffffffff81317cad>] devcgroup_inode_mknod+0x19d/0x240
 [<ffffffff812f54d8>] ? security_capable+0x18/0x20
 [<ffffffff8107e214>] ? ns_capable+0x44/0x80
 [<ffffffff811f3b0e>] vfs_mknod+0x7e/0x140
 [<ffffffff81452967>] handle_create.isra.2+0x77/0x210
 [<ffffffff810ddd6c>] ? lock_release_holdtime.part.26+0xcc/0x140
 [<ffffffff810afbf1>] ? get_parent_ip+0x11/0x50
 [<ffffffff816bacd9>] ? sub_preempt_count+0x79/0xd0
 [<ffffffff81452c47>] devtmpfsd+0x147/0x170
 [<ffffffff81452b00>] ? handle_create.isra.2+0x210/0x210

Quoted 8 lines

 [<ffffffff8109bf8d>] kthread+0xed/0x100
 [<ffffffff816bacd9>] ? sub_preempt_count+0x79/0xd0
 [<ffffffff816c04a4>] kernel_thread_helper+0x4/0x10
 [<ffffffff810a9fdc>] ? finish_task_switch+0x7c/0x120
 [<ffffffff816b62db>] ? _raw_spin_unlock_irq+0x4b/0x80
 [<ffffffff816b6d37>] ? retint_restore_args+0x13/0x13
 [<ffffffff8109bea0>] ? kthread_create_on_node+0x160/0x160
 [<ffffffff816c04a0>] ? gs_change+0x13/0x13

"[PATCH] cgroup: fix invalid rcu dereference" which is being routed thru akpm's tree should fix this.

see https://patchwork.kernel.org/patch/1456571/

Quoted 40 lines · Aristeu Rozanski

Hi Dave,

Quoted 37 lines · Dave Jones

On boot in Linus' current tree..

===============================
[ INFO: suspicious RCU usage. ]
3.6.0+ #22 Not tainted
-------------------------------
include/linux/cgroup.h:566 suspicious rcu_dereference_check() usage!

other info that might help us debug this:

rcu_scheduler_active = 1, debug_locks = 1
2 locks on stack by kdevtmpfs/59:
#0: blocked: (sb_writers){.+.+.+}, instance: ffff88014595a9a0, at: [<ffffffff81207bc4>] mnt_want_write+0x24/0x50
#1: held: (&sb->s_type->i_mutex_key#3/1){+.+.+.}, instance: ffff880145b1f5c8, at: [<ffffffff811f26d4>] kern_path_create+0x84/0x170

stack backtrace:
Pid: 59, comm: kdevtmpfs Not tainted 3.6.0+ #22
Call Trace:
 [<ffffffff810df1f2>] lockdep_rcu_suspicious+0xe2/0x130
 [<ffffffff81317cad>] devcgroup_inode_mknod+0x19d/0x240
 [<ffffffff812f54d8>] ? security_capable+0x18/0x20
 [<ffffffff8107e214>] ? ns_capable+0x44/0x80
 [<ffffffff811f3b0e>] vfs_mknod+0x7e/0x140
 [<ffffffff81452967>] handle_create.isra.2+0x77/0x210
 [<ffffffff810ddd6c>] ? lock_release_holdtime.part.26+0xcc/0x140
 [<ffffffff810afbf1>] ? get_parent_ip+0x11/0x50
 [<ffffffff816bacd9>] ? sub_preempt_count+0x79/0xd0
 [<ffffffff81452c47>] devtmpfsd+0x147/0x170
 [<ffffffff81452b00>] ? handle_create.isra.2+0x210/0x210
 [<ffffffff8109bf8d>] kthread+0xed/0x100
 [<ffffffff816bacd9>] ? sub_preempt_count+0x79/0xd0
 [<ffffffff816c04a4>] kernel_thread_helper+0x4/0x10
 [<ffffffff810a9fdc>] ? finish_task_switch+0x7c/0x120
 [<ffffffff816b62db>] ? _raw_spin_unlock_irq+0x4b/0x80
 [<ffffffff816b6d37>] ? retint_restore_args+0x13/0x13
 [<ffffffff8109bea0>] ? kthread_create_on_node+0x160/0x160
 [<ffffffff816c04a0>] ? gs_change+0x13/0x13

Quoted 4 lines

"[PATCH] cgroup: fix invalid rcu dereference" which is being routed thru akpm's tree should fix this.

see https://patchwork.kernel.org/patch/1456571/

What's happening with this ?