Clark showed me this:
| BUG: sleeping function called from invalid context at
kernel/locking/rtmutex.c:974
| in_atomic(): 1, irqs_disabled(): 0, pid: 1, name: systemd
| 5 locks held by systemd/1:
| #0: (sb_writers#7){.+.+}, at: [< (ptrval)>] mnt_want_write+0x1f/0x50
| #1: (&type->i_mutex_dir_key#3/1){+.+.}, at: [< (ptrval)>]
do_rmdir+0x14d/0x1f0
| #2: (&type->i_mutex_dir_key#5){++++}, at: [< (ptrval)>]
vfs_rmdir+0x50/0x150
| #3: (cgroup_mutex){+.+.}, at: [< (ptrval)>]
cgroup_kn_lock_live+0xfb/0x200
| #4: (kernfs_rename_lock){+.+.}, at: [< (ptrval)>]
kernfs_path_from_node+0x23/0x50
| Preemption disabled at:
| [<ffffffff8dc994c5>] migrate_enable+0x95/0x340
| CPU: 1 PID: 1 Comm: systemd Not tainted 4.16.18-rt9+ #173
| Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.11.1-1 04/01/2014
| Call Trace:
| dump_stack+0x70/0xa7
| ___might_sleep+0x159/0x240
| rt_spin_lock+0x4e/0x60
| ? kernfs_path_from_node+0x23/0x50
| kernfs_path_from_node+0x23/0x50
| trace_event_raw_event_cgroup+0x54/0x110
| cgroup_rmdir+0xdb/0x1a0
| kernfs_iop_rmdir+0x53/0x80
| vfs_rmdir+0x74/0x150
| do_rmdir+0x191/0x1f0
| SyS_rmdir+0x11/0x20
| do_syscall_64+0x73/0x220
| entry_SYSCALL_64_after_hwframe+0x42/0xb7
which is the trace_cgroup_rmdir() trace event in cgroup_rmdir(). The
trace event invokes cgroup_path() which acquires a spin_lock_t and this
is invoked within a preempt_disable()ed section.
It says "Preemption disabled at" migrate_enable() but this is not true.
A printk() just before the lock reports preempt_count() of two and
sometimes one. I *think*
- one is from rcu_read_lock_sched_notrace() in __DO_TRACE()
- the second is from preempt_disable_notrace() in ring_buffer_lock_reserve()
I would prefer not to turn kernfs_rename_lock into raw_spin_lock_t. We
had a similar problem with a i915 trace event which eventually vanished
(and before I just disabled it).
So how likely are chances that we can use rcu_read_lock() in __DO_TRACE()?
And how likely are chances that the preempt_disable() in
ring_buffer_lock_reserve() could be avoided while the trace event is
invoked?
I guess nothing of this is easy peasy. Any suggestions?
Sebastian
