On Thu, 25 Feb 2016 11:38:48 -0800
"Shi, Yang" [off-list ref] wrote:

On 2/24/2016 6:40 PM, Steven Rostedt wrote:

quoted

On Wed, 24 Feb 2016 14:47:23 -0800
Yang Shi [off-list ref] wrote:
 

quoted

commit 5634cc2aa9aebc77bc862992e7805469dcf83dac ("writeback: update writeback
tracepoints to report cgroup") made writeback tracepoints report cgroup
writeback, but it may trigger the below bug on -rt kernel due to the list_lock
held for the for loop in wb_writeback().  

list_lock is a sleeping mutex, it's not disabling preemption. Moving it
doesn't make a difference.
 

quoted

BUG: sleeping function called from invalid context at kernel/locking/rtmutex.c:930
in_atomic(): 1, irqs_disabled(): 0, pid: 625, name: kworker/u16:3  

Something else disabled preemption. And note, nothing in the tracepoint
should have called a sleeping function.  

Yes, it makes me confused too. It sounds like the preempt_ip address is 
not that accurate.

Yep, but the change you made doesn't look to be the fix.

quoted

quoted

INFO: lockdep is turned off.
Preemption disabled at:[<ffffffc000374a5c>] wb_writeback+0xec/0x830

Can you disassemble the vmlinux file to see exactly where that call is.
I use gdb to find the right locations.

 gdb> li *0xffffffc000374a5c
 gdb> disass 0xffffffc000374a5c

quoted

quoted

CPU: 7 PID: 625 Comm: kworker/u16:3 Not tainted 4.4.1-rt5 #20
Hardware name: Freescale Layerscape 2085a RDB Board (DT)
Workqueue: writeback wb_workfn (flush-7:0)
Call trace:
[<ffffffc00008d708>] dump_backtrace+0x0/0x200
[<ffffffc00008d92c>] show_stack+0x24/0x30
[<ffffffc0007b0f40>] dump_stack+0x88/0xa8
[<ffffffc000127d74>] ___might_sleep+0x2ec/0x300
[<ffffffc000d5d550>] rt_spin_lock+0x38/0xb8
[<ffffffc0003e0548>] kernfs_path_len+0x30/0x90
[<ffffffc00036b360>] trace_event_raw_event_writeback_work_class+0xe8/0x2e8  

How accurate is this trace back? Here's the code that is executed in
this tracepoint:

	TP_fast_assign(
		struct device *dev = bdi->dev;
		if (!dev)
			dev = default_backing_dev_info.dev;
		strncpy(__entry->name, dev_name(dev), 32);
		__entry->nr_pages = work->nr_pages;
		__entry->sb_dev = work->sb ? work->sb->s_dev : 0;
		__entry->sync_mode = work->sync_mode;
		__entry->for_kupdate = work->for_kupdate;
		__entry->range_cyclic = work->range_cyclic;
		__entry->for_background	= work->for_background;
		__entry->reason = work->reason;
	),

See anything that would sleep?  

According to the stack backtrace, kernfs_path_len calls slepping lock, 
which is called by __trace_wb_cgroup_size(wb) in __dynamic_array(char, 
cgroup, __trace_wb_cgroup_size(wb)).

The below is the definition:

DECLARE_EVENT_CLASS(writeback_work_class,
         TP_PROTO(struct bdi_writeback *wb, struct wb_writeback_work *work),
         TP_ARGS(wb, work),
         TP_STRUCT__entry(
                 __array(char, name, 32)
                 __field(long, nr_pages)
                 __field(dev_t, sb_dev)
                 __field(int, sync_mode)
                 __field(int, for_kupdate)
                 __field(int, range_cyclic)
                 __field(int, for_background)
                 __field(int, reason)
                 __dynamic_array(char, cgroup, __trace_wb_cgroup_size(wb))

Ah, thanks for pointing that out. I missed that.

-- Steve

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