Re: [PATCH] writeback: call writeback tracepoints withoud holding list_lock in wb_writeback()

[Steven Rostedt]

On Thu, 25 Feb 2016 11:38:48 -0800
"Shi, Yang" <yang.shi@xxxxxxxxxx> wrote:

> On 2/24/2016 6:40 PM, Steven Rostedt wrote:
> > On Wed, 24 Feb 2016 14:47:23 -0800
> > Yang Shi <yang.shi@xxxxxxxxxx> wrote:
> >  
> >> 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.
> >  
> >>
> >> 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.

> 
> >
> >  
> >> 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

> >>
> >> 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