Re: [PATCH] writeback: call writeback tracepoints withoud holding list_lock in wb_writeback()
From: Steven Rostedt
Date: Thu Feb 25 2016 - 14:54:42 EST
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:
> TP_PROTO(struct bdi_writeback *wb, struct wb_writeback_work *work),
> TP_ARGS(wb, work),
> __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.