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

[Shi, Yang]

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.

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

Thanks,
Yang

> > [<ffffffc000374f90>] wb_writeback+0x620/0x830
> > [<ffffffc000376224>] wb_workfn+0x61c/0x950
> > [<ffffffc000110adc>] process_one_work+0x3ac/0xb30
> > [<ffffffc0001112fc>] worker_thread+0x9c/0x7a8
> > [<ffffffc00011a9e8>] kthread+0x190/0x1b0
> > [<ffffffc000086ca0>] ret_from_fork+0x10/0x30
> >
> > The list_lock was moved outside the for loop by commit
> > e8dfc30582995ae12454cda517b17d6294175b07 ("writeback: elevate queue_io()
> > into wb_writeback())", however, the commit log says "No behavior change", so
> > it sounds safe to have the list_lock acquired inside the for loop as it did
> > before.
> >
> > Just acquire list_lock at the necessary points and keep all writeback
> > tracepoints outside the critical area protected by list_lock in
> > wb_writeback().
>
> But list_lock itself is a sleeping lock. This doesn't make sense.
>
> This is not the bug you are looking for.
>
> -- Steve
>
> > Signed-off-by: Yang Shi <yang.shi@xxxxxxxxxx>
> > ---
> >   fs/fs-writeback.c | 12 +++++++-----
> >   1 file changed, 7 insertions(+), 5 deletions(-)
> >
> > diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
> > index 1f76d89..9b7b5f6 100644
> > --- a/fs/fs-writeback.c
> > +++ b/fs/fs-writeback.c
> > @@ -1623,7 +1623,6 @@ static long wb_writeback(struct bdi_writeback *wb,
> >   	work->older_than_this = &oldest_jif;
> >
> >   	blk_start_plug(&plug);
> > -	spin_lock(&wb->list_lock);
> >   	for (;;) {
> >   		/*
> >   		 * Stop writeback when nr_pages has been consumed
> > @@ -1661,15 +1660,19 @@ static long wb_writeback(struct bdi_writeback *wb,
> >   			oldest_jif = jiffies;
> >
> >   		trace_writeback_start(wb, work);
> > +
> > +		spin_lock(&wb->list_lock);
> >   		if (list_empty(&wb->b_io))
> >   			queue_io(wb, work);
> >   		if (work->sb)
> >   			progress = writeback_sb_inodes(work->sb, wb, work);
> >   		else
> >   			progress = __writeback_inodes_wb(wb, work);
> > -		trace_writeback_written(wb, work);
> >
> >   		wb_update_bandwidth(wb, wb_start);
> > +		spin_unlock(&wb->list_lock);
> > +
> > +		trace_writeback_written(wb, work);
> >
> >   		/*
> >   		 * Did we write something? Try for more
> > @@ -1693,15 +1696,14 @@ static long wb_writeback(struct bdi_writeback *wb,
> >   		 */
> >   		if (!list_empty(&wb->b_more_io))  {
> >   			trace_writeback_wait(wb, work);
> > +			spin_lock(&wb->list_lock);
> >   			inode = wb_inode(wb->b_more_io.prev);
> > -			spin_lock(&inode->i_lock);
> >   			spin_unlock(&wb->list_lock);
> > +			spin_lock(&inode->i_lock);
> >   			/* This function drops i_lock... */
> >   			inode_sleep_on_writeback(inode);
> > -			spin_lock(&wb->list_lock);
> >   		}
> >   	}
> > -	spin_unlock(&wb->list_lock);
> >   	blk_finish_plug(&plug);
> >
> >   	return nr_pages - work->nr_pages;