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

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.

quoted

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

quoted

[<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

quoted

Signed-off-by: Yang Shi <redacted>
---
  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;

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