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

From: Shi, Yang
Date: Thu Feb 25 2016 - 14:38:55 EST

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:

struct device *dev = bdi->dev;
if (!dev)
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))


[<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

Just acquire list_lock at the necessary points and keep all writeback
tracepoints outside the critical area protected by list_lock in

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;

- 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);
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_lock(&inode->i_lock);
/* This function drops i_lock... */
- spin_lock(&wb->list_lock);
- spin_unlock(&wb->list_lock);

return nr_pages - work->nr_pages;