[PATCH AUTOSEL 4.19 050/219] bcache: do not mark writeback_running too early

Sasha Levin
2019-11-22T05:50:13Z

From: Shenghui Wang <shhuiw@foxmail.com>

[ Upstream commit 79b791466e525c98f6aeee9acf5726e7b27f4231 ]

A fresh backing device is not attached to any cache_set, and
has no writeback kthread created until first attached to some
cache_set.

But bch_cached_dev_writeback_init run
"
dc->writeback_running = true;
WARN_ON(test_and_clear_bit(BCACHE_DEV_WB_RUNNING,
&dc->disk.flags));
"
for any newly formatted backing devices.

For a fresh standalone backing device, we can get something like
following even if no writeback kthread created:
------------------------
/sys/block/bcache0/bcache# cat writeback_running
1
/sys/block/bcache0/bcache# cat writeback_rate_debug
rate: 512.0k/sec
dirty: 0.0k
target: 0.0k
proportional: 0.0k
integral: 0.0k
change: 0.0k/sec
next io: -15427384ms

The none ZERO fields are misleading as no alive writeback kthread yet.

Set dc->writeback_running false as no writeback thread created in
bch_cached_dev_writeback_init().

We have writeback thread created and woken up in bch_cached_dev_writeback
_start(). Set dc->writeback_running true before bch_writeback_queue()
called, as a writeback thread will check if dc->writeback_running is true
before writing back dirty data, and hung if false detected.

After the change, we can get the following output for a fresh standalone
backing device:
-----------------------
/sys/block/bcache0/bcache$ cat writeback_running
0
/sys/block/bcache0/bcache# cat writeback_rate_debug
rate: 0.0k/sec
dirty: 0.0k
target: 0.0k
proportional: 0.0k
integral: 0.0k
change: 0.0k/sec
next io: 0ms

v1 -> v2:
Set dc->writeback_running before bch_writeback_queue() called,

Signed-off-by: Shenghui Wang <shhuiw@foxmail.com>
Signed-off-by: Coly Li <colyli@suse.de>

Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
drivers/md/bcache/writeback.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/md/bcache/writeback.c b/drivers/md/bcache/writeback.c
index ba5395fd386d5..b5fc3c6c7178e 100644
--- a/drivers/md/bcache/writeback.c
+++ b/drivers/md/bcache/writeback.c
@@ -781,7 +781,7 @@ void bch_cached_dev_writeback_init(struct cached_dev *dc)
bch_keybuf_init(&dc->writeback_keys);

dc->writeback_metadata = true;
- dc->writeback_running = true;
+ dc->writeback_running = false;
dc->writeback_percent = 10;
dc->writeback_delay = 30;
atomic_long_set(&dc->writeback_rate.rate, 1024);
@@ -810,6 +810,7 @@ int bch_cached_dev_writeback_start(struct cached_dev *dc)
destroy_workqueue(dc->writeback_write_wq);
return PTR_ERR(dc->writeback_thread);
}
+ dc->writeback_running = true;

WARN_ON(test_and_set_bit(BCACHE_DEV_WB_RUNNING, &dc->disk.flags));
schedule_delayed_work(&dc->writeback_rate_update,
--
2.20.1