[PATCH v5 03/10] bcache: quit dc->writeback_thread when BCACHE_DEV_DETACHING is set

Coly Li
2018-02-06T04:21:37Z

In patch "bcache: fix cached_dev->count usage for bch_cache_set_error()",
cached_dev_get() is called when creating dc->writeback_thread, and
cached_dev_put() is called when exiting dc->writeback_thread. This
modification works well unless people detach the bcache device manually by
'echo 1 > /sys/block/bcache<N>/bcache/detach'
Because this sysfs interface only calls bch_cached_dev_detach() which wakes
up dc->writeback_thread but does not stop it. The reason is, before patch
"bcache: fix cached_dev->count usage for bch_cache_set_error()", inside
bch_writeback_thread(), if cache is not dirty after writeback,
cached_dev_put() will be called here. And in cached_dev_make_request() when
a new write request makes cache from clean to dirty, cached_dev_get() will
be called there. Since we don't operate dc->count in these locations,
refcount d->count cannot be dropped after cache becomes clean, and
cached_dev_detach_finish() won't be called to detach bcache device.

This patch fixes the issue by checking whether BCACHE_DEV_DETACHING is
set inside bch_writeback_thread(). If this bit is set and cache is clean
(no existing writeback_keys), break the while-loop, call cached_dev_put()
and quit the writeback thread.

Please note if cache is still dirty, even BCACHE_DEV_DETACHING is set the
writeback thread should continue to perform writeback, this is the original
design of manually detach.

It is safe to do the following check without locking, let me explain why,
+ if (!test_bit(BCACHE_DEV_DETACHING, &dc->disk.flags) &&
+ (!atomic_read(&dc->has_dirty) || !dc->writeback_running)) {

If the kenrel thread does not sleep and continue to run due to conditions
are not updated in time on the running CPU core, it just consumes more CPU
cycles and has no hurt. This should-sleep-but-run is safe here. We just
focus on the should-run-but-sleep condition, which means the writeback
thread goes to sleep in mistake while it should continue to run.
1, First of all, no matter the writeback thread is hung or not, kthread_stop() from
cached_dev_detach_finish() will wake up it and terminate by making
kthread_should_stop() return true. And in normal run time, bit on index
BCACHE_DEV_DETACHING is always cleared, the condition
!test_bit(BCACHE_DEV_DETACHING, &dc->disk.flags)
is always true and can be ignored as constant value.
2, If one of the following conditions is true, the writeback thread should
go to sleep,
"!atomic_read(&dc->has_dirty)" or "!dc->writeback_running)"
each of them independently controls the writeback thread should sleep or
not, let's analyse them one by one.
2.1 condition "!atomic_read(&dc->has_dirty)"
If dc->has_dirty is set from 0 to 1 on another CPU core, bcache will
call bch_writeback_queue() immediately or call bch_writeback_add() which
indirectly calls bch_writeback_queue() too. In bch_writeback_queue(),
wake_up_process(dc->writeback_thread) is called. It sets writeback
thread's task state to TASK_RUNNING and following an implicit memory
barrier, then tries to wake up the writeback thread.
In writeback thread, its task state is set to TASK_INTERRUPTIBLE before
doing the condition check. If other CPU core sets the TASK_RUNNING state
after writeback thread setting TASK_INTERRUPTIBLE, the writeback thread
will be scheduled to run very soon because its state is not
TASK_INTERRUPTIBLE. If other CPU core sets the TASK_RUNNING state before
writeback thread setting TASK_INTERRUPTIBLE, the implict memory barrier
of wake_up_process() will make sure modification of dc->has_dirty on
other CPU core is updated and observed on the CPU core of writeback
thread. Therefore the condition check will correctly be false, and
continue writeback code without sleeping.
2.2 condition "!dc->writeback_running)"
dc->writeback_running can be changed via sysfs file, every time it is
modified, a following bch_writeback_queue() is alwasy called. So the
change is always observed on the CPU core of writeback thread. If
dc->writeback_running is changed from 0 to 1 on other CPU core, this
condition check will observe the modification and allow writeback
thread to continue to run without sleeping.
Now we can see, even without a locking protection, multiple conditions
check is safe here, no deadlock or process hang up will happen.

I compose a separte patch because that patch "bcache: fix cached_dev->count
usage for bch_cache_set_error()" already gets a "Reviewed-by:" from Hannes
Reinecke. Also this fix is not trivial and good for a separate patch.

Signed-off-by: Coly Li <colyli@suse.de>
Cc: Michael Lyle <mlyle@lyle.org>
Cc: Hannes Reinecke <hare@suse.com>
Cc: Huijun Tang <tang.junhui@zte.com.cn>
---
drivers/md/bcache/writeback.c | 20 +++++++++++++++++---
1 file changed, 17 insertions(+), 3 deletions(-)

diff --git a/drivers/md/bcache/writeback.c b/drivers/md/bcache/writeback.c
index b280c134dd4d..4dbeaaa575bf 100644
--- a/drivers/md/bcache/writeback.c
+++ b/drivers/md/bcache/writeback.c
@@ -565,9 +565,15 @@ static int bch_writeback_thread(void *arg)
while (!kthread_should_stop()) {
down_write(&dc->writeback_lock);
set_current_state(TASK_INTERRUPTIBLE);
- if (!atomic_read(&dc->has_dirty) ||
- (!test_bit(BCACHE_DEV_DETACHING, &dc->disk.flags) &&
- !dc->writeback_running)) {
+ /*
+ * If the bache device is detaching, skip here and continue
+ * to perform writeback. Otherwise, if no dirty data on cache,
+ * or there is dirty data on cache but writeback is disabled,
+ * the writeback thread should sleep here and wait for others
+ * to wake up it.
+ */
+ if (!test_bit(BCACHE_DEV_DETACHING, &dc->disk.flags) &&
+ (!atomic_read(&dc->has_dirty) || !dc->writeback_running)) {
up_write(&dc->writeback_lock);

if (kthread_should_stop()) {
@@ -587,6 +593,14 @@ static int bch_writeback_thread(void *arg)
atomic_set(&dc->has_dirty, 0);
SET_BDEV_STATE(&dc->sb, BDEV_STATE_CLEAN);
bch_write_bdev_super(dc, NULL);
+ /*
+ * If bcache device is detaching via sysfs interface,
+ * writeback thread should stop after there is no dirty
+ * data on cache. BCACHE_DEV_DETACHING flag is set in
+ * bch_cached_dev_detach().
+ */
+ if (test_bit(BCACHE_DEV_DETACHING, &dc->disk.flags))
+ break;
}

up_write(&dc->writeback_lock);
--
2.16.1