[PATCH 4.14 147/183] bcache: properly set task state in bch_writeback_thread()

From: Greg Kroah-Hartman
Date: Wed Apr 25 2018 - 06:50:20 EST

4.14-stable review patch. If anyone has any objections, please let me know.


From: Coly Li <colyli@xxxxxxx>

[ Upstream commit 99361bbf26337186f02561109c17a4c4b1a7536a ]

Kernel thread routine bch_writeback_thread() has the following code block,

447 down_write(&dc->writeback_lock);
448~450 if (check conditions) {
451 up_write(&dc->writeback_lock);
452 set_current_state(TASK_INTERRUPTIBLE);
454 if (kthread_should_stop())
455 return 0;
457 schedule();
458 continue;
459 }

If condition check is true, its task state is set to TASK_INTERRUPTIBLE
and call schedule() to wait for others to wake up it.

There are 2 issues in current code,
1, Task state is set to TASK_INTERRUPTIBLE after the condition checks, if
another process changes the condition and call wake_up_process(dc->
writeback_thread), then at line 452 task state is set back to
TASK_INTERRUPTIBLE, the writeback kernel thread will lose a chance to be
waken up.
2, At line 454 if kthread_should_stop() is true, writeback kernel thread
will return to kernel/kthread.c:kthread() with TASK_INTERRUPTIBLE and
call do_exit(). It is not good to enter do_exit() with task state
TASK_INTERRUPTIBLE, in following code path might_sleep() is called and a
warning message is reported by __might_sleep(): "WARNING: do not call
blocking ops when !TASK_RUNNING; state=1 set at [xxxx]".

For the first issue, task state should be set before condition checks.
Ineed because dc->writeback_lock is required when modifying all the
conditions, calling set_current_state() inside code block where dc->
writeback_lock is hold is safe. But this is quite implicit, so I still move
set_current_state() before all the condition checks.

For the second issue, frankley speaking it does not hurt when kernel thread
exits with TASK_INTERRUPTIBLE state, but this warning message scares users,
makes them feel there might be something risky with bcache and hurt their
data. Setting task state to TASK_RUNNING before returning fixes this

In alloc.c:allocator_wait(), there is also a similar issue, and is also
fixed in this patch.

v3: merge two similar fixes into one patch
v2: fix the race issue in v1 patch.
v1: initial buggy fix.

Signed-off-by: Coly Li <colyli@xxxxxxx>
Reviewed-by: Hannes Reinecke <hare@xxxxxxx>
Reviewed-by: Michael Lyle <mlyle@xxxxxxxx>
Cc: Michael Lyle <mlyle@xxxxxxxx>
Cc: Junhui Tang <tang.junhui@xxxxxxxxxx>
Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>
Signed-off-by: Sasha Levin <alexander.levin@xxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
drivers/md/bcache/alloc.c | 4 +++-
drivers/md/bcache/writeback.c | 7 +++++--
2 files changed, 8 insertions(+), 3 deletions(-)

--- a/drivers/md/bcache/alloc.c
+++ b/drivers/md/bcache/alloc.c
@@ -287,8 +287,10 @@ do { \
break; \
mutex_unlock(&(ca)->set->bucket_lock); \
- if (kthread_should_stop()) \
+ if (kthread_should_stop()) { \
+ set_current_state(TASK_RUNNING); \
return 0; \
+ } \
schedule(); \
mutex_lock(&(ca)->set->bucket_lock); \
--- a/drivers/md/bcache/writeback.c
+++ b/drivers/md/bcache/writeback.c
@@ -420,18 +420,21 @@ static int bch_writeback_thread(void *ar

while (!kthread_should_stop()) {
+ set_current_state(TASK_INTERRUPTIBLE);
if (!atomic_read(&dc->has_dirty) ||
(!test_bit(BCACHE_DEV_DETACHING, &dc->disk.flags) &&
!dc->writeback_running)) {
- set_current_state(TASK_INTERRUPTIBLE);

- if (kthread_should_stop())
+ if (kthread_should_stop()) {
+ set_current_state(TASK_RUNNING);
return 0;
+ }

+ set_current_state(TASK_RUNNING);

searched_full_index = refill_dirty(dc);