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);
453
454 if (kthread_should_stop())
455 return 0;
456
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
problem.
Changelog:
v2: fix the race issue in v1 patch.
v1: initial buggy fix.
Signed-off-by: Coly Li <colyli@xxxxxxx>
Cc: Michael Lyle <mlyle@xxxxxxxx>
Cc: Hannes Reinecke <hare@xxxxxxx>
Cc: Junhui Tang <tang.junhui@xxxxxxxxxx>
---
drivers/md/bcache/writeback.c | 7 +++++--
1 file changed, 5 insertions(+), 2 deletions(-)
diff --git a/drivers/md/bcache/writeback.c b/drivers/md/bcache/writeback.c
index 0ade883b6316..f1d2fc15abcc 100644
--- a/drivers/md/bcache/writeback.c
+++ b/drivers/md/bcache/writeback.c
@@ -564,18 +564,21 @@ 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)) {
up_write(&dc->writeback_lock);
- set_current_state(TASK_INTERRUPTIBLE);
- if (kthread_should_stop())
+ if (kthread_should_stop()) {
+ set_current_state(TASK_RUNNING);
return 0;
+ }
schedule();
continue;
}
+ set_current_state(TASK_RUNNING);
searched_full_index = refill_dirty(dc);
--
2.15.1
