On 04/01/2018 1:08 AM, Michael Lyle wrote:

On 01/03/2018 06:03 AM, Coly Li wrote:

quoted

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

452                         set_current_state(TASK_INTERRUPTIBLE);
453
454                         if (kthread_should_stop())
455                                 return 0;
456
457                         schedule();
458                         continue;

At line 452, its status is set to TASK_INTERRUPTIBLE, and at line 454 if
kthread_should_stop() is true, a "return 0" at line 455 will to function
kernel/kthread.c:kthread() 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]".

Indeed 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.

In this patch, TASK_INTERRUPTIBLE is set after kthread_should_stop(),
so writeback kernel thread can exist and enter do_exit() with
TASK_RUNNING state. Warning message from might_sleep() is removed.

Signed-off-by: Coly Li <redacted>

Hi Mike,

I feel uncomfortable with this patch, can you help me to double check ?

In my patch, I set task state to TASK_INTERRUPTIBLE after checking the
following conditions,
469	if (!atomic_read(&dc->has_dirty) ||
470	    (!test_bit(BCACHE_DEV_DETACHING, &dc->disk.flags) &&
471	     !dc->writeback_running)) {

And set_current_state(TASK_INTERRUPTIBLE) is after
up_write(&dc->writeback_lock), there should be a potential race window
between up_write() and set_current_state() in my patch. If
wake_up_process(dc->writeback_thread) is called in this window, and then
current state is set to TASK_INTERRUPTIBLE again, writeback thread will
lose a chance to be waken up.

It seems I need to call set_current_state(TASK_INTERRUPTIBLE) before
up_write(&dc->writeback_lock), then dc->writeback_lock will protect and
avoid the race. Or move set_current_state(TASK_INTERRUPTIBLE) to the
location before the condition checks in line 469-471.

This race window is very small but I believe it exists, I will fix it in
v2 patch set.

Thanks.

Coly Li