Re: [PATCH v1 01/10] bcache: exit bch_writeback_thread() with proper task state

Coly Li
2018-01-05T17:05:22Z

On 04/01/2018 1:08 AM, Michael Lyle wrote:
> On 01/03/2018 06:03 AM, Coly Li wrote:
>> 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 <colyli@suse.de>

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