Re: [PATCH 06/10] exit: Implement kthread_exit

Eric W. Biederman
2022-01-10T15:05:34Z

Al Viro <viro@zeniv.linux.org.uk> writes:

> On Sat, Jan 08, 2022 at 12:35:40PM -0600, Eric W. Biederman wrote:
>
>> There are kernel threads started by modules that do:
>> complete(...);
>> return 0;
>>
>> That should be at a minimum calling complete_and_exit. Possibly should
>> be restructured to use kthread_stop().
>>
>> Some of those users of the now removed thread_exit() in staging are
>> among the offenders.
>>
>> However thread_exit() was implemented as:
>> #define thread_exit() complete_and_exit(NULL, 0)
>>
>> Which does nothing with a completion, it was just a really funny way to
>> spell "do_exit(0)".
>
> Yes. And there's a plenty of cargo-culting in that area.
>
>> While I agree digging through all of the kernel threads and finding the
>> ones that should be calling complete_and_exit is a fine idea. It is
>> a concern independent of these patches.
>
> BTW, could somebody explain how could this
> /*
> * Prevent the kthread exits directly, and make sure when kthread_stop()
> * is called to stop a kthread, it is still alive. If a kthread might be
> * stopped by CACHE_SET_IO_DISABLE bit set, wait_for_kthread_stop() is
> * necessary before the kthread returns.
> */
> static inline void wait_for_kthread_stop(void)
> {
> while (!kthread_should_stop()) {
> set_current_state(TASK_INTERRUPTIBLE);
> schedule();
> }
> }
>
> in drivers/md/bcache/bcache.h possibly avoid losing wakeups?
>
> AFAICS, it can be called while in TASK_RUNNING. Suppose kthread_stop()
> gets called just after the check for kthread_should_stop(). Our thread
> is still in TASK_RUNNING; kthread_stop() sets the flag for the next
> kthread_should_stop() to observe and does wake_up_process() to our
> thread. Which does nothing. Now our thread goes into TASK_INTERRUPTIBLE
> and calls schedule(). Sure, as soon as it gets woken up it'll call
> kthread_should_stop(), get true from it and that's it. What's going
> to wake it up, though?
>
> The same goes for e.g. fs/btrfs/disk-io.c:cleaner_kthread():
> if (kthread_should_stop())
> return 0;
> if (!again) {
> set_current_state(TASK_INTERRUPTIBLE);
> schedule();
> __set_current_state(TASK_RUNNING);
> }
> can't be right. Similar fun exists in e.g. fs/jfs, etc.
>
> Am I missing something?

Those examples look as suspect to me as they do to you.

Eric