Re: [PATCH 6/9] sched: add a sched_work list

[Jens Axboe <axboe@kernel.dk>]

On 2/20/20 2:53 PM, Jens Axboe wrote:
> On 2/20/20 2:17 PM, Peter Zijlstra wrote:
>> On Thu, Feb 20, 2020 at 01:31:48PM -0700, Jens Axboe wrote:
>>> This is similar to the task_works, and uses the same infrastructure, but
>>> the sched_work list is run when the task is being scheduled in or out.
>>>
>>> The intended use case here is for core code to be able to add work
>>> that should be automatically run by the task, without the task needing
>>> to do anything. This is done outside of the task, one example would be
>>> from waitqueue handlers, or anything else that is invoked out-of-band
>>> from the task itself.
>>>
>>
>>
>>> diff --git a/kernel/task_work.c b/kernel/task_work.c
>>> index 3445421266e7..ba62485d5b3d 100644
>>> --- a/kernel/task_work.c
>>> +++ b/kernel/task_work.c
>>> @@ -3,7 +3,14 @@
>>>  #include <linux/task_work.h>
>>>  #include <linux/tracehook.h>
>>>  
>>> -static struct callback_head work_exited; /* all we need is ->next == NULL */
>>> +static void task_exit_func(struct callback_head *head)
>>> +{
>>> +}
>>> +
>>> +static struct callback_head work_exited = {
>>> +	.next	= NULL,
>>> +	.func	= task_exit_func,
>>> +};
>>
>> Do we really need this? It seems to suggest we're trying to execute
>> work_exited, which would be an error.
>>
>> Doing so would be the result of calling sched_work_run() after
>> exit_task_work(). I suppose that's actually possible.. the problem is
>> that that would reset sched_work to NULL and re-allow queueing works,
>> which would then leak.
>>
>> I'll look at it in more detail tomorrow, I'm tired...
> 
> Let me try and instrument it, I definitely hit it on task exit
> but might have been induced by some earlier bugs.

I suspect I hit this before we added sched_work_run() to
exit_task_work(). I re-ran all the testing, and it's definitely
not trigger now.

So I'll remove it for now.

-- 
Jens Axboe