[PATCH v3] workqueue: Warn flushing of kernel-global workqueues

[Tetsuo Handa]

On 2022/02/24 7:29, Tejun Heo wrote:
> On Thu, Feb 24, 2022 at 07:26:30AM +0900, Tetsuo Handa wrote:
>>> The patch seems pretty wrong. What's problematic is system workqueue flushes
>>> (which flushes the entire workqueue), not work item flushes.
>>
>> Why? My understanding is that
>>
>>   flushing a workqueue waits for completion of all work items in that workqueue
>>
>>   flushing a work item waits for for completion of that work item using
>>   a workqueue specified as of queue_work()
>>
>> and
>>
>>   if a work item in some workqueue is blocked by other work in that workqueue
>>   (e.g. max_active limit, work items on that workqueue and locks they need),
>>   it has a risk of deadlock
>>
>> . Then, how can flushing a work item using system-wide workqueues be free of deadlock risk?
>> Isn't it just "unlikely to deadlock" rather than "impossible to deadlock"?
> 
> If we're jamming system_wq with a combination of work items which need more
> than max_active to make forward progress, we're stuck regardless of flushes.
> What's needed at that point is increasing max_active (or something along
> that line).

Then, what about this?
If this looks OK, I'll test this patch using linux-next via my tree.