Re: [PATCH] sched: flush plug in schedule_preempt_disabled() to prevent deadlock

[Xiaosen]

https://lore.kernel.org/lkml/20260427183848.698551-2-jstultz@xxxxxxxxxx/
The above change can resolve the deadlock I reported before by setting
task's state to TASK_RUNNING before switching context.

There is the likely alternative fix below.
https://lore.kernel.org/lkml/20260512025635.2840817-1-jstultz@xxxxxxxxxx/

Regards,
Xiaosen

On 5/13/2026 4:08 PM, Ming Lei wrote:
> On Wed, May 13, 2026 at 09:30:39AM +0200, Peter Zijlstra wrote:
>> On Wed, May 13, 2026 at 10:07:03AM +0800, Ming Lei wrote:
>>> On Tue, May 12, 2026 at 07:16:36AM -1000, Tejun Heo wrote:
>>>> Hello, Ming.
>>>>
>>>> On Tue, May 12, 2026 at 11:45:14PM +0800, Ming Lei wrote:
>>>>> On Tue, May 12, 2026 at 02:40:21PM +0200, Peter Zijlstra wrote:
>>>>>> On Tue, May 12, 2026 at 02:04:32PM +0200, Peter Zijlstra wrote:
>>>>>>> On Tue, May 12, 2026 at 04:59:39PM +0800, Ming Lei wrote:
>>>>>>>> On preemptible kernels, a deadlock can occur when a task with plugged IO
>>>>>>>> calls schedule_preempt_disabled():
>>>>>>>>
>>>>>>>>   schedule_preempt_disabled()
>>>>>>>>     sched_preempt_enable_no_resched()  // preemption now enabled
>>>>>>>>     schedule()                         // <-- preemption can happen here
>>>>>>>>       sched_submit_work()
>>>>>>>>         blk_flush_plug()
>>>>>>>>
>>>>>>>> After sched_preempt_enable_no_resched() re-enables preemption, the task
>>>>>>>> can be preempted (e.g., by a higher-priority RT task) before reaching
>>>>>>>> blk_flush_plug() in sched_submit_work(). Since the task's state is
>>>>>>>> already TASK_UNINTERRUPTIBLE (set by the mutex/rwsem slowpath caller),
>>>>>>>> requests in current->plug remain unflushed for an unbounded time.
>>>>>>>>
>>>>>>>> If another task depends on those plugged requests to make progress (e.g.,
>>>>>>>> to release a lock the sleeping task needs), a deadlock results:
>>>>>>>>
>>>>>>>>   - Task A (writeback worker): holds plugged IO, preempted before
>>>>>>>>     flushing, stuck on run queue behind higher-priority work
>>>>>>>>   - Task B: waiting for IO completion from Task A's plug, holds a lock
>>>>>>>>     that Task A needs to be woken up
>>>>
>>>> My memory is hazy around io_schedule but the above reads really weird to me.
>>>> A task, regardless of its current state stays on the runqueue when
>>>> preempted, so the condition is temporary. As soon as the preempted task can
>>>> get CPU, it should unwind the situation. That's not a deadlock. Is the
>>>> problem that there can be preemption-induced delay in flushing the plugs?
>>>
>>> IMO, preempting a `!TASK_RUNNING` task can be thought as effective sleep,
>>
>> No it cannot be. Preemption ignores task state.
> 
> Yeah, I get similar conclusion too with AI's assistance.
> 
> But both two reports show that the preempted task aren't switched back for
> long enough time, can you share any idea for Michael & Xiaosen to investigate
> further from scheduler side?
> 
> https://lore.kernel.org/linux-block/20260417082744.30124-1-michael@xxxxxxxxxxxxxxxxx/
> 
> https://lore.kernel.org/linux-block/5660795d-87de-46f5-add4-7729a02225ef@xxxxxxxxxxxxxxxx/
> 
> 
> Thanks,
> Ming