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

[Peter Zijlstra <peterz@infradead.org>]

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.