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

[Peter Zijlstra]

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
> > 
> > Both reported deadlocks involve mutex/rwsem slowpaths, which are the
> > primary callers of schedule_preempt_disabled() with non-running task
> > state.
> > 
> > Fix by flushing the plug in schedule_preempt_disabled() while
> > preemption is still disabled. This ensures the plug is empty before the
> > preemption window opens.
> 
> How is this different from any path calling schedule()? That would be
> subject to exactly the same issue.
> 
> The patch cannot be correct.

Also, is there a reason io_schedule_prepare() has a blk_flush_plug()
call?

io_schedule()
  token = io_schedule_prepare()
    blk_flush_plug(current->plug, true);
  schedule()
    if (!task_is_running(tsk))
      sched_submit_work()
        blk_flush_plug(tsk->plug, true);

Why isn't the one in sched_submit_work() sufficient? This thing either
needs a comment justifying its existence, or get removed.