sched: flush plug in schedule_preempt_disabled() to prevent deadlock#821
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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.
Fixes: 73c1010 ("block: initial patch for on-stack per-task plugging")
Reported-by: Michael Wu <michael@allwinnertech.com>
Tested-by: Michael Wu <michael@allwinnertech.com>
Reported-by: Xiaosen He <xiaosen.he@oss.qualcomm.com>
Link: https://lore.kernel.org/linux-block/20260417082744.30124-1-michael@allwinnertech.com/
Signed-off-by: Ming Lei <tom.leiming@gmail.com>
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Pull request for series with
subject: sched: flush plug in schedule_preempt_disabled() to prevent deadlock
version: 1
url: https://patchwork.kernel.org/project/linux-block/list/?series=1093387