sched: disable preemption around blk_flush_plug in sched_submit_work#765
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On preemptible kernels, a three-way deadlock can occur involving blk_mq_freeze_queue and blk_mq_dispatch_list: - Task A holds a filesystem lock (e.g., f2fs io_rwsem) and enters __bio_queue_enter(), waiting for mq_freeze_depth == 0 - Task B holds mq_freeze_depth=1 (elevator_change) and waits for q_usage_counter to reach zero in blk_mq_freeze_queue_wait() - Task C is going to sleep waiting for the filesystem lock. Before sleeping, schedule() calls sched_submit_work() -> blk_flush_plug() -> blk_mq_dispatch_list(), which acquires q_usage_counter via percpu_ref_get(). If Task C gets preempted before percpu_ref_put(), it will not be scheduled back because the task is already in uninterruptible sleep state (TASK_UNINTERRUPTIBLE). This means it holds the percpu_ref indefinitely, preventing freeze from completing. This is fundamentally an ABBA deadlock between queue freeze and the filesystem lock, exposed by preemption creating an artificial hold on q_usage_counter during the plug flush. Fix by disabling preemption around blk_flush_plug() in sched_submit_work(). The _notrace variants are used since this runs in scheduler context. preempt_enable_no_resched_notrace() is correct because we are already inside __schedule() and about to pick the next task. Fixes: 73c1010 ("block: initial patch for on-stack per-task plugging") Reported-by: Michael Wu <[email protected]> Tested-by: Michael Wu <[email protected]> Link: https://lore.kernel.org/linux-block/[email protected]/ Signed-off-by: Ming Lei <[email protected]>
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Pull request for series with
subject: sched: disable preemption around blk_flush_plug in sched_submit_work
version: 1
url: https://patchwork.kernel.org/project/linux-block/list/?series=1084708