block: fix deadlock between blk_mq_freeze_queue and blk_mq_dispatch_list#747
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Kernel: Linux version 6.18.16 Platform: Android A three-way deadlock can occur between blk_mq_freeze_queue and blk_mq_dispatch_list involving percpu_ref reference counting and rwsem synchronization: - Task A holds io_rwsem (e.g., F2FS write path) and enters __bio_queue_enter(), where it acquires percpu_ref and waits 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 scheduled out via schedule() while waiting for io_rwsem. Before switching, __blk_flush_plug() triggers blk_mq_dispatch_list() which acquires percpu_ref via percpu_ref_get(). If preempt_schedule_notrace() is triggered before percpu_ref_put(), Task C holds the reference while blocked on the rwsem. Since Task C cannot release its percpu_ref while blocked, Task B cannot unfreeze the queue, and Task A cannot proceed to release the io_rwsem, creating a circular dependency deadlock. Change: Fix by disabling preemption in blk_mq_dispatch_list() when called from schedule() (from_sched=true), ensuring percpu_ref_get() and percpu_ref_put() are atomic with respect to context switches. With from_sched=true, blk_mq_run_hw_queue() dispatches asynchronously via kblockd, so no driver callbacks run in this context and preempt_disable() is safe. Detailed scenario description: When process 1838 performs f2fs_submit_page_write, it obtains io_rwsem via f2fs_down_write_trace. When process 1865 performs f2fs_down_write_trace and wants to obtain io_rwsem, it needs to wait for process 1838 to release it, so it can only be scheduled out via schedule. Before being scheduled out, it clears the plug via __blk_flush_plug, so it will run to blk_mq_dispatch_list. Process 619 is modifying the I/O scheduling algorithm, calling elevator_change to set mq_freeze_depth=1. After that, blk_mq_freeze_queue_wait will wait for the reference count of q_usage_counter to return to zero. Coincidentally, process 1838 needs to wait for mq_freeze_depth=0 when it reaches __bio_queue_enter, so it can only wait to be woken up after q_freeze_depth=0. At this time, process 1865, when blk_mq_dispatch_list reaches the point where percpu_ref_get increments the q_usage_counter reference, and before percpu_ref_put, it calls preempt_schedule_notrace to schedule the process out due to preemption, causing q_usage_counter to never reach zero. At this point, process 1865 depends on io_rwsem to wake up, process 1838 depends on mq_freeze_depth=0 to wake up, and process 619 depends on q_usage_counter being zero to wake up and unfreeze (setting mq_freeze_depth=0), resulting in a deadlock between these three processes. Stack traces from the deadlock: Task 1838 (Back-P10-3) - holds io_rwsem, waiting for queue unfreeze: Call trace: __switch_to+0x1a4/0x35c __schedule+0x8e0/0xec4 schedule+0x54/0xf8 __bio_queue_enter+0xbc/0x19c blk_mq_submit_bio+0x118/0x814 __submit_bio+0x9c/0x234 submit_bio_noacct_nocheck+0x10c/0x2d4 submit_bio_noacct+0x354/0x544 submit_bio+0x1e8/0x208 f2fs_submit_write_bio+0x44/0xe4 __submit_merged_bio+0x40/0x114 f2fs_submit_page_write+0x3f0/0x7e0 do_write_page+0x180/0x2fc f2fs_outplace_write_data+0x78/0x100 f2fs_do_write_data_page+0x3b8/0x500 f2fs_write_single_data_page+0x1ac/0x6e0 f2fs_write_data_pages+0x838/0xdfc do_writepages+0xd0/0x19c filemap_write_and_wait_range+0x204/0x274 f2fs_commit_atomic_write+0x54/0x960 __f2fs_ioctl+0x2128/0x42c8 f2fs_ioctl+0x38/0xb4 __arm64_sys_ioctl+0xa0/0xf4 Task 619 (android.hardwar) - holds mq_freeze_depth=1, waiting for percpu_ref: Call trace: __switch_to+0x1a4/0x35c __schedule+0x8e0/0xec4 schedule+0x54/0xf8 blk_mq_freeze_queue_wait+0x68/0xb0 blk_mq_freeze_queue_nomemsave+0x68/0x7c elevator_change+0x70/0x14c elv_iosched_store+0x1b0/0x234 queue_attr_store+0xe0/0x134 sysfs_kf_write+0x98/0xbc kernfs_fop_write_iter+0x118/0x1e8 vfs_write+0x2e8/0x448 ksys_write+0x78/0xf0 __arm64_sys_write+0x1c/0x2c Task 1865 (sp-control-1) - holds percpu_ref, preempted in dispatch_list: Call trace: __switch_to+0x1a4/0x35c __schedule+0x8e0/0xec4 preempt_schedule_notrace+0x60/0x7c blk_mq_dispatch_list+0x5c0/0x690 blk_mq_flush_plug_list+0x13c/0x170 __blk_flush_plug+0x11c/0x17c schedule+0x40/0xf8 schedule_preempt_disabled+0x24/0x40 rwsem_down_write_slowpath+0x61c/0xc88 down_write+0x3c/0x158 f2fs_down_write_trace+0x30/0x84 f2fs_submit_page_write+0x78/0x7e0 do_write_page+0x180/0x2fc f2fs_outplace_write_data+0x78/0x100 f2fs_do_write_data_page+0x3b8/0x500 f2fs_write_single_data_page+0x1ac/0x6e0 f2fs_write_data_pages+0x838/0xdfc do_writepages+0xd0/0x19c filemap_write_and_wait_range+0x204/0x274 f2fs_commit_atomic_write+0x54/0x960 __f2fs_ioctl+0x2128/0x42c8 f2fs_ioctl+0x38/0xb4 __arm64_sys_ioctl+0xa0/0xf4 Signed-off-by: Michael Wu <michael@allwinnertech.com>
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Pull request for series with
subject: block: fix deadlock between blk_mq_freeze_queue and blk_mq_dispatch_list
version: 1
url: https://patchwork.kernel.org/project/linux-block/list/?series=1082402