On 6/10/18 2:38 PM, Roman Pen wrote: > It is not allowed to reinit q->tag_set_list list entry while RCU grace > period has not completed yet, otherwise the following soft lockup in > blk_mq_sched_restart() happens: > > [ 1064.252652] watchdog: BUG: soft lockup - CPU#12 stuck for 23s! [fio:9270] > [ 1064.254445] task: ffff99b912e8b900 task.stack: ffffa6d54c758000 > [ 1064.254613] RIP: 0010:blk_mq_sched_restart+0x96/0x150 > [ 1064.256510] Call Trace: > [ 1064.256664] <IRQ> > [ 1064.256824] blk_mq_free_request+0xea/0x100 > [ 1064.256987] msg_io_conf+0x59/0xd0 [ibnbd_client] > [ 1064.257175] complete_rdma_req+0xf2/0x230 [ibtrs_client] > [ 1064.257340] ? ibtrs_post_recv_empty+0x4d/0x70 [ibtrs_core] > [ 1064.257502] ibtrs_clt_rdma_done+0xd1/0x1e0 [ibtrs_client] > [ 1064.257669] ib_create_qp+0x321/0x380 [ib_core] > [ 1064.257841] ib_process_cq_direct+0xbd/0x120 [ib_core] > [ 1064.258007] irq_poll_softirq+0xb7/0xe0 > [ 1064.258165] __do_softirq+0x106/0x2a2 > [ 1064.258328] irq_exit+0x92/0xa0 > [ 1064.258509] do_IRQ+0x4a/0xd0 > [ 1064.258660] common_interrupt+0x7a/0x7a > [ 1064.258818] </IRQ> > > Meanwhile another context frees other queue but with the same set of > shared tags: > > [ 1288.201183] INFO: task bash:5910 blocked for more than 180 seconds. > [ 1288.201833] bash D 0 5910 5820 0x00000000 > [ 1288.202016] Call Trace: > [ 1288.202315] schedule+0x32/0x80 > [ 1288.202462] schedule_timeout+0x1e5/0x380 > [ 1288.203838] wait_for_completion+0xb0/0x120 > [ 1288.204137] __wait_rcu_gp+0x125/0x160 > [ 1288.204287] synchronize_sched+0x6e/0x80 > [ 1288.204770] blk_mq_free_queue+0x74/0xe0 > [ 1288.204922] blk_cleanup_queue+0xc7/0x110 > [ 1288.205073] ibnbd_clt_unmap_device+0x1bc/0x280 [ibnbd_client] > [ 1288.205389] ibnbd_clt_unmap_dev_store+0x169/0x1f0 [ibnbd_client] > [ 1288.205548] kernfs_fop_write+0x109/0x180 > [ 1288.206328] vfs_write+0xb3/0x1a0 > [ 1288.206476] SyS_write+0x52/0xc0 > [ 1288.206624] do_syscall_64+0x68/0x1d0 > [ 1288.206774] entry_SYSCALL_64_after_hwframe+0x3d/0xa2 > > What happened is the following: > > 1. There are several MQ queues with shared tags. > 2. One queue is about to be freed and now task is in > blk_mq_del_queue_tag_set(). > 3. Other CPU is in blk_mq_sched_restart() and loops over all queues in > tag list in order to find hctx to restart. > > Because linked list entry was modified in blk_mq_del_queue_tag_set() > without proper waiting for a grace period, blk_mq_sched_restart() > never ends, spining in list_for_each_entry_rcu_rr(), thus soft lockup. > > Fix is simple: reinit list entry after an RCU grace period elapsed. Good catch, applied. -- Jens Axboe
