一次奇葩的spin_lock_irq / spin_unlock_irq使用不当导致的系统卡死分析

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这是在调试内核block层时遇到的一例奇葩的soft lock锁死问题(内核版本centos 8.3，4.18.0-240)，现场如下：

• [  760.247152] watchdog: BUG: soft lockup - CPU#0 stuck for 23s! [kworker/0:1:2635]
• ……………..
• [  760.247184] CPU: 0 PID: 2635 Comm: kworker/0:1 Kdump: loaded Tainted: G            E    ---------r-  - 4.18.0 #4
• [  760.247184] Hardware name: VMware, Inc. VMware Virtual Platform/440BX Desktop Reference Platform, BIOS 6.00 02/27/2020
• [  760.247190] Workqueue: events netstamp_clear
• [  760.247193] RIP: 0010:smp_call_function_many+0x1ec/0x250
• [  760.247194] Code: c7 e8 08 fa 74 00 3b 05 96 ee 2b 01 0f 83 99 fe ff ff 48 63 d0 48 8b 0b 48 03 0c d5 60 d8 15 bd 8b 51 18 83 e2 01 74 0a f3 90 <8b> 51 18 83 e2 01 75 f6 eb c7 48 c7 c2 00 4c 41 bd 4c 89 f6 89 df
• [  760.247195] RSP: 0018:ffffa3b342ac3dd0 EFLAGS: 00000202 ORIG_RAX: ffffffffffffff13
• [  760.247196] RAX: 0000000000000001 RBX: ffff92adf5c2ae80 RCX: ffff92adf5c704e0
• [  760.247196] RDX: 0000000000000001 RSI: 0000000000000000 RDI: ffff92acc7d536c0
• [  760.247196] RBP: ffffffffbc027610 R08: 000000000002f060 R09: ffffffffbc04fc8a
• [  760.247197] R10: ffffeec908b18a00 R11: 0000000000000000 R12: 0000000000000000
• [  760.247197] R13: 0000000000000001 R14: 0000000000000080 R15: 0000000000000001
• [  760.247198] FS:  0000000000000000(0000) GS:ffff92adf5c00000(0000) knlGS:0000000000000000
• [  760.247198] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
• [  760.247199] CR2: 00007fa9e40033c8 CR3: 000000001a00a003 CR4: 00000000003606f0
• [  760.247216] Call Trace:
• [  760.247221]  ? poke_int3_handler+0xe0/0xe0
• [  760.247222]  on_each_cpu+0x28/0x60
• [  760.247223]  text_poke_bp_batch+0x8b/0x160
• [  760.247225]  arch_jump_label_transform_apply+0x2e/0x50
• [  760.247226]  static_key_enable_cpuslocked+0x52/0x80
• [  760.247228]  static_key_enable+0x16/0x20
• [  760.247229]  process_one_work+0x1a7/0x360
• [  760.247230]  worker_thread+0x30/0x390
• [  760.247232]  ? create_worker+0x1a0/0x1a0
• [  760.247232]  kthread+0x112/0x130
• [  760.247233]  ? kthread_flush_work_fn+0x10/0x10
• [  760.247235]  ret_from_fork+0x35/0x40
• [  760.247237] Kernel panic - not syncing: softlockup: hung tasks
• [  760.247239] CPU: 0 PID: 2635 Comm: kworker/0:1 Kdump: loaded Tainted: G            EL   ---------r-  - 4.18.0 #4
• [  760.247239] Hardware name: VMware, Inc. VMware Virtual Platform/440BX Desktop Reference Platform, BIOS 6.00 02/27/2020
• [  760.247241] Workqueue: events netstamp_clear
• [  760.247241] Call Trace:
• [  760.247243]  <IRQ>
• [  760.247245]  dump_stack+0x5c/0x80
• [  760.247247]  panic+0xe7/0x2a9
• [  760.247249]  ? __switch_to_asm+0x51/0x70
• [  760.247250]  watchdog_timer_fn.cold.8+0x85/0x9e
• [  760.247251]  ? watchdog+0x30/0x30
• [  760.247253]  __hrtimer_run_queues+0x100/0x280
• [  760.247255]  hrtimer_interrupt+0x100/0x220
• [  760.247256]  ? ktime_get+0x36/0xa0
• [  760.247257]  smp_apic_timer_interrupt+0x6a/0x130
• [  760.247259]  apic_timer_interrupt+0xf/0x20
• [  760.247260]  </IRQ>
• [  760.247262] RIP: 0010:smp_call_function_many+0x1ec/0x250
• [  760.247263] Code: c7 e8 08 fa 74 00 3b 05 96 ee 2b 01 0f 83 99 fe ff ff 48 63 d0 48 8b 0b 48 03 0c d5 60 d8 15 bd 8b 51 18 83 e2 01 74 0a f3 90 <8b> 51 18 83 e2 01 75 f6 eb c7 48 c7 c2 00 4c 41 bd 4c 89 f6 89 df
• [  760.247263] RSP: 0018:ffffa3b342ac3dd0 EFLAGS: 00000202 ORIG_RAX: ffffffffffffff13
• [  760.247264] RAX: 0000000000000001 RBX: ffff92adf5c2ae80 RCX: ffff92adf5c704e0
• [  760.247265] RDX: 0000000000000001 RSI: 0000000000000000 RDI: ffff92acc7d536c0
• [  760.247266] RBP: ffffffffbc027610 R08: 000000000002f060 R09: ffffffffbc04fc8a
• [  760.247266] R10: ffffeec908b18a00 R11: 0000000000000000 R12: 0000000000000000
• [  760.247267] R13: 0000000000000001 R14: 0000000000000080 R15: 0000000000000001
• [  760.247268]  ? poke_int3_handler+0xe0/0xe0
• [  760.247269]  ? native_send_call_func_ipi+0xda/0x120
• [  760.247271]  ? poke_int3_handler+0xe0/0xe0
• [  760.247272]  on_each_cpu+0x28/0x60
• [  760.247274]  text_poke_bp_batch+0x8b/0x160
• [  760.247275]  arch_jump_label_transform_apply+0x2e/0x50
• [  760.247276]  static_key_enable_cpuslocked+0x52/0x80
• [  760.247277]  static_key_enable+0x16/0x20
• [  760.247278]  process_one_work+0x1a7/0x360
• [  760.247279]  worker_thread+0x30/0x390

先启动crash分析：crash /usr/lib/debug/lib/modules/4.18.0-240.el8.x86_64/vmlinux /var/crash/127.0.0.1-2023-02-19-02\:05\:59/vmcore，bt看下卡死时各个进程的栈回溯信息：

•     DUMPFILE: /var/crash/127.0.0.1-2023-02-19-02:05:59/vmcore  [PARTIAL DUMP]
•         CPUS: 4
•         DATE: Sun Feb 19 02:04:55 2023
•       UPTIME: 00:11:46
• LOAD AVERAGE: 21.78, 17.84, 9.85
•        TASKS: 415
•     NODENAME: localhost.localdomain
•      RELEASE: 4.18.0
•      VERSION: #4 SMP Sun Feb 19 00:38:10 PST 2023
•      MACHINE: x86_64  (3407 Mhz)
•       MEMORY: 8 GB
•        PANIC: "Kernel panic - not syncing: softlockup: hung tasks"
•          PID: 2635
•      COMMAND: "kworker/0:1"
•         TASK: ffff92acd6cf17c0  [THREAD_INFO: ffff92acd6cf17c0]
•          CPU: 0
•        STATE: TASK_RUNNING (PANIC)
• crash> bt
• PID: 2635   TASK: ffff92acd6cf17c0  CPU: 0   COMMAND: "kworker/0:1"
•  #0 [ffff92adf5c03d48] machine_kexec at ffffffffbc05bf3e
•  #1 [ffff92adf5c03da0] __crash_kexec at ffffffffbc16072d
•  #2 [ffff92adf5c03e68] panic at ffffffffbc0b5dc7
•  #3 [ffff92adf5c03ee8] watchdog_timer_fn.cold.8 at ffffffffbc196dc7
•  #4 [ffff92adf5c03f18] __hrtimer_run_queues at ffffffffbc1408a0
•  #5 [ffff92adf5c03f78] hrtimer_interrupt at ffffffffbc141070
•  #6 [ffff92adf5c03fd8] smp_apic_timer_interrupt at ffffffffbca027da
•  #7 [ffff92adf5c03ff0] apic_timer_interrupt at ffffffffbca01d6f
• --- <IRQ stack> ---
•  #8 [ffffa3b342ac3d28] apic_timer_interrupt at ffffffffbca01d6f
•     [exception RIP: smp_call_function_many+492]
•     RIP: ffffffffbc15677c  RSP: ffffa3b342ac3dd0  RFLAGS: 00000202
•     RAX: 0000000000000001  RBX: ffff92adf5c2ae80  RCX: ffff92adf5c704e0
•     RDX: 0000000000000001  RSI: 0000000000000000  RDI: ffff92acc7d536c0
•     RBP: ffffffffbc027610   R8: 000000000002f060   R9: ffffffffbc04fc8a
•     R10: ffffeec908b18a00  R11: 0000000000000000  R12: 0000000000000000
•     R13: 0000000000000001  R14: 0000000000000080  R15: 0000000000000001
•     ORIG_RAX: ffffffffffffff13  CS: 0010  SS: 0018
•  #9 [ffffa3b342ac3e10] on_each_cpu at ffffffffbc156838
• #10 [ffffa3b342ac3e30] text_poke_bp_batch at ffffffffbc027fab
• #11 [ffffa3b342ac3e70] arch_jump_label_transform_apply at ffffffffbc0251be
• #12 [ffffa3b342ac3e78] static_key_enable_cpuslocked at ffffffffbc226422
• #13 [ffffa3b342ac3e88] static_key_enable at ffffffffbc226466
• #14 [ffffa3b342ac3e98] process_one_work at ffffffffbc0d3477
• #15 [ffffa3b342ac3ed8] worker_thread at ffffffffbc0d3b40
• #16 [ffffa3b342ac3f10] kthread at ffffffffbc0d9502
• #17 [ffffa3b342ac3f50] ret_from_fork at ffffffffbca00255
• crash> bt -a
• PID: 2635   TASK: ffff92acd6cf17c0  CPU: 0   COMMAND: "kworker/0:1"
•  #0 [ffff92adf5c03d48] machine_kexec at ffffffffbc05bf3e
•  #1 [ffff92adf5c03da0] __crash_kexec at ffffffffbc16072d
•  #2 [ffff92adf5c03e68] panic at ffffffffbc0b5dc7
•  #3 [ffff92adf5c03ee8] watchdog_timer_fn.cold.8 at ffffffffbc196dc7
•  #4 [ffff92adf5c03f18] __hrtimer_run_queues at ffffffffbc1408a0
•  #5 [ffff92adf5c03f78] hrtimer_interrupt at ffffffffbc141070
•  #6 [ffff92adf5c03fd8] smp_apic_timer_interrupt at ffffffffbca027da
•  #7 [ffff92adf5c03ff0] apic_timer_interrupt at ffffffffbca01d6f
• --- <IRQ stack> ---
•  #8 [ffffa3b342ac3d28] apic_timer_interrupt at ffffffffbca01d6f
•     [exception RIP: smp_call_function_many+492]
•     RIP: ffffffffbc15677c  RSP: ffffa3b342ac3dd0  RFLAGS: 00000202
•     RAX: 0000000000000001  RBX: ffff92adf5c2ae80  RCX: ffff92adf5c704e0
•     RDX: 0000000000000001  RSI: 0000000000000000  RDI: ffff92acc7d536c0
•     RBP: ffffffffbc027610   R8: 000000000002f060   R9: ffffffffbc04fc8a
•     R10: ffffeec908b18a00  R11: 0000000000000000  R12: 0000000000000000
•     R13: 0000000000000001  R14: 0000000000000080  R15: 0000000000000001
•     ORIG_RAX: ffffffffffffff13  CS: 0010  SS: 0018
•  #9 [ffffa3b342ac3e10] on_each_cpu at ffffffffbc156838
• #10 [ffffa3b342ac3e30] text_poke_bp_batch at ffffffffbc027fab
• #11 [ffffa3b342ac3e70] arch_jump_label_transform_apply at ffffffffbc0251be
• #12 [ffffa3b342ac3e78] static_key_enable_cpuslocked at ffffffffbc226422
• #13 [ffffa3b342ac3e88] static_key_enable at ffffffffbc226466
• #14 [ffffa3b342ac3e98] process_one_work at ffffffffbc0d3477
• #15 [ffffa3b342ac3ed8] worker_thread at ffffffffbc0d3b40
• #16 [ffffa3b342ac3f10] kthread at ffffffffbc0d9502
• #17 [ffffa3b342ac3f50] ret_from_fork at ffffffffbca00255
• PID: 2857   TASK: ffff92ad555497c0  CPU: 1   COMMAND: "fio"
•  #0 [fffffe0000032e50] crash_nmi_callback at ffffffffbc04eee3
•  #1 [fffffe0000032e58] nmi_handle at ffffffffbc023703
•  #2 [fffffe0000032eb0] default_do_nmi at ffffffffbc023ade
•  #3 [fffffe0000032ed0] do_nmi at ffffffffbc023cb8
•  #4 [fffffe0000032ef0] end_repeat_nmi at ffffffffbca016d4
•     [exception RIP: native_queued_spin_lock_slowpath+32]
•     RIP: ffffffffbc111190  RSP: ffff92adf5c43e38  RFLAGS: 00000002
•     RAX: 0000000000000001  RBX: 0000000000000246  RCX: 0000000000008801
•     RDX: 0000000000000001  RSI: 0000000000000001  RDI: ffff92adc0fc6400
•     RBP: ffff92acd0a39800   R8: 0000000000000000   R9: 0000000000000000
•     R10: 0000000000000068  R11: 0000000000000000  R12: ffff92ad55426910
•     R13: ffff92adc0fc6400  R14: 000000a494918997  R15: 0000000000008801
•     ORIG_RAX: ffffffffffffffff  CS: 0010  SS: 0018
• --- <NMI exception stack> ---
•  #5 [ffff92adf5c43e38] native_queued_spin_lock_slowpath at ffffffffbc111190
•  #6 [ffff92adf5c43e38] _raw_spin_lock_irqsave at ffffffffbc8cdc12
•  #7 [ffff92adf5c43e48] bfq_finish_requeue_request at ffffffffc0664555 [bfq]
•  #8 [ffff92adf5c43ea0] blk_mq_free_request at ffffffffbc4070ca
•  #9 [ffff92adf5c43ec8] scsi_end_request at ffffffffbc5d0e7a
• #10 [ffff92adf5c43f00] scsi_io_completion at ffffffffbc5d0fc8
• #11 [ffff92adf5c43f48] blk_done_softirq at ffffffffbc405ee1
• #12 [ffff92adf5c43f80] __softirqentry_text_start at ffffffffbcc000e4
• #13 [ffff92adf5c43fe0] irq_exit at ffffffffbc0bc1d7
• #14 [ffff92adf5c43ff0] call_function_single_interrupt at ffffffffbca01e0f
• --- <IRQ stack> ---
• #15 [ffffa3b34223f728] call_function_single_interrupt at ffffffffbca01e0f
•     [exception RIP: bfq_pos_tree_add_move+109]
•     RIP: ffffffffc0667f0d  RSP: ffffa3b34223f7d0  RFLAGS: 00000246
•     RAX: ffff92adc0fc6200  RBX: ffff92acd0b89000  RCX: ffff92acd6900c38
•     RDX: 0000000000000000  RSI: ffff92acd967ba58  RDI: ffff92acd0b89038
•     RBP: ffff92adc0fc6000   R8: 0000000000000000   R9: 0000000000000000
•     R10: 0000000000000000  R11: 0000000000000000  R12: 0000000000000001
•     R13: ffff92adc0fc6000  R14: ffff92acd20593f0  R15: ffff92acd0b89058
•     ORIG_RAX: ffffffffffffff04  CS: 0010  SS: 0018
• #16 [ffffa3b34223f7f8] bfq_remove_request.cold.58 at ffffffffc06680cc [bfq]
• #17 [ffffa3b34223f878] bfq_finish_requeue_request at ffffffffc0664948 [bfq]
• #18 [ffffa3b34223f8d0] blk_mq_free_request at ffffffffbc4070ca
• #19 [ffffa3b34223f8f8] bfq_bio_merge at ffffffffc065da34 [bfq]
• #20 [ffffa3b34223f948] blk_mq_make_request at ffffffffbc40abb0
• #21 [ffffa3b34223f9d8] generic_make_request at ffffffffbc3fe85f
• #22 [ffffa3b34223fa38] submit_bio at ffffffffbc3feadc
• #23 [ffffa3b34223fa78] do_blockdev_direct_IO at ffffffffbc31ee96
• #24 [ffffa3b34223fc78] ext4_direct_IO at ffffffffc0734ea6 [ext4]
• #25 [ffffa3b34223fcf0] generic_file_read_iter at ffffffffbc22da7f
• #26 [ffffa3b34223fd38] aio_read at ffffffffbc3313a5
• #27 [ffffa3b34223fe40] io_submit_one at ffffffffbc33165b
• #28 [ffffa3b34223feb8] __x64_sys_io_submit at ffffffffbc331b82
• #29 [ffffa3b34223ff38] do_syscall_64 at ffffffffbc00419b
• #30 [ffffa3b34223ff50] entry_SYSCALL_64_after_hwframe at ffffffffbca000ad
• PID: 491    TASK: ffff92acd7bf97c0  CPU: 2   COMMAND: "kworker/2:1H"
•  #0 [fffffe000005de50] crash_nmi_callback at ffffffffbc04eee3
•  #1 [fffffe000005de58] nmi_handle at ffffffffbc023703
•  #2 [fffffe000005deb0] default_do_nmi at ffffffffbc023ade
•  #3 [fffffe000005ded0] do_nmi at ffffffffbc023cb8
•  #4 [fffffe000005def0] end_repeat_nmi at ffffffffbca016d4
•     [exception RIP: native_queued_spin_lock_slowpath+32]
•     RIP: ffffffffbc111190  RSP: ffffa3b3414c3d50  RFLAGS: 00000002
•     RAX: 0000000000000001  RBX: ffff92acd7f2bc00  RCX: ffff92adc0fc6428
•     RDX: 0000000000000001  RSI: 0000000000000001  RDI: ffff92adc0fc6400
•     RBP: ffff92adc0fc6000   R8: 0000000000000000   R9: 0000000000000000
•     R10: 0000000000000000  R11: 0000000000000000  R12: 0000000000000000
•     R13: ffff92acd20593f0  R14: ffff92adc0fc6400  R15: ffff92ad55422220
•     ORIG_RAX: ffffffffffffffff  CS: 0010  SS: 0018
• --- <NMI exception stack> ---
•  #5 [ffffa3b3414c3d50] native_queued_spin_lock_slowpath at ffffffffbc111190
•  #6 [ffffa3b3414c3d50] _raw_spin_lock_irq at ffffffffbc8cde43
•  #7 [ffffa3b3414c3d58] bfq_dispatch_request at ffffffffc0661187 [bfq]
•  #8 [ffffa3b3414c3db8] blk_mq_do_dispatch_sched at ffffffffbc40f455
•  #9 [ffffa3b3414c3e10] __blk_mq_sched_dispatch_requests at ffffffffbc40ff89
• #10 [ffffa3b3414c3e70] blk_mq_sched_dispatch_requests at ffffffffbc410010
• #11 [ffffa3b3414c3e80] __blk_mq_run_hw_queue at ffffffffbc407691
• #12 [ffffa3b3414c3e98] process_one_work at ffffffffbc0d3477
• #13 [ffffa3b3414c3ed8] worker_thread at ffffffffbc0d3b40
• #14 [ffffa3b3414c3f10] kthread at ffffffffbc0d9502
• #15 [ffffa3b3414c3f50] ret_from_fork at ffffffffbca00255
• PID: 2861   TASK: ffff92ad55522f80  CPU: 3   COMMAND: "fio"
•  #0 [fffffe0000088e50] crash_nmi_callback at ffffffffbc04eee3
•  #1 [fffffe0000088e58] nmi_handle at ffffffffbc023703
•  #2 [fffffe0000088eb0] default_do_nmi at ffffffffbc023ade
•  #3 [fffffe0000088ed0] do_nmi at ffffffffbc023cb8
•  #4 [fffffe0000088ef0] end_repeat_nmi at ffffffffbca016d4
•     [exception RIP: native_queued_spin_lock_slowpath+32]
•     RIP: ffffffffbc111190  RSP: ffff92adf5cc3e38  RFLAGS: 00000002
•     RAX: 0000000000000001  RBX: 0000000000000246  RCX: 0000000000008801
•     RDX: 0000000000000001  RSI: 0000000000000001  RDI: ffff92adc0fc6400
•     RBP: ffff92acd6900c00   R8: 0000000000000000   R9: 0000000000000000
•     R10: 0000000000000068  R11: 0000000000000000  R12: ffff92acd0b02d10
•     R13: ffff92adc0fc6400  R14: 000000a49490f1bc  R15: 0000000000008801
•     ORIG_RAX: ffffffffffffffff  CS: 0010  SS: 0018
• --- <NMI exception stack> ---
•  #5 [ffff92adf5cc3e38] native_queued_spin_lock_slowpath at ffffffffbc111190
•  #6 [ffff92adf5cc3e38] _raw_spin_lock_irqsave at ffffffffbc8cdc12
•  #7 [ffff92adf5cc3e48] bfq_finish_requeue_request at ffffffffc0664555 [bfq]
•  #8 [ffff92adf5cc3ea0] blk_mq_free_request at ffffffffbc4070ca
•  #9 [ffff92adf5cc3ec8] scsi_end_request at ffffffffbc5d0e7a
• #10 [ffff92adf5cc3f00] scsi_io_completion at ffffffffbc5d0fc8
• #11 [ffff92adf5cc3f48] blk_done_softirq at ffffffffbc405ee1
• #12 [ffff92adf5cc3f80] __softirqentry_text_start at ffffffffbcc000e4
• #13 [ffff92adf5cc3fe0] irq_exit at ffffffffbc0bc1d7
• #14 [ffff92adf5cc3ff0] call_function_single_interrupt at ffffffffbca01e0f
• --- <IRQ stack> ---
• #15 [ffffa3b34225f898] call_function_single_interrupt at ffffffffbca01e0f
•     [exception RIP: bfq_bio_merge+5]
•     RIP: ffffffffc065d955  RSP: ffffa3b34225f948  RFLAGS: 00000286
•     RAX: ffffffffc065d950  RBX: ffff92acd20593f0  RCX: 0000000000000000
•     RDX: ffff92acd967e400  RSI: ffff92ad6e641e00  RDI: ffff92acd7f2bc00
•     RBP: ffffa3b34225fa30   R8: 0000000000000001   R9: 0000000000000001
•     R10: 0000000000008000  R11: 0000000000000040  R12: 0000000000000000
•     R13: ffff92ad6e641e00  R14: 0000000000000001  R15: ffff92ad6e592800
•     ORIG_RAX: ffffffffffffff04  CS: 0010  SS: 0018
• #16 [ffffa3b34225f948] blk_mq_make_request at ffffffffbc40abb0
• #17 [ffffa3b34225f9d8] generic_make_request at ffffffffbc3fe85f
• #18 [ffffa3b34225fa38] submit_bio at ffffffffbc3feadc
• #19 [ffffa3b34225fa78] do_blockdev_direct_IO at ffffffffbc31ee96
• #20 [ffffa3b34225fc78] ext4_direct_IO at ffffffffc0734ea6 [ext4]
• #21 [ffffa3b34225fcf0] generic_file_read_iter at ffffffffbc22da7f
• #22 [ffffa3b34225fd38] aio_read at ffffffffbc3313a5
• #23 [ffffa3b34225fe40] io_submit_one at ffffffffbc33165b
• #24 [ffffa3b34225feb8] __x64_sys_io_submit at ffffffffbc331b82
• #25 [ffffa3b34225ff38] do_syscall_64 at ffffffffbc00419b
• #26 [ffffa3b34225ff50] entry_SYSCALL_64_after_hwframe at ffffffffbca000ad

根据这些信息，发现pid是491和2857这两个进程嫌疑很大。先看下491这个进程的栈回溯：

crash> bt 491

PID: 491    TASK: ffff92acd7bf97c0  CPU: 2   COMMAND: "kworker/2:1H"

 #0 [fffffe000005de50] crash_nmi_callback at ffffffffbc04eee3

 #1 [fffffe000005de58] nmi_handle at ffffffffbc023703

 #2 [fffffe000005deb0] default_do_nmi at ffffffffbc023ade

 #3 [fffffe000005ded0] do_nmi at ffffffffbc023cb8

 #4 [fffffe000005def0] end_repeat_nmi at ffffffffbca016d4

    [exception RIP: native_queued_spin_lock_slowpath+32]

    RIP: ffffffffbc111190  RSP: ffffa3b3414c3d50  RFLAGS: 00000002

    RAX: 0000000000000001  RBX: ffff92acd7f2bc00  RCX: ffff92adc0fc6428

    RDX: 0000000000000001  RSI: 0000000000000001  RDI: ffff92adc0fc6400

    RBP: ffff92adc0fc6000   R8: 0000000000000000   R9: 0000000000000000

    R10: 0000000000000000  R11: 0000000000000000  R12: 0000000000000000

    R13: ffff92acd20593f0  R14: ffff92adc0fc6400  R15: ffff92ad55422220

    ORIG_RAX: ffffffffffffffff  CS: 0010  SS: 0018

--- <NMI exception stack> ---

 #5 [ffffa3b3414c3d50] native_queued_spin_lock_slowpath at ffffffffbc111190

 #6 [ffffa3b3414c3d50] _raw_spin_lock_irq at ffffffffbc8cde43

 #7 [ffffa3b3414c3d58] bfq_dispatch_request at ffffffffc0661187 [bfq]

 #8 [ffffa3b3414c3db8] blk_mq_do_dispatch_sched at ffffffffbc40f455

 #9 [ffffa3b3414c3e10] __blk_mq_sched_dispatch_requests at ffffffffbc40ff89

#10 [ffffa3b3414c3e70] blk_mq_sched_dispatch_requests at ffffffffbc410010

#11 [ffffa3b3414c3e80] __blk_mq_run_hw_queue at ffffffffbc407691

#12 [ffffa3b3414c3e98] process_one_work at ffffffffbc0d3477

#13 [ffffa3b3414c3ed8] worker_thread at ffffffffbc0d3b40

#14 [ffffa3b3414c3f10] kthread at ffffffffbc0d9502

#15 [ffffa3b3414c3f50] ret_from_fork at ffffffffbca00255

2857进程最需要分析看看，这个嫌疑更大

• PID: 2857   TASK: ffff92ad555497c0  CPU: 1   COMMAND: "fio"
•  #0 [fffffe0000032e50] crash_nmi_callback at ffffffffbc04eee3
•  #1 [fffffe0000032e58] nmi_handle at ffffffffbc023703
•  #2 [fffffe0000032eb0] default_do_nmi at ffffffffbc023ade
•  #3 [fffffe0000032ed0] do_nmi at ffffffffbc023cb8
•  #4 [fffffe0000032ef0] end_repeat_nmi at ffffffffbca016d4
•     [exception RIP: native_queued_spin_lock_slowpath+32]
•     RIP: ffffffffbc111190  RSP: ffff92adf5c43e38  RFLAGS: 00000002
•     RAX: 0000000000000001  RBX: 0000000000000246  RCX: 0000000000008801
•     RDX: 0000000000000001  RSI: 0000000000000001  RDI: ffff92adc0fc6400
•     RBP: ffff92acd0a39800   R8: 0000000000000000   R9: 0000000000000000
•     R10: 0000000000000068  R11: 0000000000000000  R12: ffff92ad55426910
•     R13: ffff92adc0fc6400  R14: 000000a494918997  R15: 0000000000008801
•     ORIG_RAX: ffffffffffffffff  CS: 0010  SS: 0018
• --- <NMI exception stack> ---
•  #5 [ffff92adf5c43e38] native_queued_spin_lock_slowpath at ffffffffbc111190
•  #6 [ffff92adf5c43e38] _raw_spin_lock_irqsave at ffffffffbc8cdc12
•  //这里获取spin_lock_irq(&bfqd->lock) bfqd->lock 锁失败，之后就大面积出现获取 bfqd->lock 锁失败而卡死
•  #7 [ffff92adf5c43e48] bfq_finish_requeue_request at ffffffffc0664555 [bfq]
•  #8 [ffff92adf5c43ea0] blk_mq_free_request at ffffffffbc4070ca
•  #9 [ffff92adf5c43ec8] scsi_end_request at ffffffffbc5d0e7a
• #10 [ffff92adf5c43f00] scsi_io_completion at ffffffffbc5d0fc8
• #11 [ffff92adf5c43f48] blk_done_softirq at ffffffffbc405ee1
• #12 [ffff92adf5c43f80] __softirqentry_text_start at ffffffffbcc000e4
• #13 [ffff92adf5c43fe0] irq_exit at ffffffffbc0bc1d7
• #14 [ffff92adf5c43ff0] call_function_single_interrupt at ffffffffbca01e0f
• --- <IRQ stack> ---
• #15 [ffffa3b34223f728] call_function_single_interrupt at ffffffffbca01e0f
•     //这里竟然产生了中断
•     [exception RIP: bfq_pos_tree_add_move+109]
•     RIP: ffffffffc0667f0d  RSP: ffffa3b34223f7d0  RFLAGS: 00000246
•     RAX: ffff92adc0fc6200  RBX: ffff92acd0b89000  RCX: ffff92acd6900c38
•     RDX: 0000000000000000  RSI: ffff92acd967ba58  RDI: ffff92acd0b89038
•     RBP: ffff92adc0fc6000   R8: 0000000000000000   R9: 0000000000000000
•     R10: 0000000000000000  R11: 0000000000000000  R12: 0000000000000001
•     R13: ffff92adc0fc6000  R14: ffff92acd20593f0  R15: ffff92acd0b89058
•     ORIG_RAX: ffffffffffffff04  CS: 0010  SS: 0018
• #16 [ffffa3b34223f7f8] bfq_remove_request.cold.58 at ffffffffc06680cc [bfq]
• #17 [ffffa3b34223f878] bfq_finish_requeue_request at ffffffffc0664948 [bfq]
• #18 [ffffa3b34223f8d0] blk_mq_free_request at ffffffffbc4070ca
•                        //这里 spin_lock_irq(&bfqd->lock) 加bfqd->lock锁关中断
• #19 [ffffa3b34223f8f8] bfq_bio_merge at ffffffffc065da34 [bfq]
• #20 [ffffa3b34223f948] blk_mq_make_request at ffffffffbc40abb0
• #21 [ffffa3b34223f9d8] generic_make_request at ffffffffbc3fe85f
• #22 [ffffa3b34223fa38] submit_bio at ffffffffbc3feadc
• #23 [ffffa3b34223fa78] do_blockdev_direct_IO at ffffffffbc31ee96
• #24 [ffffa3b34223fc78] ext4_direct_IO at ffffffffc0734ea6 [ext4]
• #25 [ffffa3b34223fcf0] generic_file_read_iter at ffffffffbc22da7f
• #26 [ffffa3b34223fd38] aio_read at ffffffffbc3313a5
• #27 [ffffa3b34223fe40] io_submit_one at ffffffffbc33165b
• #28 [ffffa3b34223feb8] __x64_sys_io_submit at ffffffffbc331b82

2857号进程执行到bfq_bio_merge()发生的卡死，看下这个函数：

1. static bool bfq_bio_merge(struct blk_mq_hw_ctx *hctx, struct bio *bio)
2. {
3.     //这里 spin_lock_irq(&bfqd->lock) 加bfqd->lock锁关中断
4.     spin_lock_irq(&bfqd->lock);
5.     ret = blk_mq_sched_try_merge(q, bio, &free);
7.     if (free)
8.         blk_mq_free_request(free);
9.     spin_unlock_irq(&bfqd->lock);
10. }

先spin_lock_irq(&bfqd->lock) 加bfqd->lock锁并关中断，然后继续执行到 bfq_bio_merge ->blk_mq_free_request->bfq_finish_requeue_request->bfq_remove_request->bfq_pos_tree_add_move函数时，却产生了中断，中断函数里执行bfq_finish_requeue_request->spin_lock_irqsave(&bfqd->lock, flags) 再次获取bfqd->lock锁失败而卡死。这些都发生在cpu1上的线程，先 spin_lock_irq(&bfqd->lock) 获取 bfqd->lock 锁，然后产生中断后又执行 spin_lock_irq(&bfqd->lock)获取bfqd->lock 锁失败。这样肯定会失败呀，因为两次获取同一把锁！

有一个很大的疑问，明明 bfq_bio_merge()里的spin_lock_irq(&bfqd->lock) 是关闭中断的!谁开了中断？导致后续执行bfq_bio_merge ->blk_mq_free_request->bfq_finish_requeue_request->bfq_remove_request->bfq_pos_tree_add_move时，产生了中断，中断函数里执行bfq_finish_requeue_request->spin_lock_irqsave(&bfqd->lock, flags) 再次获取bfqd->lock锁失败而卡死。

排查发现，竟然是 2857号进程先执行bfq_bio_merge->blk_mq_sched_try_merge->attempt_back_merge->attempt_merge->blk_account_io_merge函数意外的开启了中断，如下红色代码：

1. static void blk_account_io_merge(struct request *req)                                                                                                                                     
2. {
3.     if (blk_do_io_stat(req)) {
4.             struct hd_struct *part;
6.             part_stat_lock();
7.             part = req->part;
8.             part_dec_in_flight(req->q, part, rq_data_dir(req));
9.             hd_struct_put(part);
10.             part_stat_unlock();
11.             if(req->rq_disk && req->rq_disk->process_io.enable && req->p_process_rq_stat){
12.                 spin_lock_irq(&(req->rq_disk->process_io.process_io_insert_lock));
13.                 list_del(&req->p_process_rq_stat->process_io_insert);
14.                 //这里开启了中断!!!!!!!!!!!!!!!!!!!!
15.                 spin_unlock_irq(&(req->rq_disk->process_io.process_io_insert_lock));
16.                 kmem_cache_free(req->rq_disk->process_io.process_rq_stat_cachep,req->p_process_rq_stat);
17.                 atomic_dec(&(req->rq_disk->process_io.rq_in_queue));
18.                 req->p_process_rq_stat = NULL;
19.             }
20.     }
21. }

竟然是我在 blk_account_io_merge()执行了 spin_unlock_irq 开启了中断!服了，又是自己埋了一个坑然后自己跳进去！再次惊醒！这又是不熟悉上下文导致的!上层bfq_bio_merge()执行 spin_lock_irq(&bfqd->lock) 后，底层调用我的 blk_account_io_merge函数里的spin_unlock_irq，莫名其妙开启了本地中断！导致后续一系列不符合逻辑的获取bfqd->lock 锁失败的奇葩问题。

这是一个很大的教训， spin_lock_irq/ spin_unlock_irq 用起来有风险呀！最保险的是使用 spin_lock_irqsave/spin_unlock_irqrestore。因为spin_unlock_irqrestore 会恢复 spin_lock_irqsave 执行时的cpu中断状态，就不会有这个问题了。而spin_unlock_irq不管关中断前，中断是否关闭或开启，都强制开启中断！这样就会导致中断状态错乱，发生未知问题！比如，本应该是关闭中断的，但spin_unlock_irq却开启了中断，就会出现卡死。

这篇关于一次奇葩的spin_lock_irq / spin_unlock_irq使用不当导致的系统卡死分析的文章就介绍到这儿，希望我们推荐的文章对编程师们有所帮助！

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