On 11/14/18 8:20 AM, Jens Axboe wrote:
> On 11/14/18 1:25 AM, Ming Lei wrote:
>> c2856ae2f315d ("blk-mq: quiesce queue before freeing queue") has
>> already fixed this race, however the implied synchronize_rcu()
>> in blk_mq_quiesce_queue() can slow down LUN probe a lot, so caused
>> performance regression.
>>
>> Then 1311326cf4755c7 ("blk-mq: avoid to synchronize rcu inside blk_cleanup_queue()")
>> tried to quiesce queue for avoiding unnecessary synchronize_rcu()
>> only when queue initialization is done, because it is usual to see
>> lots of inexistent LUNs which need to be probed.
>>
>> However, turns out it isn't safe to quiesce queue only when queue
>> initialization is done. Because when one SCSI command is completed,
>> the user of sending command can be waken up immediately, then the
>> scsi device may be removed, meantime the run queue in scsi_end_request()
>> is still in-progress, so kernel panic can be caused.
>>
>> In Red Hat QE lab, there are several reports about this kind of kernel
>> panic triggered during kernel booting.
>>
>> This patch tries to address the issue by grabing one queue usage
>> counter during freeing one request and the following run queue.
>
> Thanks applied, this bug was elusive but ever present in recent
> testing that we did internally, it's been a huge pain in the butt.
> The symptoms were usually a crash in blk_mq_get_driver_tag() with
> hctx->tags == NULL, or a crash inside deadline request insert off
> requeue.
I'm still hitting some weird crashes even with this applied, like
this one:
BUG: unable to handle kernel NULL pointer dereference at 0000000000000148
PGD 0 P4D 0.
Oops: 0000 [#1] SMP PTI
CPU: 37 PID: 763 Comm: kworker/37:1H Not tainted 4.20.0-rc3-00649-ge64d9a554a91-dirty #14
Hardware name: Wiwynn Leopard-Orv2/Leopard-DDR BW, BIOS LBM08 03/03/2017
Workqueue: kblockd blk_mq_run_work_fn
RIP: 0010:blk_mq_get_driver_tag+0x81/0x120
Code: 24 10 48 89 7c 24 20 74 21 83 fa ff 0f 95 c0 48 8b 4c 24 28 65 48 33 0c 25 28 00 00 00 0f 85 96 00 00 00 48 83 c4 30 5b 5d c3 <48> 8b 87 48 01 00 00 8b 40 04 39 43 20 72 37 f6 87 b0 00 00 00 02
RSP: 0018:ffffc90004aabd30 EFLAGS: 00010246
RAX: 0000000000000003 RBX: ffff888465ea1300 RCX: ffffc90004aabde8
RDX: 00000000ffffffff RSI: ffffc90004aabde8 RDI: 0000000000000000
RBP: 0000000000000000 R08: ffff888465ea1348 R09: 0000000000000000
R10: 0000000000001000 R11: 00000000ffffffff R12: ffff888465ea1300
R13: 0000000000000000 R14: ffff888465ea1348 R15: ffff888465d10000
FS: 0000000000000000(0000) GS:ffff88846f9c0000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000148 CR3: 000000000220a003 CR4: 00000000003606e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
blk_mq_dispatch_rq_list+0xec/0x480
? elv_rb_del+0x11/0x30
blk_mq_do_dispatch_sched+0x6e/0xf0
blk_mq_sched_dispatch_requests+0xfa/0x170
__blk_mq_run_hw_queue+0x5f/0xe0
process_one_work+0x154/0x350
worker_thread+0x46/0x3c0
kthread+0xf5/0x130
? process_one_work+0x350/0x350
? kthread_destroy_worker+0x50/0x50
ret_from_fork+0x1f/0x30
Modules linked in: sb_edac x86_pkg_temp_thermal intel_powerclamp coretemp kvm_intel kvm switchtec irqbypass iTCO_wdt iTCO_vendor_support efivars cdc_ether usbnet mii cdc_acm i2c_i801 lpc_ich mfd_core ipmi_si ipmi_devintf ipmi_msghandler acpi_cpufreq button sch_fq_codel nfsd nfs_acl lockd grace auth_rpcgss oid_registry sunrpc nvme nvme_core fuse sg loop efivarfs autofs4
CR2: 0000000000000148
---[ end trace 340a1fb996df1b9b ]---
RIP: 0010:blk_mq_get_driver_tag+0x81/0x120
Code: 24 10 48 89 7c 24 20 74 21 83 fa ff 0f 95 c0 48 8b 4c 24 28 65 48 33 0c 25 28 00 00 00 0f 85 96 00 00 00 48 83 c4 30 5b 5d c3 <48> 8b 87 48 01 00 00 8b 40 04 39 43 20 72 37 f6 87 b0 00 00 00 02
which doesn't look that great... Are we sure this patch closed the window
completely?
One thing I'm pondering is we're running the queue async, so the
ref get will protect whatever blk_mq_run_hw_queues() does, but
what is preventing the queue from going away as soon as we've
returned from that call? Meanwhile we still have the work item
queued up, and it'll run, and go boom like above.
--
Jens Axboe
