Re: [PATCH V2] SCSI: fix queue cleanup race before queue initialization is done

[Ming Lei <ming.lei@redhat.com>]

On Wed, Nov 21, 2018 at 02:47:35PM -0700, Jens Axboe wrote:
> 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?

I mentioned this patch is just one workaround, see my comment before:

https://marc.info/?l=linux-scsi&m=154224379320094&w=2

> 
> 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.

blk_sync_queue() supposes to drain the queued work, but it can be
queued after blk_sync_queue() returns.

Or maybe we can try the following patch?
--

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 0375c3bd410e..986c211ca74c 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1570,6 +1570,9 @@ static void blk_mq_run_work_fn(struct work_struct *work)
 
 	hctx = container_of(work, struct blk_mq_hw_ctx, run_work.work);
 
+	if (unlikely(blk_queue_dead(hctx->queue)))
+		return;
+
 	/*
 	 * If we are stopped, don't run the queue.
 	 */


Thanks,
Ming