On Wed, Nov 21, 2018 at 06:42:51PM -0700, Jens Axboe wrote:

On 11/21/18 6:00 PM, Ming Lei wrote:

quoted

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

quoted

On 11/14/18 8:20 AM, Jens Axboe wrote:

quoted

On 11/14/18 1:25 AM, Ming Lei wrote:

quoted

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

quoted

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.

It's definitely broken. Big time. And we need to do something about
it NOW.

quoted

Or maybe we can try the following patch?

I'm going to start backing out the sync removal patches instead of
adding items to the hot path...

Yeah, I agree to do the following to bring back the sync, even though it slows
down boot.

diff --git a/block/blk-core.c b/block/blk-core.c
index 04f5be473638..f6943f4a4d16 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c

@@ -355,7 +355,7 @@ void blk_cleanup_queue(struct request_queue *q)
         * We rely on driver to deal with the race in case that queue
         * initialization isn't done.
         */
-       if (queue_is_mq(q) && blk_queue_init_done(q))
+       if (queue_is_mq(q))
                blk_mq_quiesce_queue(q);

Ted, I saw your email, I'm looking into it. Sounds like a regression
between 4.18 and 4.19. The sync issue could still be it, as it can
cause memory corruption, and that could lead to other corruption
issues.

But I don't understand why this sync issue is related data corruption
because FS IO only starts after blk_queue_init_done() becomes true.
And blk_mq_quiesce_queue() and the following blk_sync_queue() should
have drained everything queued.

Thanks,
Ming