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To:     Jens Axboe <axboe@kernel.dk>, Ming Lei <ming.lei@redhat.com>,
        "linux-block@vger.kernel.org" <linux-block@vger.kernel.org>
Cc:     Christoph Hellwig <hch@lst.de>,
        Christian Borntraeger <borntraeger@de.ibm.com>,
        Heiko Carstens <heiko.carstens@de.ibm.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Martin Schwidefsky <schwidefsky@de.ibm.com>
From:   Stefan Haberland <sth@linux.vnet.ibm.com>
Subject: 4.16-RC7 WARNING: CPU: 2 PID: 0 at block/blk-mq.c:1400
 __blk_mq_delay_run_hw_queue
Date:   Tue, 27 Mar 2018 12:04:20 +0200
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Hi,

I get the following warning in __blk_mq_delay_run_hw_queue when the 
scheduler is set to mq-deadline for DASD devices on s390.

What I see is that for whatever reason there is a hctx nr 0 which has no 
hctx->tags pointer set.
 From my observation it is always hctx nr 0 which has a tags NULL 
pointer in it and I see other hctx which have the hctx->tags pointer set 
correctly.


[    2.169986] WARNING: CPU: 0 PID: 0 at block/blk-mq.c:1402 
__blk_mq_delay_run_hw_queue+0xe8/0x118
[    2.170007] Modules linked in:
[    2.170014] CPU: 0 PID: 0 Comm: swapper/0 Not tainted 
4.16.0-rc7-04107-g91a05d9e1d6b-dirty #147
[    2.170019] Hardware name: IBM 2964 N96 702 (z/VM 6.4.0)
[    2.170024] Krnl PSW : 0000000076fd6c7f 00000000c244c24d 
(__blk_mq_delay_run_hw_queue+0xe8/0x118)
[    2.170035]            R:0 T:1 IO:0 EX:0 Key:0 M:1 W:0 P:0 AS:3 CC:0 
PM:0 RI:0 EA:3
[    2.170041] Krnl GPRS: 00000000599ec58a 0000000002a94000 
0000000002a94000 0000000000000001
[    2.170047]            0000000000000000 000000006e761e98 
0000000002a96980 0000000002a96800
[    2.170052]            0000000002d87ce0 00000000737fbda8 
0000000000000000 0000000000000001
[    2.170058]            0000000000000001 0000000000aedd10 
00000000737fbc38 00000000737fbc00
[    2.170069] Krnl Code: 00000000006ea3c8: ebaff0a00004 lmg     
%r10,%r15,160(%r15)
                           00000000006ea3ce: c0f4ffff5e0d brcl    15,6d5fe8
                          #00000000006ea3d4: a7f40001 brc     15,6ea3d6
                          >00000000006ea3d8: e340f0c00004        lg      
%r4,192(%r15)
                           00000000006ea3de: ebaff0a00004 lmg     
%r10,%r15,160(%r15)
                           00000000006ea3e4: 07f4 bcr     15,%r4
                           00000000006ea3e6: 41b01100 la      %r11,256(%r1)
                           00000000006ea3ea: 182a lr      %r2,%r10
[    2.170158] Call Trace:
[    2.170205] ([<0000000002a96800>] 0x2a96800)
[    2.170248]  [<00000000006ea4c0>] blk_mq_run_hw_queue+0xa0/0x100
[    2.170262]  [<00000000006ea59c>] blk_mq_run_hw_queues+0x7c/0x98
[    2.170295]  [<00000000006e88f6>] __blk_mq_complete_request+0x10e/0x1e0
[    2.170300]  [<00000000006e9e30>] blk_mq_complete_request+0x80/0xa0
[    2.170307]  [<000000000087fad0>] dasd_block_tasklet+0x218/0x480
[    2.170415]  [<000000000017c3f8>] tasklet_hi_action+0xa0/0x138
[    2.170434]  [<0000000000a91c10>] __do_softirq+0xc8/0x540
[    2.170471]  [<000000000017bd4e>] irq_exit+0x136/0x140
[    2.170478]  [<000000000010c912>] do_IRQ+0x8a/0xb8
[    2.170518]  [<0000000000a90ee0>] io_int_handler+0x138/0x2e0
[    2.170524]  [<0000000000102cd0>] enabled_wait+0x58/0x128
[    2.170562] ([<0000000000102cb8>] enabled_wait+0x40/0x128)
[    2.170577]  [<000000000010319a>] arch_cpu_idle+0x32/0x48
[    2.170604]  [<0000000000a8f636>] default_idle_call+0x3e/0x58
[    2.170613]  [<00000000001cd5d2>] do_idle+0xda/0x190
[    2.170621]  [<00000000001cd93e>] cpu_startup_entry+0x3e/0x48
[    2.170633]  [<0000000000e5ebf4>] start_kernel+0x47c/0x490
[    2.170641]  [<0000000000100020>] _stext+0x20/0x80
[    2.170650] 2 locks held by swapper/0/0:
[    2.170658]  #0:  (&(&dq->lock)->rlock){..-.}, at: 
[<00000000b45eaf9e>] dasd_block_tasklet+0x1cc/0x480
[    2.170676]  #1:  (rcu_read_lock){....}, at: [<00000000bc7fa045>] 
hctx_lock+0x34/0x110
[    2.170750] Last Breaking-Event-Address:
[    2.170758]  [<00000000006ea3d4>] __blk_mq_delay_run_hw_queue+0xe4/0x118
[    2.170803] ---[ end trace 1073cf0de1fd32d0 ]---


Regards,
Stefan