WARNING: refcount bug in blk_mq_free_request (2)

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit:    e21a712a Linux 5.3-rc3
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=10cf349a600000
kernel config:  https://syzkaller.appspot.com/x/.config?x=a4c9e9f08e9e8960
dashboard link: https://syzkaller.appspot.com/bug?extid=f4316dab9d4518b755eb
compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=117a1906600000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=11aa11aa600000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+f4316dab9d4518b755eb@xxxxxxxxxxxxxxxxxxxxxxxxx

------------[ cut here ]------------
refcount_t: underflow; use-after-free.
WARNING: CPU: 1 PID: 16 at lib/refcount.c:190 refcount_sub_and_test_checked  
lib/refcount.c:190 [inline]
WARNING: CPU: 1 PID: 16 at lib/refcount.c:190  
refcount_sub_and_test_checked+0x1d0/0x200 lib/refcount.c:180
Kernel panic - not syncing: panic_on_warn set ...
CPU: 1 PID: 16 Comm: ksoftirqd/1 Not tainted 5.3.0-rc3 #98
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS  
Google 01/01/2011
Call Trace:
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0x172/0x1f0 lib/dump_stack.c:113
 panic+0x2dc/0x755 kernel/panic.c:219
 __warn.cold+0x20/0x4c kernel/panic.c:576
 report_bug+0x263/0x2b0 lib/bug.c:186
 fixup_bug arch/x86/kernel/traps.c:179 [inline]
 fixup_bug arch/x86/kernel/traps.c:174 [inline]
 do_error_trap+0x11b/0x200 arch/x86/kernel/traps.c:272
 do_invalid_op+0x37/0x50 arch/x86/kernel/traps.c:291
 invalid_op+0x23/0x30 arch/x86/entry/entry_64.S:1026
RIP: 0010:refcount_sub_and_test_checked lib/refcount.c:190 [inline]
RIP: 0010:refcount_sub_and_test_checked+0x1d0/0x200 lib/refcount.c:180
Code: 1d 7e b3 64 06 31 ff 89 de e8 9c a3 35 fe 84 db 75 94 e8 53 a2 35 fe  
48 c7 c7 80 02 c6 87 c6 05 5e b3 64 06 01 e8 18 15 07 fe <0f> 0b e9 75 ff  
ff ff e8 34 a2 35 fe e9 6e ff ff ff 48 89 df e8 b7
RSP: 0018:ffff8880a990fbb0 EFLAGS: 00010286
RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
RDX: 0000000000000100 RSI: ffffffff815c3ba6 RDI: ffffed1015321f68
RBP: ffff8880a990fc48 R08: ffff8880a9900440 R09: ffffed1015d24101
R10: ffffed1015d24100 R11: ffff8880ae920807 R12: 00000000ffffffff
R13: 0000000000000001 R14: ffff8880a990fc20 R15: 0000000000000000
 refcount_dec_and_test_checked+0x1b/0x20 lib/refcount.c:220
 blk_mq_free_request+0x3b8/0x580 block/blk-mq.c:524
 __blk_mq_end_request block/blk-mq.c:550 [inline]
 blk_mq_end_request+0x456/0x560 block/blk-mq.c:559
 nbd_complete_rq+0x42/0x50 drivers/block/nbd.c:322
 blk_done_softirq+0x2fe/0x4d0 block/blk-softirq.c:37
 __do_softirq+0x262/0x98c kernel/softirq.c:292
 run_ksoftirqd kernel/softirq.c:603 [inline]
 run_ksoftirqd+0x8e/0x110 kernel/softirq.c:595
 smpboot_thread_fn+0x6a3/0xa40 kernel/smpboot.c:165
 kthread+0x361/0x430 kernel/kthread.c:255
 ret_from_fork+0x24/0x30 arch/x86/entry/entry_64.S:352
Kernel Offset: disabled
Rebooting in 86400 seconds..


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