WARNING: refcount bug in blk_mq_free_request (2)

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit: 90203176 Add linux-next specific files for 20190625
git tree: linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=11d868ada00000
kernel config: https://syzkaller.appspot.com/x/.config?x=efa119147b3bee3
dashboard link: https://syzkaller.appspot.com/bug?extid=3464c2fd1d4a51d3688c
compiler: gcc (GCC) 9.0.0 20181231 (experimental)
CC: [ax...@kernel.dk linux...@vger.kernel.org
linux-...@vger.kernel.org]

Unfortunately, I don't have any reproducer for this crash yet.

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+3464c2...@syzkaller.appspotmail.com

blk_update_request: I/O error, dev nbd0, sector 4 op 0x0:(READ) flags 0x0
phys_seg 1 prio class 0
Buffer I/O error on dev nbd0, logical block 2, async page read
blk_update_request: I/O error, dev nbd0, sector 6 op 0x0:(READ) flags 0x0
phys_seg 1 prio class 0
Buffer I/O error on dev nbd0, logical block 3, async page read
------------[ cut here ]------------
refcount_t: underflow; use-after-free.
WARNING: CPU: 0 PID: 2494 at lib/refcount.c:190
refcount_sub_and_test_checked lib/refcount.c:190 [inline]
WARNING: CPU: 0 PID: 2494 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: 0 PID: 2494 Comm: kworker/0:1H Not tainted 5.2.0-rc6-next-20190625 #22
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
Google 01/01/2011
Workqueue: kblockd blk_mq_run_work_fn
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+0x14/0x20 arch/x86/entry/entry_64.S:986
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 69 05 64 06 31 ff 89 de e8 9c 74 38 fe 84 db 75 94 e8 53 73 38 fe
48 c7 c7 a0 fa c4 87 c6 05 49 05 64 06 01 e8 a8 0b 0a fe <0f> 0b e9 75 ff
ff ff e8 34 73 38 fe e9 6e ff ff ff 48 89 df e8 a7
RSP: 0018:ffff8880a378f878 EFLAGS: 00010282
RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
RDX: 0000000000000000 RSI: ffffffff815c11f6 RDI: ffffed10146f1f01
RBP: ffff8880a378f910 R08: ffff8880a37a4040 R09: ffffed1015d060b1
R10: ffffed1015d060b0 R11: ffff8880ae830587 R12: 00000000ffffffff
R13: 0000000000000001 R14: ffff8880a378f8e8 R15: 0000000000000000
refcount_dec_and_test_checked+0x1b/0x20 lib/refcount.c:220
blk_mq_free_request+0x3b8/0x580 block/blk-mq.c:528
__blk_mq_end_request block/blk-mq.c:554 [inline]
blk_mq_end_request+0x456/0x560 block/blk-mq.c:563
blk_mq_dispatch_rq_list+0xaf8/0x19e0 block/blk-mq.c:1281
blk_mq_do_dispatch_sched+0x1a3/0x430 block/blk-mq-sched.c:115
blk_mq_sched_dispatch_requests+0x3f4/0x6d0 block/blk-mq-sched.c:211
__blk_mq_run_hw_queue+0x1cc/0x2e0 block/blk-mq.c:1393
blk_mq_run_work_fn+0x5c/0x70 block/blk-mq.c:1626
process_one_work+0x9af/0x1740 kernel/workqueue.c:2269
worker_thread+0x98/0xe40 kernel/workqueue.c:2415
kthread+0x361/0x430 kernel/kthread.c:255
ret_from_fork+0x24/0x30 arch/x86/entry/entry_64.S:353
Kernel Offset: disabled
Rebooting in 86400 seconds..


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[syzbot]

Sending this report upstream.