WARNING in blk_mq_init_sched

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit:    f7c3bf8f Merge tag 'gfs2-for-5.4' of git://git.kernel.org/..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=15f5baf9600000
kernel config:  https://syzkaller.appspot.com/x/.config?x=50d4af03d68a470c
dashboard link: https://syzkaller.appspot.com/bug?extid=b2c197f98f86543b69c8
compiler:       clang version 9.0.0 (/home/glider/llvm/clang  
80fee25776c2fb61e74c1ecb1a523375c2500b69)

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+b2c197f98f86543b69c8@xxxxxxxxxxxxxxxxxxxxxxxxx

------------[ cut here ]------------
WARNING: CPU: 1 PID: 25817 at block/blk-mq-sched.c:558  
blk_mq_sched_free_requests block/blk-mq-sched.c:558 [inline]
WARNING: CPU: 1 PID: 25817 at block/blk-mq-sched.c:558  
blk_mq_init_sched+0xad6/0xc00 block/blk-mq-sched.c:543
Kernel panic - not syncing: panic_on_warn set ...
CPU: 1 PID: 25817 Comm: syz-executor.4 Not tainted 5.3.0+ #0
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+0x1d8/0x2f8 lib/dump_stack.c:113
 panic+0x25c/0x799 kernel/panic.c:219
 __warn+0x22f/0x230 kernel/panic.c:576
 report_bug+0x190/0x290 lib/bug.c:186
 fixup_bug arch/x86/kernel/traps.c:179 [inline]
 do_error_trap+0xd7/0x440 arch/x86/kernel/traps.c:272
 do_invalid_op+0x36/0x40 arch/x86/kernel/traps.c:291
 invalid_op+0x23/0x30 arch/x86/entry/entry_64.S:1028
RIP: 0010:blk_mq_sched_free_requests block/blk-mq-sched.c:558 [inline]
RIP: 0010:blk_mq_init_sched+0xad6/0xc00 block/blk-mq-sched.c:543
Code: f6 e8 9e 03 00 00 49 83 c6 10 4c 89 f7 e8 82 08 37 04 e9 ce fd ff ff  
e8 c8 81 3f fe 48 c7 c7 72 5c 35 88 31 c0 e8 1d ae 28 fe <0f> 0b e9 ce f9  
ff ff e8 ae 81 3f fe 48 c7 c7 72 5c 35 88 31 c0 e8
RSP: 0018:ffff88802225fbb8 EFLAGS: 00010246
RAX: 0000000000000024 RBX: 0000000000000000 RCX: 489d2508ed9c7100
RDX: ffffc9000e9a6000 RSI: 0000000000009af7 RDI: 0000000000009af8
RBP: ffff88802225fc50 R08: ffffffff815c9744 R09: ffffed1015d66090
R10: ffffed1015d66090 R11: 0000000000000000 R12: dffffc0000000000
R13: ffff888026958990 R14: ffff888026958080 R15: ffff8880269580d0
 elevator_init_mq+0x317/0x450 block/elevator.c:719
 __device_add_disk+0x6d/0x1140 block/genhd.c:705
 device_add_disk+0x2a/0x40 block/genhd.c:763
 add_disk include/linux/genhd.h:429 [inline]
 loop_add+0x5d1/0x780 drivers/block/loop.c:2051
 loop_control_ioctl+0x422/0x640 drivers/block/loop.c:2174
 do_vfs_ioctl+0x744/0x1730 fs/ioctl.c:46
 ksys_ioctl fs/ioctl.c:713 [inline]
 __do_sys_ioctl fs/ioctl.c:720 [inline]
 __se_sys_ioctl fs/ioctl.c:718 [inline]
 __x64_sys_ioctl+0xe3/0x120 fs/ioctl.c:718
 do_syscall_64+0xf7/0x1c0 arch/x86/entry/common.c:290
 entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x459a09
Code: fd b7 fb ff c3 66 2e 0f 1f 84 00 00 00 00 00 66 90 48 89 f8 48 89 f7  
48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff  
ff 0f 83 cb b7 fb ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007fce60497c78 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 0000000000459a09
RDX: 0000000000000000 RSI: 0000000000004c80 RDI: 0000000000000006
RBP: 000000000075bfc8 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00007fce604986d4
R13: 00000000004c3118 R14: 00000000004d69f8 R15: 00000000ffffffff
Kernel Offset: disabled
Rebooting in 86400 seconds..


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