BUG: corrupted list in p9_conn_cancel

Hello,

syzbot found the following crash on:

HEAD commit:    ca04b3cca11a Merge tag 'armsoc-fixes' of git://git.kernel...
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=132bb168400000
kernel config:  https://syzkaller.appspot.com/x/.config?x=2ca6c7a31d407f86
dashboard link: https://syzkaller.appspot.com/bug?extid=ad0832746849421bba05
compiler:       gcc (GCC) 8.0.1 20180413 (experimental)
userspace arch: i386
syzkaller repro:https://syzkaller.appspot.com/x/repro.syz?x=16c4c2c2400000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=165c81b2400000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+ad0832746849421bba05@syzkaller.appspotmail.com

list_del corruption, ffff880045022ee8->next is LIST_POISON1  
(dead000000000100)
------------[ cut here ]------------
kernel BUG at lib/list_debug.c:47!
invalid opcode: 0000 [#1] SMP KASAN
CPU: 0 PID: 26 Comm: kworker/0:2 Not tainted 4.18.0-rc3+ #40
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS  
Google 01/01/2011
Workqueue: events p9_poll_workfn
RIP: 0010:__list_del_entry_valid.cold.1+0x26/0x58 lib/list_debug.c:45
Code: 02 fe 0f 0b 4c 89 e2 48 89 de 48 c7 c7 a0 65 1a 88 e8 4a 6e 02 fe 0f  
0b 4c 89 ea 48 89 de 48 c7 c7 40 65 1a 88 e8 36 6e 02 fe <0f> 0b 48 89 de  
48 c7 c7 60 66 1a 88 e8 25 6e 02 fe 0f 0b 48 89 de
RSP: 0018:ffff8801d9467318 EFLAGS: 00010282
RAX: 000000000000004e RBX: ffff880045022ee8 RCX: 0000000000000000
RDX: 0000000000000000 RSI: ffffffff81631821 RDI: 0000000000000001
RBP: ffff8801d9467330 R08: ffff8801d94346c0 R09: ffffed003b5c4fc0
R10: ffffed003b5c4fc0 R11: ffff8801dae27e07 R12: dead000000000200
R13: dead000000000100 R14: ffff8801d9467388 R15: 1ffff1003b28ce9e
FS:  0000000000000000(0000) GS:ffff8801dae00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000008064ed0 CR3: 0000000008e6a000 CR4: 00000000001406f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
  __list_del_entry include/linux/list.h:117 [inline]
  list_del include/linux/list.h:125 [inline]
  p9_conn_cancel+0x8d6/0xd30 net/9p/trans_fd.c:224
  p9_poll_mux net/9p/trans_fd.c:620 [inline]
  p9_poll_workfn+0x4b2/0x6d0 net/9p/trans_fd.c:1107
  process_one_work+0xc73/0x1ba0 kernel/workqueue.c:2153
  worker_thread+0x189/0x13c0 kernel/workqueue.c:2296
  kthread+0x345/0x410 kernel/kthread.c:246
  ret_from_fork+0x3a/0x50 arch/x86/entry/entry_64.S:412
Modules linked in:
Dumping ftrace buffer:
    (ftrace buffer empty)
---[ end trace 13d6ed0983a959b1 ]---
RIP: 0010:__list_del_entry_valid.cold.1+0x26/0x58 lib/list_debug.c:45
Code: 02 fe 0f 0b 4c 89 e2 48 89 de 48 c7 c7 a0 65 1a 88 e8 4a 6e 02 fe 0f  
0b 4c 89 ea 48 89 de 48 c7 c7 40 65 1a 88 e8 36 6e 02 fe <0f> 0b 48 89 de  
48 c7 c7 60 66 1a 88 e8 25 6e 02 fe 0f 0b 48 89 de
RSP: 0018:ffff8801d9467318 EFLAGS: 00010282
RAX: 000000000000004e RBX: ffff880045022ee8 RCX: 0000000000000000
RDX: 0000000000000000 RSI: ffffffff81631821 RDI: 0000000000000001
RBP: ffff8801d9467330 R08: ffff8801d94346c0 R09: ffffed003b5c4fc0
R10: ffffed003b5c4fc0 R11: ffff8801dae27e07 R12: dead000000000200
R13: dead000000000100 R14: ffff8801d9467388 R15: 1ffff1003b28ce9e
FS:  0000000000000000(0000) GS:ffff8801dae00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000008064ed0 CR3: 0000000008e6a000 CR4: 00000000001406f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400


---
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syzbot suspects this bug was fixed by commit:

commit 9f476d7c540cb57556d3cc7e78704e6cd5100f5f
Author: Tomas Bortoli [off-list ref]
Date:   Mon Jul 23 18:42:53 2018 +0000

     net/9p/trans_fd.c: fix race by holding the lock

bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=11adad06600000
start commit:   c25c74b7 Merge tag 'trace-v4.18-rc3-2' of git://git.kernel..
git tree:       upstream
kernel config:  https://syzkaller.appspot.com/x/.config?x=25856fac4e580aa7
dashboard link: https://syzkaller.appspot.com/bug?extid=ad0832746849421bba05
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=13d8db0c400000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=10c12a58400000

If the result looks correct, please mark the bug fixed by replying with:

#syz fix: net/9p/trans_fd.c: fix race by holding the lock

For information about bisection process see: https://goo.gl/tpsmEJ#bisection