[syzbot] INFO: task can't die in p9_client_rpc (3)

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit:    a1f92694 Add linux-next specific files for 20210518
git tree:       linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=10805a64300000
kernel config:  https://syzkaller.appspot.com/x/.config?x=d612e75ffd53a6d3
dashboard link: https://syzkaller.appspot.com/bug?extid=716aab0e63b2895e1811

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

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+716aab0e63b2895e1811@xxxxxxxxxxxxxxxxxxxxxxxxx

INFO: task syz-executor.2:17696 can't die for more than 143 seconds.
task:syz-executor.2  state:D stack:27376 pid:17696 ppid:  8468 flags:0x00004004
Call Trace:
 context_switch kernel/sched/core.c:4688 [inline]
 __schedule+0xb38/0x58c0 kernel/sched/core.c:5945
 schedule+0xcf/0x270 kernel/sched/core.c:6024
 p9_client_rpc+0x405/0x1240 net/9p/client.c:759
 p9_client_flush+0x1f9/0x430 net/9p/client.c:667
 p9_client_rpc+0xfe3/0x1240 net/9p/client.c:784
 p9_client_version net/9p/client.c:955 [inline]
 p9_client_create+0xae1/0x1110 net/9p/client.c:1055
 v9fs_session_init+0x1dd/0x17b0 fs/9p/v9fs.c:406
 v9fs_mount+0x79/0x9c0 fs/9p/vfs_super.c:126
 legacy_get_tree+0x105/0x220 fs/fs_context.c:592
 vfs_get_tree+0x89/0x2f0 fs/super.c:1498
 do_new_mount fs/namespace.c:2905 [inline]
 path_mount+0x132a/0x1fa0 fs/namespace.c:3235
 do_mount fs/namespace.c:3248 [inline]
 __do_sys_mount fs/namespace.c:3456 [inline]
 __se_sys_mount fs/namespace.c:3433 [inline]
 __x64_sys_mount+0x27f/0x300 fs/namespace.c:3433
 do_syscall_64+0x31/0xb0 arch/x86/entry/common.c:47
 entry_SYSCALL_64_after_hwframe+0x44/0xae
RIP: 0033:0x4665d9
RSP: 002b:00007f42a9fe9188 EFLAGS: 00000246 ORIG_RAX: 00000000000000a5
RAX: ffffffffffffffda RBX: 000000000056bf80 RCX: 00000000004665d9
RDX: 0000000020000200 RSI: 0000000020000000 RDI: 0000000000000000
RBP: 00000000004bfcb9 R08: 0000000020000440 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 000000000056bf80
R13: 00007ffdec5a8f0f R14: 00007f42a9fe9300 R15: 0000000000022000

Showing all locks held in the system:
1 lock held by khungtaskd/1641:
 #0: ffffffff8c17afe0 (rcu_read_lock){....}-{1:2}, at: debug_show_all_locks+0x53/0x260 kernel/locking/lockdep.c:6333
1 lock held by in:imklog/8364:
 #0: ffff888025c12370 (&f->f_pos_lock){+.+.}-{3:3}, at: __fdget_pos+0xe9/0x100 fs/file.c:990
4 locks held by rs:main Q:Reg/8365:
 #0: ffff8880b9c35718 (&rq->__lock){-.-.}-{2:2}, at: raw_spin_rq_lock_nested+0x2b/0x120 kernel/sched/core.c:460
 #1: ffff8880b9c1f988 (&per_cpu_ptr(group->pcpu, cpu)->seq){-.-.}-{0:0}, at: psi_task_switch+0x24c/0x670 kernel/sched/psi.c:872
 #2: ffff88802e7cf230 (&sb->s_type->i_mutex_key#10){++++}-{3:3}, at: inode_lock include/linux/fs.h:774 [inline]
 #2: ffff88802e7cf230 (&sb->s_type->i_mutex_key#10){++++}-{3:3}, at: ext4_buffered_write_iter+0xb6/0x4d0 fs/ext4/file.c:263
 #3: ffff88802e7cf4f8 (&ei->i_raw_lock){+.+.}-{2:2}, at: spin_lock include/linux/spinlock.h:359 [inline]
 #3: ffff88802e7cf4f8 (&ei->i_raw_lock){+.+.}-{2:2}, at: ext4_do_update_inode fs/ext4/inode.c:5033 [inline]
 #3: ffff88802e7cf4f8 (&ei->i_raw_lock){+.+.}-{2:2}, at: ext4_mark_iloc_dirty+0x213/0x38d0 fs/ext4/inode.c:5724
2 locks held by agetty/8383:
 #0: ffff888015b94098 (&tty->ldisc_sem){++++}-{0:0}, at: tty_ldisc_ref_wait+0x22/0x80 drivers/tty/tty_ldisc.c:253
 #1: ffffc90000fdc2e8 (&ldata->atomic_read_lock){+.+.}-{3:3}, at: n_tty_read+0xcf0/0x1230 drivers/tty/n_tty.c:2113

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