syzbot

===============================
[ INFO: suspicious RCU usage. ]
4.9.202+ #0 Not tainted
-------------------------------
include/linux/radix-tree.h:199 suspicious rcu_dereference_check() usage!

other info that might help us debug this:


rcu_scheduler_active = 2, debug_locks = 0
2 locks held by syz-executor184/2062:
 #0:  (&sb->s_type->i_mutex_key#10){+.+.+.}, at: [<00000000e5465ac4>] inode_lock include/linux/fs.h:771 [inline]
 #0:  (&sb->s_type->i_mutex_key#10){+.+.+.}, at: [<00000000e5465ac4>] shmem_add_seals+0x166/0x1020 mm/shmem.c:2610
 #1:  (&(&mapping->tree_lock)->rlock){..-...}, at: [<000000008749736d>] spin_lock_irq include/linux/spinlock.h:332 [inline]
 #1:  (&(&mapping->tree_lock)->rlock){..-...}, at: [<000000008749736d>] shmem_tag_pins mm/shmem.c:2465 [inline]
 #1:  (&(&mapping->tree_lock)->rlock){..-...}, at: [<000000008749736d>] shmem_wait_for_pins mm/shmem.c:2506 [inline]
 #1:  (&(&mapping->tree_lock)->rlock){..-...}, at: [<000000008749736d>] shmem_add_seals+0x342/0x1020 mm/shmem.c:2622

stack backtrace:
CPU: 1 PID: 2062 Comm: syz-executor184 Not tainted 4.9.202+ #0
 ffff8801cf257ca0 ffffffff81b55d2b ffff8801c977ec28 0000000000000000
 0000000000000002 00000000000000c7 ffff8801d209df00 ffff8801cf257cd0
 ffffffff81406867 ffffea00074d45c0 dffffc0000000000 ffff8801cf257d78
Call Trace:
 [<000000000d8f2bba>] __dump_stack lib/dump_stack.c:15 [inline]
 [<000000000d8f2bba>] dump_stack+0xcb/0x130 lib/dump_stack.c:56
 [<00000000ff27527b>] lockdep_rcu_suspicious.cold+0x10a/0x149 kernel/locking/lockdep.c:4458
 [<0000000019b3d172>] radix_tree_deref_slot include/linux/radix-tree.h:199 [inline]
 [<0000000019b3d172>] shmem_tag_pins mm/shmem.c:2467 [inline]
 [<0000000019b3d172>] shmem_wait_for_pins mm/shmem.c:2506 [inline]
 [<0000000019b3d172>] shmem_add_seals+0xa44/0x1020 mm/shmem.c:2622
 [<000000003a7a5a43>] shmem_fcntl+0xf7/0x130 mm/shmem.c:2657
 [<0000000