============================================
WARNING: possible recursive locking detected
syzkaller #0 Not tainted
--------------------------------------------
syz.0.17/5939 is trying to acquire lock:
ffff8880131acd00 (&sb->s_type->i_mutex_key#12){++++}-{4:4}, at: inode_lock_shared include/linux/fs.h:1039 [inline]
ffff8880131acd00 (&sb->s_type->i_mutex_key#12){++++}-{4:4}, at: lookup_slow+0x42/0x70 fs/namei.c:1935

but task is already holding lock:
ffff8880131adc18 (&sb->s_type->i_mutex_key#12){++++}-{4:4}, at: inode_lock include/linux/fs.h:1024 [inline]
ffff8880131adc18 (&sb->s_type->i_mutex_key#12){++++}-{4:4}, at: cache_write net/sunrpc/cache.c:961 [inline]
ffff8880131adc18 (&sb->s_type->i_mutex_key#12){++++}-{4:4}, at: cache_write_procfs+0xaa/0x170 net/sunrpc/cache.c:1561

other info that might help us debug this:
 Possible unsafe locking scenario:

       CPU0
       ----
  lock(&sb->s_type->i_mutex_key#12);
  lock(&sb->s_type->i_mutex_key#12);

 *** DEADLOCK ***

 May be due to missing lock nesting notation

locks held by syz.0.17/5939: 2, last CPU#2:
 #0: ffff8880256da460 (sb_writers#3){.+.+}-{0:0}, at: ksys_write+0x12a/0x250 fs/read_write.c:739
 #1: ffff8880131adc18 (&sb->s_type->i_mutex_key#12){++++}-{4:4}, at: inode_lock include/linux/fs.h:1024 [inline]
 #1: ffff8880131adc18 (&sb->s_type->i_mutex_key#12){++++}-{4:4}, at: cache_write net/sunrpc/cache.c:961 [inline]
 #1: ffff8880131adc18 (&sb->s_type->i_mutex_key#12){++++}-{4:4}, at: cache_write_procfs+0xaa/0x170 net/sunrpc/cache.c:1561

stack backtrace:
CPU: 2 UID: 0 PID: 5939 Comm: syz.0.17 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
Call Trace:
 <TASK>
 __dump_stack lib/dump_stack.c:94 [inline]
 dump_stack_lvl+0x100/0x190 lib/dump_stack.c:120
 print_deadlock_bug.cold+0xbd/0xca kernel/locking/lockdep.c:3057
 check_deadlock kernel/locking/lockdep.c:3109 [inline]
 validate_chain kernel/locking/lockdep.c:3911 [inline]
 __lock_acquire+0x1343/0x1ec0 kernel/locking/lockdep.c:5254
 lock_acquire kernel/locking/lockdep.c:5908 [inline]
 lock_acquire+0x1d1/0x380 kernel/locking/lockdep.c:5865
 down_read+0x99/0x4c0 kernel/locking/rwsem.c:1574
 inode_lock_shared include/linux/fs.h:1039 [inline]
 lookup_slow+0x42/0x70 fs/namei.c:1935
 walk_component fs/namei.c:2282 [inline]
 lookup_last fs/namei.c:2789 [inline]
 path_lookupat+0x5e8/0xc40 fs/namei.c:2813
 filename_lookup+0x202/0x590 fs/namei.c:2842
 kern_path+0x37/0x50 fs/namei.c:3036
 svc_export_parse+0x1fe/0x17d0 fs/nfsd/export.c:1341
 cache_do_downcall+0xc3/0x130 net/sunrpc/cache.c:922
 cache_downcall net/sunrpc/cache.c:944 [inline]
 cache_write net/sunrpc/cache.c:962 [inline]
 cache_write_procfs+0xfb/0x170 net/sunrpc/cache.c:1561
 pde_write fs/proc/inode.c:330 [inline]
 proc_reg_write+0x240/0x330 fs/proc/inode.c:342
 vfs_write+0x2aa/0x1050 fs/read_write.c:685
 ksys_write+0x12a/0x250 fs/read_write.c:739
 do_syscall_32_irqs_on arch/x86/entry/syscall_32.c:79 [inline]
 __do_fast_syscall_32+0x13a/0x8b0 arch/x86/entry/syscall_32.c:291
 do_fast_syscall_32+0x32/0x70 arch/x86/entry/syscall_32.c:316
 entry_SYSENTER_compat_after_hwframe+0x84/0x8e
RIP: 0023:0xf7f45fec
Code: Unable to access opcode bytes at 0xf7f45fc2.
RSP: 002b:00000000fff65cac EFLAGS: 00000292 ORIG_RAX: 0000000000000004
RAX: ffffffffffffffda RBX: 0000000000000004 RCX: 0000000080000b00
RDX: 0000000000000025 RSI: 0000000000000000 RDI: 0000000000000000
RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
 </TASK>