Re: [PATCH] debugfs: fix potential infinite loop in debugfs_remove_recursive

["yukuai (C)" <yukuai3@huawei.com>]

On 2019/11/14 10:43, Steven Rostedt wrote:
> On Thu, 14 Nov 2019 10:01:23 +0800
> "yukuai (C)" <yukuai3@huawei.com> wrote:
> 
> 
>> Do you agree with that list_empty(&chile->d_subdirs) here is not
>> appropriate? Since it can't skip the subdirs that is not
>> simple_positive(simple_positive() will return false), which is the
>> reason of infinite loop.
> 
> I do agree that simple_empty() is wrong, for the reasons you pointed out.
> 
>>>> +		if (!simple_empty(child)) {
>>>
>>> Have you tried this with lockdep enabled? I'm thinking that you might
>>> get a splat with holding parent->d_lock and simple_empty(child) taking
>>> the child->d_lock.
>> The locks are taken and released in the right order:
>> take parent->d_lock
>> 	take child->d_lock
>> 		list_for_each_entry(c, &child->d_sundirs, d_child)
>> 			take c->d_lock
>> 			release c->d_lock
>> 	release child->d_lock
>> release parent->d_lock
>> I don't see anything wrong, am I missing something?
> 
> It should be fine, my worry is that we may be missing a lockdep
> annotation, that might confuse lockdep, as lockdep may see this as the
> same type of lock being taken, and wont know the order.
> 
> Have you tried this patch with lockdep enabled and tried to hit this
> code path?
> 
> -- Steve
> 
> .
> 
You are right, I get the results with lockdep enabled:
[   64.314748] ============================================
[   64.315568] WARNING: possible recursive locking detected
[   64.316549] 5.4.0-rc7-dirty #5 Tainted: G           O
[   64.317398] --------------------------------------------
[   64.318230] rmmod/2607 is trying to acquire lock:
[   64.318982] ffff88812c8d01e8 
(&(&dentry->d_lockref.lock)->rlock){+.+.}, at: simple_empty+0x2c/0xf0
[   64.320539]
[   64.320539] but task is already holding lock:
[   64.321466] ffff88812c8d00a8 
(&(&dentry->d_lockref.lock)->rlock){+.+.}, at: 
debugfs_remove_recursive+0x7a/0x260
[   64.323066]
[   64.323066] other info that might help us debug this:
[   64.324200]  Possible unsafe locking scenario:
[   64.324200]
[   64.325166]        CPU0
[   64.325569]        ----
[   64.325966]   lock(&(&dentry->d_lockref.lock)->rlock);
[   64.326790]   lock(&(&dentry->d_lockref.lock)->rlock);
[   64.327604]
[   64.327604]  *** DEADLOCK ***
[   64.327604]
[   64.328675]  May be due to missing lock nesting notation
[   64.328675]
[   64.329758] 2 locks held by rmmod/2607:
[   64.330373]  #0: ffff88812c8ba630 (&sb->s_type->i_mutex_key#3){++++}, 
at: debugfs_remove_recursive+0x6a/0x260
[   64.331997]  #1: ffff88812c8d00a8 
(&(&dentry->d_lockref.lock)->rlock){+.+.}, at: 
debugfs_remove_recursive+0x7a/0x260
[   64.333713]
[   64.333713] stack backtrace:
[   64.334412] CPU: 2 PID: 2607 Comm: rmmod Tainted: G           O 
5.4.0-rc7-dirty #5
[   64.335688] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), 
BIOS rel-1.12.0-0-ga698c8995f-prebuilt.qemu.org 04/01/2014
[   64.337582] Call Trace:
[   64.337999]  dump_stack+0xd6/0x13a
[   64.338554]  __lock_acquire+0x19b1/0x1a70
[   64.339205]  lock_acquire+0x10a/0x2a0
[   64.339821]  ? simple_empty+0x2c/0xf0
[   64.340503]  _raw_spin_lock+0x50/0xd0
[   64.341097]  ? simple_empty+0x2c/0xf0
[   64.341693]  simple_empty+0x2c/0xf0
[   64.342258]  debugfs_remove_recursive+0xef/0x260
[   64.343755]  hisi_sas_test_exit+0x26/0x30 [hisi_sas_test]
[   64.344732]  __x64_sys_delete_module+0x258/0x330
[   64.345474]  ? do_syscall_64+0x74/0x530
[   64.346094]  ? trace_hardirqs_on+0x6a/0x1e0
[   64.346766]  do_syscall_64+0xcc/0x530
[   64.347349]  entry_SYSCALL_64_after_hwframe+0x49/0xbe
[   64.348265] RIP: 0033:0x7f2418459fc7
[   64.348841] Code: 73 01 c3 48 8b 0d c1 de 2b 00 f7 d8 64 89 01 48 83 
c8 ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 b8 b0 00 008
[   64.351789] RSP: 002b:00007fffbc2b0ab8 EFLAGS: 00000206 ORIG_RAX: 
00000000000000b0
[   64.353106] RAX: ffffffffffffffda RBX: 00007fffbc2b0b18 RCX: 
00007f2418459fc7
[   64.354238] RDX: 000000000000000a RSI: 0000000000000800 RDI: 
000055b1e1d84258
[   64.355377] RBP: 000055b1e1d841f0 R08: 00007fffbc2afa31 R09: 
0000000000000000
[   64.356628] R10: 00007f24184cc260 R11: 0000000000000206 R12: 
00007fffbc2b0ce0
[   64.357760] R13: 00007fffbc2b13d6 R14: 000055b1e1d83010 R15: 
000055b1e1d841f0

The warning will disappeare by adding 
lockdep_set_novalidate_class(&child->d_lock) before calling 
simple_empty(child). But I'm not sure It's the right modfication.

Thanks
Yu Kuai