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Subject: Re: [PATCH 1/4] fs/dcache: Limit numbers of negative dentries
To: Miklos Szeredi <mszeredi@redhat.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>,
        Jonathan Corbet <corbet@lwn.net>,
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From: Waiman Long <longman@redhat.com>
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Date: Thu, 20 Jul 2017 11:46:38 -0400
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On 07/20/2017 11:08 AM, Miklos Szeredi wrote:
> On Thu, Jul 20, 2017 at 4:21 PM, Waiman Long <longman@redhat.com> wrote:
>> On 07/20/2017 03:20 AM, Miklos Szeredi wrote:
>>> On Wed, Jul 19, 2017 at 10:42 PM, Waiman Long <longman@redhat.com> wrote:
>>>>>> @@ -603,7 +698,13 @@ static struct dentry *dentry_kill(struct dentry *dentry)
>>>>>>
>>>>>>         if (!IS_ROOT(dentry)) {
>>>>>>                 parent = dentry->d_parent;
>>>>>> -               if (unlikely(!spin_trylock(&parent->d_lock))) {
>>>>>> +               /*
>>>>>> +                * Force the killing of this negative dentry when
>>>>>> +                * DCACHE_KILL_NEGATIVE flag is set.
>>>>>> +                */
>>>>>> +               if (unlikely(dentry->d_flags & DCACHE_KILL_NEGATIVE)) {
>>>>>> +                       spin_lock(&parent->d_lock);
>>>>> This looks like d_lock ordering problem (should be parent first, child
>>>>> second).  Why is this needed, anyway?
>>>>>
>>>> Yes, that is a bug. I should have used lock_parent() instead.
>>> lock_parent() can release dentry->d_lock, which means it's perfectly
>>> useless for this.
>> As the reference count is kept at 1 in dentry_kill(), the dentry won't
>> go away even if the dentry lock is temporarily released.
> It won't go away, but anything else might happen to it (ref grabbed by
> somebody else, instantiated, etc).  Don't see how it's going to be
> better than the existing trylock.
>
> Thanks,
> Miklos

In the unlikely event that the reference count or the d_flags changes,
we can abort the killing.

Cheers,
Longman