linux-kernel - Re: [PATCH 1/4] fs/dcache: Limit numbers of negative dentries

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Date:   Thu, 20 Jul 2017 11:46:38 -0400
From:   Waiman Long <longman@...hat.com>
To:     Miklos Szeredi <mszeredi@...hat.com>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        Jonathan Corbet <corbet@....net>,
        lkml <linux-kernel@...r.kernel.org>, linux-doc@...r.kernel.org,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH 1/4] fs/dcache: Limit numbers of negative dentries

On 07/20/2017 11:08 AM, Miklos Szeredi wrote:
> On Thu, Jul 20, 2017 at 4:21 PM, Waiman Long <longman@...hat.com> wrote:
>> On 07/20/2017 03:20 AM, Miklos Szeredi wrote:
>>> On Wed, Jul 19, 2017 at 10:42 PM, Waiman Long <longman@...hat.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